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endoderm

endoderm

endoderm Sentence Examples

  • The endoderm is shaded, the ectoderm left clear.

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  • 40); in Geryonia, however, it remains double, and the centripetal canals arise by parting of the two layers; (4) excretory endoderm, lining pores at the margin of the umbrella, occurring in certain Leptomedusae as socalled " marginal tubercles," opening, on the one hand, into the ring-canal and, on the other hand, to the exterior by " marginal funnels," which debouch into the sub-umbral cavity above the velum.

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  • - Portion;of the body-wall of Hydra, showing ectoderm cells above, separated by " structureless lamella " from three flagellate endoderm cells below.

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  • - Portion;of the body-wall of Hydra, showing ectoderm cells above, separated by " structureless lamella " from three flagellate endoderm cells below.

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  • - Vacuolated Endoderm Cells of cartilaginous consistence from the axis of the tentacle of a Medusa (Cunina).

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  • Further, two distinct types of otocyst can be recognized in the Hydro medusae; that of the Leptolinae, in which the entire organ is ectodermal, concrement-cells and all, and the organ is not a tentaculocyst; and that of the Trachylinae, in which the organ is a tentaculocyst, and the concrement-cells are endodermal, derived from the endoderm of the modified tentacle, while the rest of the organ is ectodermal.

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  • - Tentaculocyst (statorhabd) ous with the endoderm of Cunina solnzaris.

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  • In the endoderm large concretions are formed (con.).

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  • The process carrying the otolith outer side of a or concretion hk, formed by endoderm cells, is tentacle, two enclosed by an upgrowth forming the " vesicle," nerves run round which is not yet quite closed in at the top. the base of the (After Hertwig.) tentacle to it.

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  • The endoderm of the medusa shows the same general types of structure as in the polyp, described above.

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  • We can distinguish (I) digestive endoderm, in the stomach, often with special glandular elements; (2) circu-, latory endoderm, in the radial and ring canals; (3) supporting endoderm in the axes of the tentacles and in the endodermlamella; the latter is primitively a double layer of cells, produced by concrescence OC-- = w.?"

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  • The reproductive cells may be regarded as belonging primarily to neither ectoderm nor endoderm, though lodged in the ectoderm in all Hydromedusae.

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  • Cordylus of el, Endoderm lamella.

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  • en, Endoderm lining the enteric cavity.

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  • e, Wandering endoderm cells of the gelatinous substance.

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  • have become obliterated by coalescence of their walls, so that the entire endoderm of the umbrella is in the condition of the endodermlamella.

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  • The endodermal spadix (sp) of the sporosac represents the endoderm of the manubrium; the ectodermal lining of the sporosac (ex.) represents the ex-umbral ectoderm of the medusa; and the intervening layers, together with the sub-umbral cavity, have disappeared.

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  • e.l, Endoderm lamella.

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  • The tissues of the bud become differentiated into ectoderm and endoderm, and the endoderm of the bud becomes secondarily continuous with that of the parent, but no part of the parental endoderm contributes to the building up of the daughter-polyp. Lang regarded this method of budding as universal in polyps, a notion disproved by O.

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  • In individuals either of the male or female sex, germ-cells which are quite undifferentiated and neutral in character, become amoeboid, and wander into the endoderm.

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  • endoderm, containing the st, Stomach, which in H ac coelenteric cavity (cod), quires a secondary com while the outer layer munication with the diges furnishes the future ectotive cavity of the mother.

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  • Endoderm.

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  • The thick black line represents endoderm, the thinner line ectoderm.

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  • The endoderm of the planula now acquires a cavity, and at the narrower pole a mouth is formed, giving rise to the primary siphon.

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  • The invaginated cells (derived from the division of the four big cells) form the endoderm or arch-enteron; the outer cells are the ectoderm.

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  • ing gs, the bilobed arch sh, The primitive shell-sac or enteron or lateral vesicles shell-gland ° of invaginated endoderm, pi, The rectal peduncle or whichwill develop into liver.

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  • The body-cavity and the muscular, fibrous and vascular tissues are traced partly to two symmetrically disposed " mesoblasts," which bud off from the invaginated arch-enteron, partly to cells derived from the ectoderm, which at a very early stage is connected by long processes with the invaginated endoderm.

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  • It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula.

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  • He finds that the endoderm arises may be readily distinguished, six of which subsequently enter into from an anterior and a posterior rudiment derived from the " endothe formation of the head, three going to the thorax and twelve to blast," that many of the cells of these rudiments wander into the the abdomen.

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  • size compared with the thorax or abdomen, but in the embryo it On the whole it seems likely that the endoderm is represented in forms a much larger portion of the body than it does in the adult.

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  • en, Endoderm.

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  • There is some evidence that in this group the ectoderm of the oesophagus is chiefly concerned with digestion, whereas the endoderm of the intestine is limited to the absorption of the soluble products.

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  • On the ether hand, a survey of the facts of cellular embryology which were accumulated in regard to a variety of classes within a few years of Kovalevsky's work led to a generalization, independently arrived at by Haeckel and Lankester, to the effect that a lower grade of animals may be distinguished, the Protozoa or Plastidozoa, which consist either of single cells or colonies of equiformal cells, and a higher grade, the Metazoa or Enterozoa, in which the egg-cell by " cell division " gives rise to two layers of cells, the endoderm and the ectoderm, surrounding a primitive digestive chamber, the archenteron.

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  • (I) The body is built up of two layers only, an external protective and sensory layer, the ectoderm, and an internal digestive layer, the endoderm.

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  • (3) The generative cells are produced in either the ectoderm or endoderm, and not in a third layer arising in the embryo, distinct from the two primary layers; in other words, there is no mesoderm or coelom.

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  • In this stage the body is composed of two layers, ectoderm (d) externally, and endoderm (c) internally, surrounding a central cavity, the archenteron (b), which communicates with the exterior by a pore (a), the blastopore.

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  • ep, Ectoderm; hy, endoderm; al, enteric cavity.

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  • Round the blastopore hollow outgrowths, variable in number, arise by the evagination of the entire body-wall, both ectoderm and endoderm.

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  • Between the ectoderm and endoderm a gelatinous supporting layer, termed the mesogloea, makes its appearance.

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  • c, Endoderm.

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  • Further, in the hydropolyp the digestive cavity either remains simple and undivided and circular in transverse section, or may show ridges projecting internally, which in this case are formed of endoderm alone, without any participation of the mesogloea.

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  • As in all Hydrozoa (q.v.) the body wall is composed of two cell-layers, the ectoderm and endoderm.

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  • It may be traversed by processes of the cells of the ectoderm and endoderm, or it may contain cells which have migrated into it from these two layers.

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  • The ectoderm covers the whole external surface of the animal, while the endoderm lines the coelenteron or gastrovascular space; the two layers meet each other, and become continuous, at the edge of the mouth.

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  • From the ring-canal are given off tentacle-canals which run down the axis of each tentacle; in many cases, however, the cavity of the tentacle is obliterated and instead of a canal the tentacle contains a solid core of endoderm.

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  • The gonads or generative organs may be produced either in the ectoderm or the endoderm.

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  • As a result of this change of form the gastric cavity or coelenteron becomes of compressed lenticular form, and the endoderm lining it can be distinguished as an upper or exumbral layer and a lower or subumbral layer.

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  • The next event is a great growth in thickness of the gelatinous mesogloea, especially on the exumbral side; as a result the flattened coelenteron is still further compressed so that in certain spots its cavity is obliterated, and its exumbral and subumbral layers of endoderm come into contact and undergo concrescence.

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  • The two apposed layers of endoderm in the cathammal area undergo complete fusion to form a single layer of epithelium, the endoderm-lamella of the adult medusa.

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  • This cavity is excavated in a third mass of cells distinct from the cells lining the gut, forming the endoderm, and the cells covering the surface of the body, the ectoderm.

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  • The invaginated layer is the enteric cell-layer or endoderm; the outer cell-layer is the dermic cell-layer or ectoderm.

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  • The cavity communicating with the blastopore and lined by the endoderm is the archenteron.

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  • In eggs which contain a larger quantity of food-yolk, the process by which the endoderm is enveloped by the ectoderm is somewhat different.

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  • Between ectoderm and endoderm a third intermediate cell-layer _r B (After Lankester, 15.) FIG.

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  • the cleavage of the original en, Endoderm or enteric cell layer.

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  • The mesoderm arises for the most part from the endoderm.

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  • The Coelentera, as contrasted with other Metazoa (but not Parazoa), consist of two layers of cells only, an outer layer or ectoderm, an inner layer or endoderm.

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  • In the remaining Metazoa certain cells are budded off at an early stage of development from one or both of the two original layers, to form later a third layer, the mesoderm, which lies between the ectoderm and endoderm; such forms have therefore received the name Triploblastica.

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  • In the Coelentera the ectoderm and endoderm are set apart from one another at a very early period in the life-history; generally either by delamination or invagination, processes described in the article Embryology.

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  • The endoderm is generally also an epithelium one cell in thickness, the cells being digestive, secretory and sometimes muscular.

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  • The mesogloea is in itself an inert non-cellular secretion, but the immigration of muscular and other cells into its substance, from both ectoderm and endoderm, gives it in many cases a strong resemblance to the mesoderm of Triploblastica, - a resemblance which, while probably superficial, may yet serve to indicate the path of evolution of the mesoderm.

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  • The Coelentera may thus be briefly defined as Metazoa which exhibit two embryonic cell-layers only, - the ectoderm and endoderm, - their body-cavities being referable to a single cavity or coelenteron in the endoderm.

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  • The Scyphozoa have the following features in common: - They typically exhibit an ectodermal stomodaeum; partitions or mesenteries project into their coelenteron from the body-wall, and on these are generally concentrated digestive cells (to form mesenterial filaments, phacellae or gastric filaments, &c.); the external musculature of the body-wall is circular (except in Cerianthus); the internal, longitudinal; and the sexual cells probably always arise in the endoderm.

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  • The sexual cells of the medusoid lie in the endoderm on interradii, that is, on the second set of radii accentuated in the course of development.

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  • As regards the other three groups, however, it is easy to conceive of them as derived from an ancestor, represented to-day to some extent by the planula-larva, which was Coelenterate in so far as it was composed of an ectoderm and endoderm, and had an internal digestive cavity (I.

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  • At the parting of the ways which led, on the one hand, to modern Scyphomedusae, on the other to Anthozoa (III.), it is probable that the common ancestor was marked by incipient mesenteries and by the limitation of the sexual cells to endoderm.

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  • The gonads are formed in the endoderm (hence " Entocarpeae "), and the generative products are shed into the gastric cavity and pass to the exterior by way of the mouth.

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  • A true velum, such as is found in Hydromedusae, never contains endoderm.

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  • The gonads are folds of the endoderm containing generative cells, and are primitively four in number, situated interradially, but each gonad may be divided into two by the partition which separates two adjacent lobes of the stomach, that is to say, by one of the areas of concrescence between exumbral and subumbral endoderm, whence arises a condition with eight gonads which is by no means uncommon.

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  • 6) and consists of a short, hollow rod, the wall of which is composed of the two bodylayers, ectoderm and endoderm, enclosing a cavity continuous with that of the gastrovascular system.

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  • At the apex of the rhopalium the endoderm is greatly thickened and consists of concrementcells secreting otoliths (Con).

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  • L.o.c., Endoderm lamella (line of concrescence of the walls of the enteric cavity of the umbrella, whereby its single chamber is broken up into four pouches).

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  • disc. The view is from the aboral surface, magnified H, Bridge between the two End, Endoderm.

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  • The subumbral ocellus of Aurelia is found to be of the inverted type, with the visual cones turned away from the light, as in Tiaropsis amongst Hydromedusae, and here also the pigment is furnished by the endoderm, forming a cup into which the ectodermal visual cells project (Schewiakoff ['31) In) the Stauromedusae tentaculocysts are either absent altogether, as in - St.

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  • Each taeniola bears a strongly developed longitudinal muscle-band, stated by Claus and Chun to be developed from the endoderm, like the retractor muscles of the anthopolyp, but by other investigators it is affirmed that each retractor muscle of the scyphistoma arises from the lining of a funnel-shaped ectodermal ingrowth (" Septaltrichter ") growing down from the peristome inside each taeniola, in a manner similar to the infundibular cavities of Lucernaria, which in their turn are homologous with the sub f genital cavities of Scypho l A .` medusae.

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  • hy, Endoderm.

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  • The diagnostic features of the class Scyphozoa thus constituted are supposed to be (I) an ectodermal oesophagus or stomodaeum, (2) a gastric cavity subdivided by mesenteries, (3) gonads formed in the endoderm.

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  • As in all Arthropoda, it is composed of three divisions, a fore-gut or stomodaeum, ectodermal in origin and lined by an inturning of the chitinous cuticle, a mid-gut formed by endoderm and without a cuticular lining, and a hind-gut or proctodaeum, which, like the fore-gut, is ectodermal and is lined by cuticle.

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  • One large cell, or megamere, remains for some time unsegmented but is finally segmented and forms the endoderm cells which are invaginated.

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  • The outer layer is known technically as the ectoderm, the inner layer as the endoderm.

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  • Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell-layers of the body-wall; the mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains skeletal elements formed by cells which have migrated into it from the ectoderm.

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  • By means of circularly disposed muscular fibrils formed from the endoderm the tentacles can be protracted or thrust out after contraction.

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  • 4 organs or gonads are borne on the mesenteries, the germinal cells being derived from the inner layer or endoderm.

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  • The inner layer or endoderm is also a cellular layer, and is chiefly made up of columnar cells, each bearing a cilium at its free extremity and terminating internally in a long muscular fibre.

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  • In Actinians the epithelio-muscular cells of the endoderm are crowded with yellow spherical bodies, which are unicellular plants or Algae, living symbiotically in the tissues of the zooid.

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  • The endoderm contains in addition gland cells and nervous elements.

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  • In the course of development, however, cells from the ectoderm and endoderm may migrate into it.

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  • These form a more or less complicated canal system, lined by endoderm, and communicating with the cavities of the zooids.

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  • 2.-I, Portion of epithelium from the tentacle of an Actinian, showing three supporting cells and one sense cell (sc); 2, a cnidoblast with enclosed nematocyst from the same specimen; 3 and 4, two forms of gland cell from the stomodaeum; 5a, 5b, epithelio-muscular cells from the tentacle in different states of contraction; 5c, an epithelio-muscular cell from the endoderm, containing a symbiotic zooxanthella; 6, a ganglion cell from the ectoderm of the peristome.

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  • ec, Ectoderm; en, endoderm; mg, mesogloea; m, m, mesenteries; s, septum; b, basal plate formed of ellipsoids of carbonate of lime secreted by the basal ectoderm; ep, epitheca.

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  • (Af ter von Koch.) formed, the endoderm of the basal disk lying above the basal plate is raised up in the form of radiating folds.

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  • The ectoderm beneath each fold becomes detached from the surface of the basal plate, and both it and the mesogloea are folded conformably with the endoderm.

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  • 16 and 17 that every ec septum is covered by a fold of endoderm, mesogloea, and ectoderm, and is in fact pushed into the cavity of the zooid from without.

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  • (C original; the rest after von Koch.) aporose corals, the only communication between the cavity of the edge-zone and the general cavity of the zooid is by way of the lip of the calicle; in the latter, or perforate corals, the theca is permeated by numerous branching and anastomosing canals lined by endoderm, which place the cavity of the edge-zone in communication with the general cavity of the zooid.

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  • Corals have been divided into A porosa and Perforata, according as the theca and septa are compact and solid, or are perforated by pores containing canals lined by endoderm.

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  • The various divisions of the perivisceral cavity develop as a series of spaces between the ectoderm and endoderm, and later in the mesoderm.

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  • A, Eafly stage; no trace of the vascular space; endoderm and ectoderm in contact.

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  • B, Endoderm has separated from the dorsal and ventral ectoderm.

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  • The first cells of this sort to form in the embryo are called the ectoderm, the mesoderm and the endoderm.

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  • In contrast the bottle cells, which form anterior endoderm, behave normally.

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  • Active cell migration drives the unilateral cell movements of the anterior visceral endoderm.

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  • The solution to this problem awaited identification of the substances produced by the future endoderm.

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  • endoderm formation in the major animal models.

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  • endoderm cells, the water is forced to move through the cytoplasm.

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  • First, how do liver precursors adopt their specific fate within the foregut endoderm?

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  • foregut endoderm?

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  • organogenesis in the gastrointestinal endoderm and limb development.

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  • The endoderm of the polyp is typically a flagellated epithelium of large cells (fig.

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  • In different parts of the coelenteron the endoderm may be of three principal types - (i) digestive endoderm, the primitive type, with cells of large size and considerably vacuolated, found in the hydranth; some of these cells may become special glandular cells, without flagella or contractile processes; (2) circulatory endoderm, without vacuoles and without basal contractile processes, found in the hydrorhiza and hydrocaulus; (3) supporting endoderm (fig.

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  • - Vacuolated Endoderm Cells of cartilaginous consistence from the axis of the tentacle of a Medusa (Cunina).

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  • Further, two distinct types of otocyst can be recognized in the Hydro medusae; that of the Leptolinae, in which the entire organ is ectodermal, concrement-cells and all, and the organ is not a tentaculocyst; and that of the Trachylinae, in which the organ is a tentaculocyst, and the concrement-cells are endodermal, derived from the endoderm of the modified tentacle, while the rest of the organ is ectodermal.

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  • - Tentaculocyst (statorhabd) ous with the endoderm of Cunina solnzaris.

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  • In the endoderm large concretions are formed (con.).

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  • The process carrying the otolith outer side of a or concretion hk, formed by endoderm cells, is tentacle, two enclosed by an upgrowth forming the " vesicle," nerves run round which is not yet quite closed in at the top. the base of the (After Hertwig.) tentacle to it.

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  • The endoderm of the medusa shows the same general types of structure as in the polyp, described above.

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  • We can distinguish (I) digestive endoderm, in the stomach, often with special glandular elements; (2) circu-, latory endoderm, in the radial and ring canals; (3) supporting endoderm in the axes of the tentacles and in the endodermlamella; the latter is primitively a double layer of cells, produced by concrescence OC-- = w.?"

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  • 40); in Geryonia, however, it remains double, and the centripetal canals arise by parting of the two layers; (4) excretory endoderm, lining pores at the margin of the umbrella, occurring in certain Leptomedusae as socalled " marginal tubercles," opening, on the one hand, into the ring-canal and, on the other hand, to the exterior by " marginal funnels," which debouch into the sub-umbral cavity above the velum.

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  • As has been described above, the endoderm may also contribute to the sense-organs, but such contributions are always of an accessory nature, for instance, concrement-cells in the otocysts, pigment in the ocelli, and never of sensory nature, sense-cells being Hydromedusae are of separate sexes, the only known exception being Amphogona apsteini, one of the Trachomedusae (Browne [9]).

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  • The reproductive cells may be regarded as belonging primarily to neither ectoderm nor endoderm, though lodged in the ectoderm in all Hydromedusae.

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  • Cordylus of el, Endoderm lamella.

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  • en, Endoderm lining the enteric cavity.

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  • e, Wandering endoderm cells of the gelatinous substance.

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  • have become obliterated by coalescence of their walls, so that the entire endoderm of the umbrella is in the condition of the endodermlamella.

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  • The endodermal spadix (sp) of the sporosac represents the endoderm of the manubrium; the ectodermal lining of the sporosac (ex.) represents the ex-umbral ectoderm of the medusa; and the intervening layers, together with the sub-umbral cavity, have disappeared.

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  • of both layers, ecto;?, s.c. ?? ?.; derm and endoderm, s.c. n s ?;, v N containing a prolonga.

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  • The ectodermal ingrowth is the entocodon (Gc.); it bulges into, and pushes down, the endoderm at the apex of the bud, and if solid it soon acquires a cavity (fig.

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  • The cavity of the entocodon increases continually in size, while the endoderm pushes up at the sides of it to form a cup with hollow walls, enclosing but not quite surrounding the Gc entocodon, which C remains in contact at its outer side with the ectoderm covering the bud (fig.

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  • The endoderm is shaded, the ectoderm left clear.

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  • e.l, Endoderm lamella.

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  • The tissues of the bud become differentiated into ectoderm and endoderm, and the endoderm of the bud becomes secondarily continuous with that of the parent, but no part of the parental endoderm contributes to the building up of the daughter-polyp. Lang regarded this method of budding as universal in polyps, a notion disproved by O.

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  • 47, A), and becomes separated into a superficial layer, future ectoderm, surrounding a central mass, future endoderm (fig.

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  • In individuals either of the male or female sex, germ-cells which are quite undifferentiated and neutral in character, become amoeboid, and wander into the endoderm.

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  • endoderm, containing the st, Stomach, which in H ac coelenteric cavity (cod), quires a secondary com while the outer layer munication with the diges furnishes the future ectotive cavity of the mother.

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  • We may distinguish the following series of stages: (I) ovum; (2) cleavage, leading to formation of a blastula; (3) formation of an inner mass or parenchyma, the future endoderm, by immigration or delamination, leading to the so-called parenchymula-stage; (4) formation of an archenteric cavity, the future coelenteron, by a splitting of the internal parenchyma, and of a blastopore, the future mouth, by perforation at one pole, leading to the gastrula-stage; (5) the outgrowth of tentacles round the mouth (blastopore), leading to the actinula-stage; and (6) the actinula becomes the polyp or medusa in the manner described elsewhere (see articles Hydrozoa, POLYP and Medusa).

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  • It would be necessary to regard this structure as a secondary extension of the endoderm in the tentacle-web, on Allman's theory, or between the outgrowths of the hydrorhiza, on Mechnikov's hypothesis.

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  • It consists of a hollow tube, or tubes, of which the wall is made up of the two body-layers, ectoderm and endoderm, and the cavity is a continuation of the digestive cavities of the nutritive and other appendages, i.e.

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  • The thick black line represents endoderm, the thinner line ectoderm.

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  • - The fertilized ovum gives rise to a parenchymula, with solid endoderm, which is set free as a free-swimming planula larva, in the manner already described (see Hydrozoa).

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  • The endoderm of the planula now acquires a cavity, and at the narrower pole a mouth is formed, giving rise to the primary siphon.

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  • The invaginated cells (derived from the division of the four big cells) form the endoderm or arch-enteron; the outer cells are the ectoderm.

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  • ing gs, the bilobed arch sh, The primitive shell-sac or enteron or lateral vesicles shell-gland ° of invaginated endoderm, pi, The rectal peduncle or whichwill develop into liver.

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  • The other extreme end closes, but the invaginated endoderm cells remain in continuity with this extremity of the blastopore, and form the " rectal peduncle " or " pedicle of invagination " of Lankester, although the endoderm cells retain no contact with the middle region of the now closed-up blastopore.

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  • The body-cavity and the muscular, fibrous and vascular tissues are traced partly to two symmetrically disposed " mesoblasts," which bud off from the invaginated arch-enteron, partly to cells derived from the ectoderm, which at a very early stage is connected by long processes with the invaginated endoderm.

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  • It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula.

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  • - The segments are perceptible at a very regarded as the true endoderm in the hexapod embryo, for he states early stage of the development as a number of transverse bands (1897) that in the bristle-tail Lepisma and in dragon-flies they give arranged in a linear sequence.

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  • He finds that the endoderm arises may be readily distinguished, six of which subsequently enter into from an anterior and a posterior rudiment derived from the " endothe formation of the head, three going to the thorax and twelve to blast," that many of the cells of these rudiments wander into the the abdomen.

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  • size compared with the thorax or abdomen, but in the embryo it On the whole it seems likely that the endoderm is represented in forms a much larger portion of the body than it does in the adult.

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  • en, Endoderm.

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  • There is some evidence that in this group the ectoderm of the oesophagus is chiefly concerned with digestion, whereas the endoderm of the intestine is limited to the absorption of the soluble products.

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  • On the ether hand, a survey of the facts of cellular embryology which were accumulated in regard to a variety of classes within a few years of Kovalevsky's work led to a generalization, independently arrived at by Haeckel and Lankester, to the effect that a lower grade of animals may be distinguished, the Protozoa or Plastidozoa, which consist either of single cells or colonies of equiformal cells, and a higher grade, the Metazoa or Enterozoa, in which the egg-cell by " cell division " gives rise to two layers of cells, the endoderm and the ectoderm, surrounding a primitive digestive chamber, the archenteron.

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  • (I) The body is built up of two layers only, an external protective and sensory layer, the ectoderm, and an internal digestive layer, the endoderm.

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  • (3) The generative cells are produced in either the ectoderm or endoderm, and not in a third layer arising in the embryo, distinct from the two primary layers; in other words, there is no mesoderm or coelom.

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  • In this stage the body is composed of two layers, ectoderm (d) externally, and endoderm (c) internally, surrounding a central cavity, the archenteron (b), which communicates with the exterior by a pore (a), the blastopore.

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  • ep, Ectoderm; hy, endoderm; al, enteric cavity.

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  • Round the blastopore hollow outgrowths, variable in number, arise by the evagination of the entire body-wall, both ectoderm and endoderm.

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  • Between the ectoderm and endoderm a gelatinous supporting layer, termed the mesogloea, makes its appearance.

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  • The endoderm is shaded, the ectoderm is left clear.

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  • c, Endoderm.

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  • endoderm meet at this point.

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  • Further, in the hydropolyp the digestive cavity either remains simple and undivided and circular in transverse section, or may show ridges projecting internally, which in this case are formed of endoderm alone, without any participation of the mesogloea.

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  • In the anthopolyp, on the other hand, the digestive cavity is always subdivided by so-called mesenteries, in-growths of the endoderm containing vertical lamellae of mesogloea (see Anthozoa).

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  • Diagram of the structure of a medusa; the ectoderm is left clear, the endoderm is dotted, the mesogloea is shaded black; a-b, principal axis (see Hydrozoa); to the left of this line the section is supposed to pass through an inter-radius (I.R.); to the right through a radius (R).

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  • As in all Hydrozoa (q.v.) the body wall is composed of two cell-layers, the ectoderm and endoderm.

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  • It may be traversed by processes of the cells of the ectoderm and endoderm, or it may contain cells which have migrated into it from these two layers.

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  • The ectoderm covers the whole external surface of the animal, while the endoderm lines the coelenteron or gastrovascular space; the two layers meet each other, and become continuous, at the edge of the mouth.

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  • From the ring-canal are given off tentacle-canals which run down the axis of each tentacle; in many cases, however, the cavity of the tentacle is obliterated and instead of a canal the tentacle contains a solid core of endoderm.

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  • Oesophagus, stomach, radial canals, ring-canal and tentacle-canals, constitute together the gastrovascular system and are lined throughout by endoderm, which forms also a flat sheet of cells connecting the radial canals and ring canal together like a web; this is the so-called endoderm-lamella (e.l.), a most important feature of medusan morphology, the nature of which will be apparent when the development is described.

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  • The gonads or generative organs may be produced either in the ectoderm or the endoderm.

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  • As a result of this change of form the gastric cavity or coelenteron becomes of compressed lenticular form, and the endoderm lining it can be distinguished as an upper or exumbral layer and a lower or subumbral layer.

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  • The next event is a great growth in thickness of the gelatinous mesogloea, especially on the exumbral side; as a result the flattened coelenteron is still further compressed so that in certain spots its cavity is obliterated, and its exumbral and subumbral layers of endoderm come into contact and undergo concrescence.

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  • The two apposed layers of endoderm in the cathammal area undergo complete fusion to form a single layer of epithelium, the endoderm-lamella of the adult medusa.

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  • This cavity is excavated in a third mass of cells distinct from the cells lining the gut, forming the endoderm, and the cells covering the surface of the body, the ectoderm.

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  • The invaginated layer is the enteric cell-layer or endoderm; the outer cell-layer is the dermic cell-layer or ectoderm.

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  • The cavity communicating with the blastopore and lined by the endoderm is the archenteron.

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  • In eggs which contain a larger quantity of food-yolk, the process by which the endoderm is enveloped by the ectoderm is somewhat different.

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  • Between ectoderm and endoderm a third intermediate cell-layer _r B (After Lankester, 15.) FIG.

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  • the cleavage of the original en, Endoderm or enteric cell layer.

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  • The mesoderm arises for the most part from the endoderm.

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  • ae, Arch-enteron or cavity lined by the enteric cell-layer or endoderm.

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  • The Coelentera, as contrasted with other Metazoa (but not Parazoa), consist of two layers of cells only, an outer layer or ectoderm, an inner layer or endoderm.

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  • In the remaining Metazoa certain cells are budded off at an early stage of development from one or both of the two original layers, to form later a third layer, the mesoderm, which lies between the ectoderm and endoderm; such forms have therefore received the name Triploblastica.

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  • In the Coelentera the ectoderm and endoderm are set apart from one another at a very early period in the life-history; generally either by delamination or invagination, processes described in the article Embryology.

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  • The endoderm is generally also an epithelium one cell in thickness, the cells being digestive, secretory and sometimes muscular.

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  • The mesogloea is in itself an inert non-cellular secretion, but the immigration of muscular and other cells into its substance, from both ectoderm and endoderm, gives it in many cases a strong resemblance to the mesoderm of Triploblastica, - a resemblance which, while probably superficial, may yet serve to indicate the path of evolution of the mesoderm.

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  • The Coelentera may thus be briefly defined as Metazoa which exhibit two embryonic cell-layers only, - the ectoderm and endoderm, - their body-cavities being referable to a single cavity or coelenteron in the endoderm.

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  • The Scyphozoa have the following features in common: - They typically exhibit an ectodermal stomodaeum; partitions or mesenteries project into their coelenteron from the body-wall, and on these are generally concentrated digestive cells (to form mesenterial filaments, phacellae or gastric filaments, &c.); the external musculature of the body-wall is circular (except in Cerianthus); the internal, longitudinal; and the sexual cells probably always arise in the endoderm.

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  • The sexual cells of the medusoid lie in the endoderm on interradii, that is, on the second set of radii accentuated in the course of development.

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  • As regards the other three groups, however, it is easy to conceive of them as derived from an ancestor, represented to-day to some extent by the planula-larva, which was Coelenterate in so far as it was composed of an ectoderm and endoderm, and had an internal digestive cavity (I.

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  • At the parting of the ways which led, on the one hand, to modern Scyphomedusae, on the other to Anthozoa (III.), it is probable that the common ancestor was marked by incipient mesenteries and by the limitation of the sexual cells to endoderm.

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  • The gonads are formed in the endoderm (hence " Entocarpeae "), and the generative products are shed into the gastric cavity and pass to the exterior by way of the mouth.

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  • A true velum, such as is found in Hydromedusae, never contains endoderm.

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  • The gonads are folds of the endoderm containing generative cells, and are primitively four in number, situated interradially, but each gonad may be divided into two by the partition which separates two adjacent lobes of the stomach, that is to say, by one of the areas of concrescence between exumbral and subumbral endoderm, whence arises a condition with eight gonads which is by no means uncommon.

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  • Taking as a starting-point the wide archenteric cavity which the medusa inherits primitively from the antecedent actinula-stage (see article Medusa), we find, in such a form as Tessera, four interradial areas of concrescence between the exumbral and subumbral layers of endoderm, four so-called septal nodes or " cathammata," subdividing the stomach into four wide, radially situated pouches which communicate with each other beyond the septal nodes by wide apertures constituting what is termed by courtesy a ring-canal.

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  • 6) and consists of a short, hollow rod, the wall of which is composed of the two bodylayers, ectoderm and endoderm, enclosing a cavity continuous with that of the gastrovascular system.

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  • At the apex of the rhopalium the endoderm is greatly thickened and consists of concrementcells secreting otoliths (Con).

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  • L.o.c., Endoderm lamella (line of concrescence of the walls of the enteric cavity of the umbrella, whereby its single chamber is broken up into four pouches).

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  • disc. The view is from the aboral surface, magnified H, Bridge between the two End, Endoderm.

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  • The subumbral ocellus of Aurelia is found to be of the inverted type, with the visual cones turned away from the light, as in Tiaropsis amongst Hydromedusae, and here also the pigment is furnished by the endoderm, forming a cup into which the ectodermal visual cells project (Schewiakoff ['31) In) the Stauromedusae tentaculocysts are either absent altogether, as in - St.

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  • Each taeniola bears a strongly developed longitudinal muscle-band, stated by Claus and Chun to be developed from the endoderm, like the retractor muscles of the anthopolyp, but by other investigators it is affirmed that each retractor muscle of the scyphistoma arises from the lining of a funnel-shaped ectodermal ingrowth (" Septaltrichter ") growing down from the peristome inside each taeniola, in a manner similar to the infundibular cavities of Lucernaria, which in their turn are homologous with the sub f genital cavities of Scypho l A .` medusae.

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  • hy, Endoderm.

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  • The diagnostic features of the class Scyphozoa thus constituted are supposed to be (I) an ectodermal oesophagus or stomodaeum, (2) a gastric cavity subdivided by mesenteries, (3) gonads formed in the endoderm.

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  • As in all Arthropoda, it is composed of three divisions, a fore-gut or stomodaeum, ectodermal in origin and lined by an inturning of the chitinous cuticle, a mid-gut formed by endoderm and without a cuticular lining, and a hind-gut or proctodaeum, which, like the fore-gut, is ectodermal and is lined by cuticle.

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  • One large cell, or megamere, remains for some time unsegmented but is finally segmented and forms the endoderm cells which are invaginated.

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  • The outer layer is known technically as the ectoderm, the inner layer as the endoderm.

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  • Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell-layers of the body-wall; the mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains skeletal elements formed by cells which have migrated into it from the ectoderm.

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  • By means of circularly disposed muscular fibrils formed from the endoderm the tentacles can be protracted or thrust out after contraction.

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  • 4 organs or gonads are borne on the mesenteries, the germinal cells being derived from the inner layer or endoderm.

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  • The inner layer or endoderm is also a cellular layer, and is chiefly made up of columnar cells, each bearing a cilium at its free extremity and terminating internally in a long muscular fibre.

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  • In Actinians the epithelio-muscular cells of the endoderm are crowded with yellow spherical bodies, which are unicellular plants or Algae, living symbiotically in the tissues of the zooid.

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  • The endoderm contains in addition gland cells and nervous elements.

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  • In the course of development, however, cells from the ectoderm and endoderm may migrate into it.

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  • These form a more or less complicated canal system, lined by endoderm, and communicating with the cavities of the zooids.

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  • 2.-I, Portion of epithelium from the tentacle of an Actinian, showing three supporting cells and one sense cell (sc); 2, a cnidoblast with enclosed nematocyst from the same specimen; 3 and 4, two forms of gland cell from the stomodaeum; 5a, 5b, epithelio-muscular cells from the tentacle in different states of contraction; 5c, an epithelio-muscular cell from the endoderm, containing a symbiotic zooxanthella; 6, a ganglion cell from the ectoderm of the peristome.

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  • ec, Ectoderm; en, endoderm; mg, mesogloea; m, m, mesenteries; s, septum; b, basal plate formed of ellipsoids of carbonate of lime secreted by the basal ectoderm; ep, epitheca.

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  • (Af ter von Koch.) formed, the endoderm of the basal disk lying above the basal plate is raised up in the form of radiating folds.

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  • The ectoderm beneath each fold becomes detached from the surface of the basal plate, and both it and the mesogloea are folded conformably with the endoderm.

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  • 16 and 17 that every ec septum is covered by a fold of endoderm, mesogloea, and ectoderm, and is in fact pushed into the cavity of the zooid from without.

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  • (C original; the rest after von Koch.) aporose corals, the only communication between the cavity of the edge-zone and the general cavity of the zooid is by way of the lip of the calicle; in the latter, or perforate corals, the theca is permeated by numerous branching and anastomosing canals lined by endoderm, which place the cavity of the edge-zone in communication with the general cavity of the zooid.

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  • Corals have been divided into A porosa and Perforata, according as the theca and septa are compact and solid, or are perforated by pores containing canals lined by endoderm.

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  • The enteron arises as a space in the endoderm, and an opacity - the primitive streak - appears at the hind end of the blastopore (fig.

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  • The various divisions of the perivisceral cavity develop as a series of spaces between the ectoderm and endoderm, and later in the mesoderm.

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  • A, Eafly stage; no trace of the vascular space; endoderm and ectoderm in contact.

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  • B, Endoderm has separated from the dorsal and ventral ectoderm.

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  • A process (gastrulation) leads to the formation of three distinct layers called germ layers: the ectoderm (outer layer), the mesoderm (middle layer), and the endoderm (inner layer).

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  • The endoderm forms the lining of lungs, bladder, digestive tract, tongue, tonsils, and other organs.

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  • Finally, the digestive system and some glands are developed from the endoderm cells.

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  • It would be necessary to regard this structure as a secondary extension of the endoderm in the tentacle-web, on Allman's theory, or between the outgrowths of the hydrorhiza, on Mechnikov's hypothesis.

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  • In the wall of the sack is a double layer of endoderm, the space between which is a continuation of the coelenteron.

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  • The other extreme end closes, but the invaginated endoderm cells remain in continuity with this extremity of the blastopore, and form the " rectal peduncle " or " pedicle of invagination " of Lankester, although the endoderm cells retain no contact with the middle region of the now closed-up blastopore.

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  • The endoderm is shaded, the ectoderm is left clear.

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  • endoderm meet at this point.

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  • In the wall of the sack is a double layer of endoderm, the space between which is a continuation of the coelenteron.

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