THE OVUM. 49 



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 proto- 

 plasmic 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 craoiate 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 struc- 

 ture (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 poly- 

 nucleolar. 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 con- 

 nected 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 



BE. 4 



