THE CHROMOSOME THEORY OF HEREDITY 293 



Fig. 169. — Diagram of the general plan of spermatogenesis and oogenesis in 

 animals. The somatic, or diploid, number of chromosomes is assumed to 

 be eight. Male to the left, female to the right. A, primordial germ cells; 

 B, spermatogonia and oogonia, many of which arise during the period of 

 multiplication; C, primary spermatocyte and oocyte, after the growth period, 

 with chromosomes in synapsis; D, secondary spermatocytes and oocytes 

 with haploid number of chromosomes. These cells are the product of the 

 reduction division; E, spermatids, which develop into spermatozoa; in the 

 female, one egg and three polar bodies. The cells of this stage are produced 

 by the equation division. The order of the reduction and equation divi- 

 sions is variable in different species; the two together are called the first 

 and second maturation divisions. F, union of ovum and spermatozoon 

 (fertilization); G, the diploid number of chromosomes in the daughter cells 

 formed by the first cleavage of the zygote. This number persists in all cells 

 of the body and in the germ cells until maturation. (From Woodruff.) 



