INTRODUCTION 19 



The Germ Cells. 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 

 *'yolk 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 oogonium as it lies in the ovary is 

 definitely oriented with respect to its chief axes. 



