134 GENERAL ZOOLOGY 



that this chromatin is the substance by which hereditary 

 characters are passed from one generation to the next. After 

 maturation each germ cell has but one half the amount of 

 chromatin which it possessed before maturation started. 

 This reduction of the chromatin prepares the germ cells for 

 fertilization. In consequence, when a mature egg and ma- 

 ture sperm join in fertilization each brings but one half its 

 original amount of chromatin. The fertilized egg therefore 

 contains the usual or normal supply of chromatin, one half of 

 which was in the mature egg before fertilization and the other 

 half was added by the sperm (Fig. 72, E). At the end of fer- 

 tilization the sperm and egg have completely joined to form 

 a single cell having a single nucleus. It is from this single 

 cell, the fertilized egg, that the new individual has its origin. 



Cleavage. By mitosis the nucleus of the fertilized egg 

 divides to form two nuclei and from the one cell two com- 

 plete cells are produced (Fig. 73, B), but they remain 

 attached to one another. This multiplication of cells from 

 the fertilized egg is called cleavage. After two cells have 

 been formed, each undergoes mitosis and cleavage, so that 

 from two cells four are produced (Fig. 73, C). This cleavage 

 process continues in^all the higher animals until the original 

 fertilized egg has formed a large number of cells. 



Blastula. Even as early as the eight-cell stage in the em- 

 bryo, or developing young, the cells in many species arrange 

 themselves in such a manner as to have a small cavity in the 

 interior of the group. Such a stage where the cleavage cells 

 are arranged as a single layer of cells surrounding a cavity 

 is called a blastula (Fig. 73, E). 



Gastrula. In later development one region of the blastula 

 wall begins to push inward just as a hollow rubber ball may 

 be indented (Fig. 73, F). As this ingrown region deepens 

 the cells which were in a single layer during the blastula 

 stage become recognizable as an outer-surface covering, or 



