no CYTOPLASMIC STRUCTURES [CH. vn 



not only a thin vitelline membrane but also a stronger 

 outer covering. The vitelline membrane is usually if not 

 always part of the ovum, but the outer covering (often 

 called the shell, though a true egg-shell has a different 

 origin) is most frequently secreted by the follicle cells of 

 the ovary, although in several instances, for example the 

 zona radiata of Vertebrates, its origin is uncertain, and it 

 also is possibly produced by the egg itself. Finally in a 

 considerable number of animals a true shell is secreted by 

 glands in the oviduct after the egg leaves the ovary, as, for 

 example, in Elasmobranch fishes, Reptiles, Birds, and 

 among Invertebrates in many Molluscs, some Crustacea, 

 etc. Not rarely there may be an albuminous envelope, 

 either naked or enclosed in the shell, as in Amphibia and 

 Birds. 



