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 
convex internally, appear in it and are the germinal 
spots (g.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 
vitellus (v.). 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 aérated 
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 (b), formed by the ultimate 
ramifications of the vasa deferentia. The vesicles may, 
in fact, be regarded as dilatatioas of the ends and sides 
