ORIGIN OF THE GERM-CELLS 



149 





is less than in the somatic cells (Fig. 74). Vom Rath, workin 

 in the same direction, believes that in the salamander also the 

 number of chromosomes in the early progenitors of the germ-cells 

 is one-half that characteristic of the somatic cells. ^ In both these 

 cases, the chromosomes are doubtless bivalent, representing two 



Fig. 74. — Primordial germ-cells in Cyclops. [Hackkk.] 



A. Young embryo, showing stem-cell {st). D. The stem-cell lias ciiviiled into two. giMut; 

 rise to the primordial germ-cell {g). C. Later stage, in section; tlie iirimordial germ-cell has 

 migrated into the interior and divided into two ; two groups of chromosomes in each. 



chromosomes joined together. In Ascaris, in Hke manner, each of 

 the two chromosomes of the stem-cell or primordial germ-cells is 

 probably plurivalent, and represents a combination of several units 

 of a lower order which separate during the segmenLution ol the 

 thread when the somatic mitosis occurs. 



1 Cf. p. 256, Chapter V. 



