Fig. 21. Section through a small part of the surface of an ovum of an immature female of Scyllium canicula.
fe. follicular epithelium. vt. vitelline membrane. Zn. zona radiata. yk. yolk with protoplasmic network.
In Elasmobranchii the first membrane to be formed is the vitelline membrane, which appears in some instances before the formation of the follicle—a fact which appears to shew that it is really formed as a differentiation of the protoplasm of the egg. In most Elasmobranchii this membrane attains a very considerable development. A zona radiata is generally (if not always) present in Elasmobranchii, but arises at a later period than the vitelline membrane ([fig. 21], Zn). The zona radiata always disappears long before the ovum is ripe. The vitelline membrane also gradually atrophies, though it lasts much longer than the zona radiata. When the egg is taken up by the oviduct all trace of both membranes has vanished. In Reptilia precisely the same arrangements of the membranes are found as in Elasmobranchii, except that as a rule the zona radiata is relatively more important. The vitelline membrane is thin except in the Crocodilia. The third innermost membrane is found according to Eimer in many Reptilia. In birds both vitelline membrane and zona radiata are present, but the latter atrophies early, leaving the former as the sole membrane when the egg is ripe.
In osseous fish the vitelline membrane is usually either absent or may perhaps in some instances, e.g. the Perch, be imperfectly represented. In the ripe ovum of the Herring there is a distinctly developed membrane external to the zona radiata which is probably the vitelline membrane. The zona radiata attains a very great development, and is generally provided with knobs of various shapes on its outer surface. A delicate membrane internal to this—my third membrane—has often been described, but there is still some doubt about its existence. In some cases an external less granular layer of the ovum itself has been described as a special membrane. In the Perch a peculiar mucous capsule, penetrated by irregular branched prolongations of the follicle cells, is present in addition to the ordinary membranes. In Petromyzon a zona radiata appears to be present, which in the adult is divided into two layers, both of them radiately striated according to Calberla, but according to Kupffer and Benecke the outer one is not perforated, and would appear therefore to be a vitelline membrane as defined above. A delicate membrane is formed at a comparatively late period around the ova of the Amphibia, and is stated (Waldeyer, No. 6, and Kolessnikow) to have a delicate radial striation. It probably corresponds with the zona radiata.
In Mammalia a radiately striated membrane—the zona radiata—is generally described as being present, and internal to it, in the nearly ripe egg, a delicate membrane has been shewn by E. van Beneden to exist. Externally to the zona radiata there may be observed a granular membrane irregular on its outer surface on which the cells of the discus are supported. This membrane is more or less distinctly separated from the zona radiata; and by tracing back its development it appears very probable that it is the remnant of the first-formed membrane in the very young ovum, and therefore the vitelline membrane.
A micropyle (first discovered by Ransom, No. [74]) is present in a large number of osseous fish and in Petromyzon (Calberla). Doubts have been thrown on its existence in the latter form by Kupffer and Benecke; and at any rate it would only seem to perforate the zona radiata. In the osseous fish in which it has been detected, Salmonidæ, Percidæ (Gasterosteus), Clupeidæe, etc., it forms a minute perforation of the zona radiata at the animal pole, just large enough to admit a single spermatozoon. Its characters differ slightly in different cases, but there is usually a shallow depression, in the centre of which it is situated.
The eggs of all Craniata (except Petromyzon (?)) appear to be enclosed in a cellular envelope known as the follicle. The cells which form this are, as has been already explained, derived from the germinal epithelium[25], and frequently arrange themselves around the ovum before the appearance of the growths of stroma into the epithelium. All young follicles are nearly alike, but as they grow older they exhibit various modifications in the different groups. They retain their simplest condition as a flat epithelial layer in most osseous fish and Amphibia. In most other forms the cells become at some period columnar, and are generally arranged in two or more layers. There is formed externally to the epithelium a delicate membrane—the membrana propria folliculi—which is in its turn enclosed in a vascular connective-tissue sheath.
In Elasmobranchii and many Reptilia (Lacertilia, Ophidia) some of the cells become much larger than the others, and assume a funnel-shaped form with the narrow end in contact with the egg membrane. These large cells, which have a regular arrangement in the epithelium, are probably in some way connected with the nutrition. They have only been noticed in large-yolked ova. Many observers have described prolongations of the follicle cells through the pores of the zona radiata in Aves, Reptilia and Teleostei.
The most remarkable modification of the follicle is that which is found in Mammalia. At first the follicle is similar to that of other Vertebrata, and is formed of flat cells derived from the germinal cells adjoining the ovum. These cells next become columnar and then one or two layers deep. Later they become thicker on one side than on the other, and there appears in the thickened mass a cavity, which gradually becomes more distended and is filled with an albuminous fluid. As the cavity enlarges, the ovum with several layers of cells around it forms a prominence projecting into it. The whole structure with its tunic is known as the Graafian follicle. The follicle cells are known as the membrana granulosa, and the projection, in which the ovum lies, as the discus or cumulus proligerus. The cells of the discus in immediate contiguity to the ovum usually form a more or less specialized layer and are somewhat more columnar than the adjoining cells.
The Spermatozoon.