THE OVUM. 51 



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 zoua 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, Salmonida^, 

 Percidse (Gasterosteus), Clupeidse, 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 epi- 

 thelium. 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 



1 For the different views maintained by Foulis, Krilliker, 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). 



42 



