DEVELOPMENT OF PARAVORTEX GEMELLIPARA 533 



the remaining ^'itellarial nuclei which did not previously pene- 

 trate the yolk spheres. Hallez observed also that the true embry- 

 onic cells contribute to the development of the ectoderm. This 

 ectoderm is retained permanently. The remaining 'migratory 

 cells' form another 'syncytium' about the vitelline spheres which 

 they have just left, this constituting, in Hallez's opinion, the 

 primary intestine. 



The ectoderm on the apposed surfaces of the two embryos is 

 formed by a delamination through the center of the yolk-cell 

 derived plasma with its migratory nuclei. Hallez discerned but 

 few cell membranes. 



From a study of his figures and descriptions it seems possible 

 that owing to a failure to distinguish the extremely delicate 

 outlines in the embryonic structures he has been led to the above 

 interpretation. I believe that further study may show that the 

 processes of early development in Paravortex cardii are very 

 nearly the same as those which I ha^^e found in Paravortex 

 gemellipara. 



The derivation of the ectoderm and entoderm as observed 

 in the development of P. gemellipara agrees essentially with 

 that determined by Bresslau for the several species of Rhabdo- 

 coeles which he studied. A variation in the manner of yolk 

 absorption, however, occurs in Paravortex. Whereas he found 

 the yolk cells either to retain their envelopes, as in Mesostomum 

 ehrenbergi, or to be enclosed by the ectoderm in the form of 

 numerous yolk spheres resulting from the breaking down of the 

 vitelline cells, in P. gemellipara the yolk granules are first ab- 

 sorbed by the primary entoderm cells, and these are in turn 

 ingested by the secondary entoderm. The first step of the 

 process is similar to that known to occur in the chick, where 

 the intestinal entoderm absorbs the yolk particles. 



That the yolk-absorptive function of these entoderm elements 

 is their chief capacity is further indicated in a freak embryo 

 shown m figure 53. This specimen, beside lacking the yolk- 

 filled masses so characteristic of normal embryos, exhibits a 

 remarkable distribution of the undifferentiated tissue. The 

 latter has proliferated at a normal rate, so that following its 



