THE OVUM. 6l 



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 concentra- 

 tion 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 Bam- 

 beke to penetrate into the vitelline sphere. 



In the ova of the Craniata the germinal vesicle is generally 

 polynucleolar. In Amphioxus and Petromyzon there is how- 

 ever 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 under- 

 goes changes which will be more especially detailed in the next 



