In the eggs of many Amphibia a dark granular mass known as the yolk nucleus makes its appearance; and is supposed, without any very clear evidence, to be related to the formation of the yolk.
A body in the form of a shell enclosing a dark nucleus, which is perhaps of the same nature, has been described by Eimer in the Reptilian egg: it eventually resolves itself into a number Of angular fragments. In Elasmobranchii a similar body is perhaps present.
The food-yolk just described is imbedded in the active protoplasmic portion of the body of the ovum. In the case of the mammalian ovum the food-yolk is fairly uniformly distributed, but in the case of all other craniate ova the protoplasm of the ovum is especially concentrated at one pole, which is known as the upper or animal pole, and the food-yolk is more especially concentrated at the opposite pole. The Herring’s ovum forms an apparent exception to this statement, in that the concentration of the protoplasm to form the germinal disc does not take place till after impregnation. In Amphibia the animal pole is mainly marked by the smaller size of the yolk-spherules, but in most other forms a small portion of the ovum in the region of the germinal vesicle is nearly free from yolk-spherules, and then forms a more or less specialized part known as the germinal disc. In Aves, Reptilia, and Elasmobranchii the germinal disc shades off insensibly into the yolk; but in Teleostei it is more sharply marked off, and is continued more or less completely round the periphery of the ovum. In ova with true germinal discs it is the germinal disc alone which undergoes segmentation. The protoplasm of vertebrate ova frequently exhibits a reticulate or sponge-like structure ([fig. 21]) and the reticulum in many cases, e.g. Elasmobranchii and Reptilia, serves to hold the yolk-spheres together. In the Tench it has been observed by Bambeke to penetrate into the vitelline sphere.
In the ova of the Craniata the germinal vesicle is generally polynucleolar. In Amphioxus and Petromyzon there is however but a single nucleolus, and in Mammalia there is usually one special nucleolus and two or three accessory ones. The opposite extreme is reached in many osseous fish where the nucleoli are extremely numerous. The protoplasmic reticulum of the embryonic germinal vesicle may in some instances be retained till the ovum is nearly ripe, but usually assumes a very granular form. It is at first connected with the nucleoli which form nodal points in it, but this relation cannot always be detected in the later stages. A membrane, which in the case of the larger ova becomes very thick, is always present round the germinal vesicle. It is said to be perforated in some Reptilian ova (Eimer). As to the position of the germinal vesicle, it is at first situated in the centre of the ovum, but always eventually travels to the animal pole, and as the egg becomes ripe undergoes changes which will be more especially detailed in the next chapter. In the ova with a large amount of food-yolk it assumes an eccentric position very early.
The homologies of the primary egg membranes of Craniata are still involved in some obscurity. There seem to be three membranes, which may all coexist, and of which one or more are almost always present. These membranes are—
(1) An outermost usually homogeneous non-perforated membrane, which is by most authors regarded as a chorion, but is probably a vitelline membrane—by which name I shall speak of it.
(2) A radiately striated membrane (internal to the former when the two coexist) which can be broken up into a series of separate columns. These give to the membrane its radiate striation, but it is probable that between the columns there are pores sufficiently large to admit of the passage of protoplasmic filaments. This membrane will be spoken of as the zona radiata. It is a differentiation of the outermost layer of the yolk.
(3) Within the zona radiata a third and delicate membrane is occasionally found, especially when the ovum is approaching maturity.
