778 GENERATION. 



first no special, investing membrane ; but as it develops, it presents, surround- 

 ing the vitellus, a single layer of columnar cells. At the deepest portion of 

 these cells, a homogeneous basement-membrane is gradually formed, and the 

 cells undergo a sort of cuticular transformation, becoming finally the vitelline 

 membrane. 



An important point, in this connection, is the question of the existence 

 of pores, or perforations in the vitelline membrane. As will be seen farther 

 on, there can be no doubt with regard to the actual penetration of the sper- 

 matozoids through this membrane, so that they come in contact with the 

 vitellus ; and it is in this way that the ovum is fecundated. In the osseous 

 fishes and in mollusks, there seems to be no question with regard to the 

 existence of a number of pores in the vitelline membrane ; but these are not 

 easily demonstrated in the ova of mammals. Admitting the existence of a 

 micropyle and pores in the vitelline membrane in fishes and mollusks, it is 

 certain that openings are very much more indistinct, if they can be seen at 

 all, in the ova of mammals ; but the fact of the actual penetration of sper- 

 matozoids almost of necessity presupposes the presence of orifices. It must 



be difficult, in examining a perfectly trans- 

 parent and homogeneous membrane in water, 

 which would fill up all pores, to distinguish 

 any openings, and their presence is to be 

 admitted, mainly because the spermatozoids 

 are known to pass through. The idea of 

 their existence in mammals certainly re- 

 ceives support from analogy with the lower 

 forms of animals. 



The vitellus contains the elements which 

 are to undergo development into the embry- 

 on. It is composed of a semi-fluid mass, con- 

 taining, in addition to the germinal vesicle, 



FIG. 284. Ovum of the rabbit, from a -, , 10 ^j.i 



Graafian follicle $, of an inch (2 mm.) large numbers of granules. Some of these 



in diameter (copied from Waldeyer T -, , , -, , 



and reduced). granules are large, strongly refracting, glob- 



uiar b dies > which are bri s ht and s 



abundant that they obscure the other parts 

 of the vitellus. Between these, are many 

 albuminoid granules, which are much smaller and not so distinct. 



The germinal vesicle, sometimes called the vesicle of Purkinje, is the 

 enlarged nucleus of the primordial ovum. It is a clear, globular vesicle, 

 about yfo of an inch (45 p.) in diameter, embedded in the vitellus, its position 

 varying in different ova. It presents in its interior a number of fine granules, 

 and a large, dark spot, called the germinal spot, or the spot of Wagner, which 

 measures about 3 e * 6 of an inch (7 /A) in diameter. This spot corresponds to 

 the nucleolus of the primordial ovum. In mammals the mature ovum con- 

 tains but one germinal vesicle and one germinal spot. 



Discharge of the Ovum. A ripe Graafian follicle measures | to of an 

 inch (10 to 12 mm.) in diameter, and presents a rounded elevation, contain- 



