506 STANLEY C. BALL 



lie in contact with the wall of the capsule (fig. 17). This migra- 

 tion of the embryo from an interior to a peripheral position is 

 a step characteristic of all the species considered in this paper. 

 According to Bresslau the embryonic cells migrate to the ventral 

 surface of the yolk in the species of Mesostomum and Plagios- 

 tomum which he studied. Hallez ('09) finds it assuming an 

 antero-ventral position in Para vortex cardii. The same mi- 

 gration was observed in the Dendrocoeles by Mattieson ('03) 

 and in the Triclads by Jijima ('84). The embryonic mass of 

 Paravortex gemellipara, like that of P. cardii, is forced to the 

 anterior side of the yolk. 



There is considerable variation in the amount of yolk absorbed 

 by the primary entoderm (compare figs. 18 and 20). Hence 

 there remains a larger or smaller quantity disposed, as seen in a 

 median section (fig. 17), in the form of a bi-concave lens be- 

 tween the two embryos. The fate of this mass will be discussed 

 in the following section. 



Some of the entoderm cells seen in figure 21 have already reached 

 the limit of their absorptive capacity, while others are still 

 comparatively small. It should be pointed out, however, that 

 there is a great variation in size at the time they are taken into 

 the embryo. That the quantity of yolk absorbed depends 

 rather upon the activity of the cell than upon the amount of 

 yolk in its neighborhood is suggested by the fact that small 

 primary entoderm cells are enfolded by the gastrulating embryo 

 €ven when there was plenty of material present, as indicated 

 by the quantity later found in the ectoderm. 



At the completion of the yolk absorption, and before gastru- 

 lation, degeneration of the nuclei of the primary entoderm cells 

 sets in. As represented in figure 21, their chromatin becomes 

 less distinct, while the nucleoli disappear. Finally, with the 

 dissolution of the nuclear membranes, there remain for a time 

 only indistinct vestiges of nucleoplasm with a little chromatin. 

 Rarely does any trace of the nucleus appear in the cell after 

 it has been taken into the body of the embryo. The activity 

 of the cell having ceased, the presence of the nucleus is no 

 longer necessary. 



