176 OVARY AND PAROVARIUM, BY W. WALDEYER. 



(fig. 193, G 6) appears when examined with low powers as a 

 strong perfectly transparent homogeneous lamella sharply 

 differentiated from the yolk; when torn with needles, the 

 contents issue in a stream, and the membrane which offers 

 relatively considerable resistance rolls itself back, as especially 

 O3curs in the vitellus of the Frog. The chemical characters 

 the zona pellucida are still not accurately known : it resist 

 the action of acids, especially of acetic acid, and only dissolve 

 with difficulty in alkalies. With high microscopic powers tl 

 zona pellucida of almost all animals presents a peculiar struc 

 tural character first demonstrated by J. Miiller (78) and Remj 

 (96) in the vitelline membrane of the ova of Fish. It app( 

 in fact, to be perforated in a radial direction by numerous 

 pores that are very distinct in the Osseous Fishes, and confei 

 upon the membrane in these animals, when seen on the surface, 

 a very delicate shagreened appearance. 



The pores are much finer in Mammals (fig. 193, G 6). 

 have not been able to discover them in the ripe ovum of Birds 

 though it is here very difficult to obtain sufficiently thu 

 sections of the vitelline membrane. Microscopically the vitel- 

 line membrane of the Bird appears to be composed of a vei 

 fine close felt of fibres. 



The cells of the epithelium of the ovum, of which mention 

 already been made (p. 174), and which occur in Mammals as 

 special layer of the discus proligerus, distinct from the epith< 

 lium of the follicle, lie in immediate contact with the vitellus, 

 and are firmly adherent to it. In Birds and Reptiles the ends 

 of the follicular epithelium that are turned towards the vi- 

 telline membrane divide into a number of very fine rods like 

 the free border of the intestinal epithelium. 



This basement membrane, or zona radiata, is in the above- 

 mentioned animals the precursor of the vitelline membrane ; 

 we may obviously also regard this as a homogeneous membrane 

 that is perforated by numerous fine fibres of the protoplasm of 

 the follicular epithelial cells (see fig. 194). In Fishes it is 

 indisputable that fine hair-like processes, protoplasmic fibres, 

 proceed from the epithelial cells, and penetrate the pores of the 

 vitelline membrane. I can especially recommend the ova of 

 the Perch for this investigation, which exhibit two investing 



