/ 





DEYELOPxAIENT OF THE FEOG'S EGG 



-oo>»<oo- 



CHAPTER I 



THE FORMATION OF THE SEX-CELLS 



The development of the sex-cells is generally divided into 

 three periods : 1) a multiplication-period, during which the 

 primitive germ-cells pass through a large number of divisions ; 

 2) a growth-period, in which the primitive germ-cells, that 

 have become reduced in size through repeated division, grow 

 larger ; 3) a maturation-period, when only two divisions take 

 place, between which the nucleus does not pass into a resting- 

 stage. At the end of this last division the male germ-cells 

 undergo histological changes by which they become trans- 

 formed into spermatozoa. 1 



Spermatogenesis 



The changes that take place in the testes of the frog have 

 not been so fully worked out as in some other animals ; we 

 may therefore follow, first, the method of development of the 



1 This is a modification of the terminology of v. la Valette St. George, whose 

 nomenclature of spermatogenesis is still often used. La Valette's classification 

 is as follows : — 



The primordial germ-cells give rise to spermatogonia, which cease to divide 

 after a time and increase in size. Each spermatogonium is thus converted into 

 a primary spermatocyte. Each primary spermatocyte divides into two cells, the 

 spermatocytes of the second order, and each of these divides once more, with- 

 out a resting-period, to form two spermatids. In this way four spermatids are 

 formed from each primary spermatocyte. Each spermatid is then changed 

 directly into a spermatozoon. 



B 1 



