THE OVUM. 59 



as described above. At a later period a number of the cells in 

 the deeper layer of the epithelium, as well as certain cells in the 

 superficial part, become primitive germinal cells, while the re- 

 mainder of the cells become smaller and are destined to form the 

 follicle cells. 



The most conspicuous primitive germinal cells are situated 

 in the superficial layer of epithelium ; and the primitive germinal 

 cells in the deeper layers of the germinal epithelium are not 

 nearly so marked as in most Craniata, so that it is difficult in some 

 cases to be sure of their destination till their nucleus commences 

 to undergo its characteristic metamorphosis. 



The change of the primitive ova into permanent ova takes 

 place in the same manner in Mammals as in Elasmobranchii, 

 except that the fusion of the primitive ova into polynuclear 

 masses is much rarer. The formation of the at first quite simple 

 follicles takes place while the ova are still aggregated in large 

 masses ; and the first follicles are formed in the innermost part 

 of the germinal epithelium. Soon after their formation the folli- 

 cles become isolated by connective-tissue growths. 



Post-embryonic development of the ova. 



The ova of the Vertebrata differ greatly in size and structure. 

 The differences in size depend upon the quantity of the food- 

 yolk. In the Amphioxus and Mammalia, in which the ova are 

 smallest, the comparatively insignificant amount of food-yolk is 

 distributed uniformly through the ovum. A larger quantity of 

 it is present in the ova of Amphibia, Marsipobranchii and Teleostei, 

 and it attains an immense development in the ova of Elasmo- 

 branchii, Reptilia, and Aves. 



The food-yolk originates from a differentiation of the proto- 

 plasm of the egg. It arises as a number of small highly refract- 

 ing particles in a stratum slightly below the surface. 



In the Mammalian ovum these particles spread through the protoplasm 

 of the egg, but do not attain any considerable development. In other 

 forms the case is different. In Elasmobranch Fishes the refracting particles 

 appear to develop into vesicles, in the interior of which there arise 

 solid oval or even rectangular highly refracting bodies, in the substance 

 of which a stratification may usually be observed, which gives them 



