20 GERM-CELL CYCLE IN ANIMALS 
The principal poles are dissimilar; the end of the egg 
containing most of the cytoplasm and nearer which 
lie the nucleus and centrosome is known as the 
animal pole; the other end, which is often crowded 
Fie. 4. —Germ cells. Ovarian ovum of a cat just before maturity. 
c.m, =cell membrane; mics. = microsomes; ncl = nucleolus; n. m= 
nuclear membrane; yk. al.=yolk alveoli. (From Dahlgren and 
Kepner.) 
with the yolk globules, is called the vegetative pole. 
The subject of the organization of the egg will be 
referred to more in detail later (Chapter VIII). 
The male sex cells or spermatozoa differ very 
strikingly from the eggs. They are usually of the 
