HEREDITY 315 



separated synaptic mates and the other cell the other set. The 

 daughter cells of this division are known as secondary spermato- 

 cytes, and, as we have seen, are of two sorts, one with X and one 

 with Y sex chromosomes. The division of the primary spermatocyte 

 is therefore a reduction al division; secondary spermatocytes have 

 half the S number of chromosomes. Consequently, all cells in stages 

 following the secondary spermatocyte are haploid. 



The secondary spermatocytes then divide without any further re- 

 duction or change in chromosome number. The division is in many 

 respects a typical mitotic division, each P or P' chromosome splitting 

 lengthwise during the metaphase, to contribute one-half itself to 

 each of the resulting cells. These cells are the spermatids, one-half 

 of which have all the P chromosomes, the other half having; the 

 P' chromosomes. One-half the spermatids contain PX chromosomes 

 and the other half PY chromosomes. The spermatids become trans- 

 formed by a complicated process but without further division into 

 mature, active spermatozoa. There having been two generations of 

 cells between the spermatogonia and the mature spermatozoa, one 

 spermatogonium is the origin of four spermatozoa, two with X 

 chromosomes and two with Y chromosomes. The spermatozoa of 

 most animals are therefore of two sorts in equal numbers; in Man 

 one contains twenty-three unpaired autosomes and one X chromo- 

 some, the other sort containing twenty-three unpaired autosomes 

 and one Y chromosome. 



Oogenesis (Fig. 202). The processes undergone by the chro- 

 mosomes of the ovum during maturation are in many respects 

 similar to corresponding stages in sperm maturation. The earliest 

 female germ cells are the oogonia, corresponding to the sperma- 

 togonia of the male. After the mitotic divisions of this period of 

 multiplication are ended they are primary oocytes, which corre- 

 spond to the primary spermatocytes. During the primary oocyte 

 stage, synapsis between p and m chromosomes of each synaptic 

 pair occurs and the result is two new chromosomal elements, desig- 



