THE CONTINUITY OF LIFE 



Primordial germ cells 



B 



245 



Sperm 



Spermatogonia 

 and oogonia 



Primary spermatocyte 

 and primary oocyte 



D 



Secondary spermatocytes 

 and secondary oocytes 



E 



Spermatids and 

 egg and polocytes 



F 



Fertilization 



G 

 Division of zygote 



Fig. 165. — Diagram of the general plan of spermatogenesis and oogenesis 

 in animals. Tetrad formation is disregarded. 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; E, spermatids (which become transformed into sperm) and egg 

 and three polocytes, each with the haploid number of chromosomes; F, union 

 of sperm and egg (fertilization) to form zygote with diploid number of chro- 

 mosomes; G, chromosome complex of cells after first division of the zygote, and 

 of all subsequent somatic cells, and germ cells until maturation. 



