CHAPTER VI 



THE SEGREGATION OF THE GERM CELLS IN NEM- 

 ATODES, SAGITTA, AND CERTAIN OTHER MET- 

 AZOA' 



1. The Keimbahn in the Nematoda 



The classical example of the keimbahn in animals 

 is that of Ascaris megalocephala as described by 

 Boveri (1887, 1892). The first cleavage division of 

 the egg of Ascaris results in two daughter cells, each 

 containing two long chromosomes (Fig. 51, A). In 

 the second division the chromosomes of one cell 

 divide normally and each daughter cell receives one 

 half of each (Fig. 51, B, S). The chromosomes of 

 the other cell behave differently; the thin middle 

 portion of each breaks up into granules (Fig. 51, A) 

 which split, half going to each daughter cell, but the 

 swollen ends (Fig. 51, B, C) are cast off into the cyto- 

 plasm. In the four-cell stage there are consequently 

 two cells with the full amount of chromatin and two 

 with a reduced amount. This inequality in the 

 amount of chromatin results in different-sized nuclei 

 (Fig. 51, C) ; those with entire chromosomes (S) 

 are larger than those that have lost the swollen ends 

 (C). In the third division one of the two cells w^ith 

 the two entire chromosomes loses the swollen ends 

 of each ; the other (Fig. 51, Z), 8) retains its chromo- 



174 



