ACCOUNT OF THE GERM-CELL CYCLE 29 



matogonia to form primary oocytes and primary 

 spermatocytes ; 



7. Maturation; 



8. Fertilization (if not parthenogenetic) . 



1. The Segregation of the Primordial Germ 

 Cells. 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 



