THE OVA AND THE SPERMATOZOA. 133 



surround it as a peripheral coat {ep.). This central cell 

 is the ovum. Its nucleus enlarges, and becomes what is 

 called the germinal vesicle (g-v.). At the same time 

 numerous small corpuscles, flattened externally and 

 I convex internally, appear in it and are the germinal 

 spots ig.s.). The protoplasm of the cell, as it enlarges, 

 becomes granular and opaque, assumes a deep brownish- 

 yellow colour, and is thus converted into the yelk or 

 vitcllus (y.). As the egg grows, a structureless vitelline 

 membrane is formed between the vitellus and the cells 

 which line the ovisac, and incloses the egg, as in a 

 bag. Finally, the ovisac bursts, and the egg, falling 

 into the cavity of the ovary, makes its way down the 

 oviduct, and sooner or later passes out by its aperture. 

 When they leave the oviduct, the ova are invested by 

 a viscous, transparent substance, which attaches them 

 to the swimmerets of the female, and then sets ; thus 

 each egg, inclosed in a tough case, is firmly suspended 

 by a stalk, which, on the one side, is continued into the 

 substance of the case, while, on the other, it is fixed to 

 the swimmeret. The swimmerets are kept constantly in 

 motion, so that the eggs are well supplied with aerated 

 water. 



The testis consists of an immense number of minute 

 spheroidal vesicles (fig. 33, A, a), attached like grapes to 

 the ends of short stalks (h), formed by the ultimate 

 ramifications of the vasa deferentia. The vesicles may, 

 in fact, be regarded as dilatations of the ends and sides 



