64 CHORDATA. 



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, Salmonidae, Percidae (Gasterosteus), Clupeidae, 

 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 1 , 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 mem- 

 brana propria folliculi which is in its turn enclosed in a 

 vascular connective-tissue sheath. 



In Elasmobranchii and many Reptilia (Lacertilia, Ophidid) 

 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 prolonga- 

 tions 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 



1 For the different views maintained by Foulis, Kolliker, etc. the reader is referred 

 to the writings of these authors. The grounds for the view here adopted will be found 

 in my paper (No. 64). 



