i8o 



FOOT-NOTES TO EVOLUTION. 



ing resting stage each spermatocyte now divided again, 

 as in Fig. u, E and F, each dyad now being separated 

 into halves, so that in the spermatids of Fig. n, F, but 



FlG. II. Reduction of chromosomes in the spermatogenesis of Ascaris 

 megalocepkala, var. bivalens : A, nucleus of a spermatogonium ; the 

 typical number of chromosomes (four) is seen, each split longitu- 

 dinally preparatory to the next division. B, young spindle stage of 

 primary spermatocyte ; two tetrads are present, each formed hy the 

 double longitudinal splitting of a chromatin thread. C, the tetrads 

 in the equatorial stage of the division. D, separation of dyads. 

 E, the dyads in the succeeding division of the secondary spermato- 

 cyte. F. completion of the division of the same ; each cell (sperma- 

 tid) contains the reduced number of chromosomes (two). (After 

 Brauer.) 



two chromatin masses are present. Thus the tetrads of 

 the primary spermatocyte are divided up among the four 

 spermatids, so that each of the latter receives one fourth 



