INTRODUCTION 19 
Tue Germ Critis. Eggs and spermatozoa differ 
from each other both morphologically and physiolog- 
ically. Eggs are usually spherical or oval in shape 
(Fig. 4), although they may vary greatly from the 
typical form and may even be ameboid as in certain 
coelenterates. In size they range from that of the 
mouse, which is only about 0.065 mm. in diameter, to 
that of birds, which are several inches long. The 
large volume of the latter is due to the presence of 
an enormous amount of nutritive material, and the 
general statement may be made that the size of an 
egg does not depend so much upon the size of the 
animal as upon the amount of yolk stored within it. 
The egg nucleus, which is frequently very large and 
clear, is known as the germinal vesicle; and its 
nucleolus has often been referred to as the germinal 
spot. Embedded within the cytoplasm of the ovum 
are several bodies besides the yolk globules. A 
“volk nucleus” may be present; mitochondrial 
granules or rods may occur; and special inclusions, 
which become associated with the primordial germ 
cells and have been named keimbahn-determinants, 
have been recorded in many cases. Considerable 
evidence has accumulated that the egg substance 
is not a homogeneous, isotropic mixture, but is def- 
initely organized, and that this organization is 
related to the morphology of the embryo which is 
to develop from it; hence we speak of the promor- 
phology of the egg. Eggs are said to possess polarity, 
and even the odgonium as it lies in the ovary is 
definitely oriented with respect to its chief axes. 
