120 



THE GERM- CELLS 



uncertain whether the bodies thus arising have the same physiologi- 

 cal significance as the "yolk-nuclei" of cytoplasmic origin. Calkins 

 ('95> J )> working in my laboratory, has brought forward strong evi- 

 dence that the "yolk-nucleus" of Lumbricns is derived from a sub- 

 stance nearly related with chromatin (Fig. 61). The yolk-nucleus 



Fig. 61. Young ovarian eggs of the earthworm (Lrtmbricus) , showing yolk-nucleus. 

 [CALKINS.] 



A. Very early stage ; the irregular yolk-nucleus (y. .) closely applied to the germinal vesicle 

 and staining like chromatin. B. Later stage; the yolk-nucleus separating from the germinal 

 vesicle and changing its staining-power. C. Still later stage; the yolk-nucleus broken up into 

 rounded bodies staining like the cytoplasm. 



here first appears as an irregular granular body lying directly on the 

 nuclear wall, which in some cases appears to be interrupted, as if yolk- 

 nucleus and chromatin were directly in continuity. Later the yolk- 

 nucleus separates from the germinal vesicle and lies beside it in the 

 cytoplasm. It finally breaks up into a considerable number of sec- 

 ondary yolk-nuclei scattered through the egg. The action of differ- 

 ential stains at different periods indicates that the substance of the 



