THE CONTINUITY OF LIFE 



239 



have reached testis or ovary, we are on surer ground and can trace 

 with considerable exactness their divisions and transformations 

 which give rise to the gametes: sperm and eggs. In the first place 

 the primordial germ cells proceed to divide in the testis and ovary so 

 that they produce a large number of germ cells known as sperma- 

 togonia and oogonia respectively. (Fig. 165, A, B.) 



Thus, for instance, the ovary of an adult female Frog shows 

 oogonia in various stages of development. The eggs of the next 

 breeding season are the largest cells; those of intermediate size 



Early stage in development 



Second stage 



Third stage 



Blood vessel 



Wall of ovary 



Fourth stage 



Epithelium lining 

 the coelom 



Nucleus 

 of egg 



Fifth stage 



Fig. 163. — Diagram of a section through a lobe of the ovary of a Frog, 

 showing several stages in egg development. 



represent approximately those of the following year; and many 

 smaller cells are the oogonia from which the eggs of later years 

 will arise. Furthermore there are other cells that form a matrix 

 of supporting and nutritive tissue (follicle cells, etc.) in which 

 the germ cells are embedded. The Mammalian ovary presents a 

 similar picture, except that since fewer eggs are produced at each 

 breeding period, the supply of oogonia is less numerous. Never- 

 theless, even in the human ovary there are potentially many thou- 

 sands of eggs. The testis shows a similar condition, but since so 

 many more sperm than eggs are produced, the spermatogonia 

 divide much more actively. (Figs. 133, 163.) 



1. Mitosis 



Before taking up the origin of the gametes by division from the 

 spermatogonia and oogonia, it will be necessary to describe in 



