ACCOUNT OF THE GERM-CELL CYCLE 29 
matogonia to form primary odcytes and primary 
spermatocytes ; 
7. Maturation ; 
8. Fertilization (if not parthenogenetic). 
1. THE SEGREGATION OF THE PRIMORDIAL GERM 
Cretts. This phase of the germ-cell cycle is espe- 
cially emphasized in this book (see Chapters III to VI) 
and need be referred to only casually here. The 
mature eggs of animals are organized both mor- 
phologically and physiologically ; that is, differenti- 
ations have already taken place in their protoplasmic 
contents before they are ready to begin develop- 
ment. This organization determines what sort of 
divisions the egg will undergo during the cleavage 
stages. During cleavage certain parts of the cell 
contents become separated from other parts and 
thus the differentiated substances of the egg are 
localized in definite parts of the embryo. The 
contents of the cleavage cells likewise become 
differentiated as development proceeds, until finally 
the cells produced form two or three more or less 
definite germ layers. In some cases the egg always 
divides in the same way, and the history or “cell 
lineage”’ of the cells can be followed accurately, 
and the parts of the larva to which they give rise 
can be established. This is known as determinate 
cleavage in contrast to the indeterminate type in 
which there appears to be no relation between the 
cleavage cells and the structure of the egg or larva. 
The degree of organization of the egg no doubt ac- 
counts for the differences in cleavage; those of the 
