78 EVOLUTION AND GENETICS 



the validity of this evidence is accepted, the study of 

 the cell leads to the ultimate units about which the 

 whole process of the transmission of the hereditaiy 

 factors turns. 



Before considering this somewhat technical mat- 

 ter, certain facts, which are familiar for the most part, 

 should be recalled, because, on these, rests the whole 

 of the subsequent explanation. 



The thousands of cells that make up the cell-state 

 that we call an animal or plant come from the fer- 

 tilized egg (fig. 31) . An hour or two after fertiliza- 

 tion the egg divides into tw^o cells (fig. 32). Then 

 each half divides again. Each quarter next divides. 

 The j^rocess continues until a large number of cells 

 is formed and, out of these, organs mold themselves. 



At every division of the cell the chromosomes also 

 divide. Half of these have come from the mother, 

 half from the father. Every cell contains, therefore, 

 the sum total of all the chromosomes, and if these 

 are the bearers of the hereditary qualities, every cell 

 in the body, whatever its function, has a common 

 inheritance. 



At an early stage in tlie development of the ani- 

 mal certain cells are set apart to form the organs of 

 reproduction. In some animals these cells can be 

 identified early in the cleavage (fig. 33). 



The reproductive cells are at first like all the other 

 cells in the body in that they contain a full comple- 

 ment of chromosomes, half paternal and half mater- 



