DEVELOPMENT OF PARA VORTEX GEMELLIPARA 489 



yolk in the 'balles vitellines.' To carry out this function they 

 migrate inward from the periphery. 



He finds that the cytoplasm surrounding one of these migra- 

 tory nuclei makes its way into the substance of each vitelline 

 sphere, eventually drawing with it the nucleus. Later many 

 of these nuclei again emigrate and may then enter into the 

 formation of the ectoderm or of the primary intestine. 



Thus it is to be borne in mijid that, if Hallez's interpretation 

 be correct, both the entoderm and at least a part of the ectoderm 

 cells in the definitive embryo contain nuclei and cytoplasm 

 which have undergone little change since leaving the vitellaria; 

 in other words, the relationship of these cells to those derived 

 from the egg is apparent only when one recalls that ovary and 

 vitellarium were at their beginning differentiated from the same 

 mass of cells. 



Bresslau ('04) traced the history of the yolk cells in Mesosto- 

 mum ehrenbergi, M. productum, M. lingua, Bothromesosto- 

 mum persona turn and Plagiostomum girardi. The yolk cells 

 of the former behave like tliose of P. cardii in that they soon 

 separate into two groups. The outer ones become flattened 

 so as to form a shell membrane between the capsular shell and 

 the inner yolk cells. The latter become vacuolated to such 

 a remarkable extent that the nucleus and a small amount of 

 cytoplasm is finally flattened in the form of a thin cresent on 

 one side of the vacuole As the ectoderm formed from the 

 anterior portion of the developing embryo envelops these yolk 

 cells, the nuclei of the latter gradually degenerate, but may 

 still be discerned between the larger vacuoles after the ectoderm 

 has closed over the posterior end of the embryo. 



No such shell membrane is formed in the capsule of Bothromeso- 

 stomum personatum; here the much more rapid degeneration of 

 yolk nuclei is accompanied by the appearance of multitudes of 

 vacuoles of various sizes, all much smaller than those described 

 for M. ehrenbergi. The formation of the ectoderm and enclosure 

 of the yolk globules within the embryo is essentially the same 

 as in the latter species. 



