DEVELOPMENT OF PARA VORTEX GEMELLIPARA 535 



step occurs only after they have been ingested by the secondary 

 entoderm cells, which thus resemble in behavior the cells of the 

 alimentary canal in Planocera. 



Hallez states that in P. cardii no membrane is formed over the 

 surface of the yolk mass by a part of the vitellarial cells. P. 

 gemellipara resembles P. cardii in this respect; no shell-membrane 

 arises. But Hallez thought he found a similarity between the 

 behavior of the yolk-cell nuclei of Paravortex and Mesostomum 

 in that certain of these nuclei in the former played an important 

 part in the differentiation of the definitive ectoderm and primary 

 intestine; according to Bresslau, in Mesostomum ehrenbergi, 

 the peripheral yolk cells form a shell membrane over the sur- 

 face of the remaining yolk. In Paravortex gemellipara, however, 

 every cell which enters into the structure of any organ is a de- 

 rivative of the egg. The vitellarial yolk-cell nuclei and mem- 

 branes disintegrate, leaving only the yolk as food; the nuclei 

 never contribute to the formation of living tissue. 



Of the mitochondrial mass little remains to be said. Its his- 

 tory from the first appearance in the young oocyte was traced 

 until it finally disappeared in the entoderm of the young worm. 

 The suggestion was tentatively made that the body might be 

 or might produce an enzyme which has to do with the trans- 

 formation of the yolk. But a similar transformation took 

 place in the ectoderm, a tissue which received none of the mito- 

 chondrial mass. It therefore appears that the literature con- 

 cerning the 'Dotterkern' has received another contribution, but 

 without further explanation of its action. Truly suggestive, 

 however, is the behavior of the bod}-. 



VI. SUMMARY 



1. The origin of two or more embryos found in the capsules 

 of Paravortex gemellipara is similar to their origm in other cap- 

 sule-forming Rhabdocoeles; that is, by the enclosure of two or 

 more eggs and about one hundred yolk cells in a shell secreted 

 by the shell glands. 



2. The mitochondrial mass (yolk-nucleus) was traced from its 

 origin in the young oocyte through cleavage to its final resting 



