THE OVUM. 63 



The vitelline membrane is thin except in the Crocodilia. The 

 third innermost membrane is found according to Eimer in many 

 Reptilia. In birds both vitelline membrane and zona radiata 

 are present, but the latter atrophies early, leaving the former as 

 the sole membrane when the egg is ripe. 



In osseous fish the vitelline membrane is usually either 

 absent or may perhaps in some instances, e.g. the Perch, be 

 imperfectly represented. In the ripe ovum of the Herring there 

 is a distinctly developed membrane external to the zona radiata 

 which is probably the vitelline membrane. The zona radiata 

 attains a very great development, and is generally provided with 

 knobs of various shapes on its outer surface. A delicate mem- 

 brane internal to this my third membrane has often been 

 described, but there is still some doubt about its existence. In 

 some cases an external less granular layer of the ovum itself has 

 been described as a special membrane. In the Perch a peculiar 

 mucous capsule, penetrated by irregular branched prolongations 

 of the follicle cells, is present in addition to the ordinary mem- 

 branes. 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 

 zona 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). 



