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.
- Vacuolated Endoderm Cells of cartilaginous consistence from the axis of the tentacle of a Medusa (Cunina).
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.
- Tentaculocyst (statorhabd) ous with the endoderm of Cunina solnzaris.
In the endoderm large concretions are formed (con.).
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.
The endoderm of the medusa shows the same general types of structure as in the polyp, described above.
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.?"
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.
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 ).
The reproductive cells may be regarded as belonging primarily to neither ectoderm nor endoderm, though lodged in the ectoderm in all Hydromedusae.
Cordylus of el, Endoderm lamella.
En, Endoderm lining the enteric cavity.
E, Wandering endoderm cells of the gelatinous substance.
G, With spadix prolonged e.1, Endoderm-lamella.
Have become obliterated by coalescence of their walls, so that the entire endoderm of the umbrella is in the condition of the endodermlamella.
41, D); centrally is seen the spadix (sp.), bearing the generative cells (g), and external to these (1) a layer of ectoderm representing the epithelium of the manubrium; (2) the layer of sub-umbral ectoderm; (3) the endoderm-lamella (ed.); (4) the ex-umbral ectoderm (ex.); and (5) there may or may not be present also an ectotheca.
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.
Of both layers, ecto;?, s.c. ?? ?.; derm and endoderm, s.c. n s ?;, v N containing a prolonga.
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.
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.
The next changes that take place are chiefly in the endoderm-cup (fig.
The endoderm is shaded, the ectoderm left clear.
E.l, Endoderm lamella.
Form the radial canals (r.c.), ring-canal (c.c.), and endoderm-lamella (e.l., fig.
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.
47, A), and becomes separated into a superficial layer, future ectoderm, surrounding a central mass, future endoderm (fig.
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.
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).
Apart from the weighty arguments which the development furnishes against the theories of Allman and Mechnikov, it may be pointed out that neither hypothesis gives a satisfactory explanation of a structure universally present in medusae of whatever class, namely the endoderm-lamella, discovered by the brothers O.
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.
The Hertwigs when they discovered the endoderm-lamella showed on morphological grounds that polyp and medusa are independent types, each produced by modification in different directions of a more primitive type represented in development by the actinulastage.
The tentacles are always solid, containing an axis of endoderm-cells resembling notochordal tissue or plantparenchyma, and are but moderately flexible.
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.
In the wall of the sack is a double layer of endoderm, the space between which is a continuation of the coelenteron.
By coalescence of the endoderm-layers, the coelenteron may be reduced to vessels, usually eight in number, opening into a ring-sinus surrounding the pore.
Thus the disposition of the endoderm-cavities is roughly comparable to the gastrovascular system of a medusa.
- 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).
The endoderm of the planula now acquires a cavity, and at the narrower pole a mouth is formed, giving rise to the primary siphon.
The ectoderm is indicated by close hatching, the endoderm by light hatching, the mesogloea by thick black lines, the horny skeleton of the pneumatophore and sail by dotting.
The invaginated cells (derived from the division of the four big cells) form the endoderm or arch-enteron; the outer cells are the ectoderm.
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.
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.
These two endoderm-rudiments embryonic membrane formed by delamination from the blastoderm, ultimately grow together and give rise to the epithelium of the midwhile in a few insects, including the wingless spring-tails, the emgut.
Stated that in the Muscidae, while the anterior endoderm-rudiment The embryo is invaginated into the yolk, but the surface edges of arises as Kowalevsky had observed, the posterior part of the " midthe blastoderm do not close over, so that a groove or pore puts gut " has its origin as a direct outgrowth from the proctodaeum.
- 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.
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.
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.
17), is the mouth or oral piece; the second, explained by the presence of a " latent endoderm-group " in those the antennal segment; the third, the intercalary or prae-mandibular invaginations.
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.