According to Heymons, the yolk-cells must be Metameric Segmentation.
In eggs which contain a larger quantity of food-yolk, the process by which the endoderm is enveloped by the ectoderm is somewhat different.
Proper, the other, the vitellarium, much more conspicuous and usually consisting of a definite number of large cells, producing yolk material for the growth of the egg.
Nevertheless, instances were adduced where the most careful heating of yolk of egg, milk, hay-infusions, &c., had failed, - the boiled infusions, &c., turning putrid and swarming with bacteria after a few hours.
As in other Molluscan groups, we find a wide variation in the early process of the formation of the first embryonic cells, and their arrangement as a diblastula, dependent on the greater or less amount of food-yolk which is present in the egg-cell when it commences its embryonic changes.
The female organs consist of distinct ovaries and yolk-glands, the ducts of which uniti.
The hypoblast is formed either by a definite invagination or by the immigration of isolated cells, known as vitellophags, which wander through the yolk and later become associated into a definite mesenteron, or by some combination of these two methods.
The embryo passes through three stages - (I) still enclosed within the egg and living on its own yolk; (2) free, within the vitelline mass, which is directly swallowed by the mouth; (3) there is no more vitelline mass, but the embryo is possessed of long external gills, which serve for an exchange of nutritive fluid through the maternal uterus, these gills functioning in the same way as the chorionic villi of the mammalian egg.
In Hemiptera only eleven and in Collembola only yolk, and that the mesenteric epithelium becomes reinforced by six abdominal segments have been detected.
Part by the yolk, and in part by those anterior and posterior rudiments Its composition has been the subject of prolonged difference of which usually form the mesenteron, but that in some Hexapoda opinion.
The germ band evidently marks the ventral aspect of the developing insect, whose body must be completed by the extension of the embryo so as to enclose the yolk dorsally.
Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall.
Parallels may be found in "Prairie oyster," the yolk of an egg with vinegar, pepper, &c. added; or "Scotch woodcock," a savoury of buttered eggs on anchovy toast.
Other phosphoglobulins are vitelline, found in the yolk of hens eggs, and ichthulin, found in the eggs of fish.
- A, reproductive system of Amphilina foliacea: a, glandular pit; b, opening of uterus; b', uterus (black); c, yolk-gland and its duct; d, ovary; e, e', opening and duct of vagina; f, spermotheca; g, male genital opening (gonopore); h, penis; i, vas def erens; j, testes; k, shell-gland.
Ov., ovaries; sh.g., shell gland; y.g., yolk gland; r.s., receptaculum seminis; ut., uterus; X 7.
(The ovary (a) leads into (bb) the oviduct, which is joined at (g) by the duct of the yolk-glands (h, h, Y).
The shell is thick, and operculate in some forms; thin, and provided with filaments, in others; in the latter cases it may contain only a few yolk-granules suspended in an albumen-like substance.
B, anterior portion more highly magnified (from Marshall and Hurst, after Sommer); cs, cirrus sac; d, ductus ejaculatorius; f, female aperture; o, ovary; od, oviduct; p, penis; s, shell-gland; t, anterior testis; u, uterus; va, vp, vasa deferentia; us, vesicula seminalis; y, yolk-gland; yd, its duct.
Each shell contains a single ovum and a mass of yolk-cells.
A canal (Laurer's canal) leads from the oviduct or yolk-duct to the dorsal surface.
The mesodermic portion becomes charged with a yolk-like material (y), and, on the germination of the statoblast, gives rise to the outer layer (mes) of the bud.
In most Ectoprocta, however, the development takes place internally or in an ovicell, and a considerable quantity of yolk is present.
Caldwell showed that it was filled with yolk, and finally established the fact that Platypus as well as Echidna is oviparous.
The relative size of the eggs and the amount of nutritive yolk which they contain are generally much greater in those forms which have a direct development.
The Australasian species come nearest to laying eggs, inasmuch as the eggs are large, full of yolk, and enclosed in a shell; but development normally takes place in the uterus, though abnormally, incompletely developed eggs are extruded.
Ova large and heavily charged with yolk, and provided with a stoutish shell.
Ova minute, with little food-yolk; embryos in the uterus at very different stages of development.
In diameter, with little yolk, and the embryos provided with large trophic vesicles (Willey).
Ova large, with much yolk and thick membrane, like those of Australasian species; embryos with slit-like blastopore and of very different ages in the same uterus, probably born all the year round.
Development and Metamorphosis.-In a great number of batrachians, including most of the European species, the egg is small and the food-yolk is in insufficient quantity to form an external appendage of the embryo.
But in a few European and North American species, and in a great many inhabitants of the tropics, the egg is large and a considerable portion of it persists for a long time as a yolk-sac. Although the segmentation is always complete, it is very irregular in these types, some of which make a distinct approach to the meroblastic egg.
In the Cape species the eggs are smaller, though still of considerable size; the yolk is much less developed, and the egg membrane is thinner though dense.
Maculosa, but as a rule only one fully develops in each uterus, the embryo being nourished on the yolk of the other eggs, which more or less dissolve to form a large mass of nutrient matter.
Y, The yolk-like mesodermic mass.
Intestine; sc; posterior suckers; yk, yolk-glands.
Here the two elements, ovum and yolk-cells, are surrounded by a shell of operculate or of spindle-capped types.
Coincidently, to allow of fertilization and the escape of excess of yolk, and of spermatozoa, other accessory ducts open at this point.
The fertilized ova, provided with yolk and a shell, are next transferred to the "uterus" along which they travel to the exterior.
The ovary (a) leads into (bb) the oviduct, which is joined at (g) by the duct of the yolk-glands (h).
From the oviduct a long duct full of yolk passes backwards almost to the hinder end of the body and ends blindly in a globular dilatation just below the skin.
The egg consists of a fertilized ovum and a mass of yolk-cells.