DEVELOPMENT OF PARAVORTEX GEMELLIPARA 509 



entered the ectoderm cells of the embryo pictured in figure 18. 

 Such individuals as those in figures 22 and 23 must previously 

 have had left over after the absorptive action of the entoderm 

 an amount of free yolk comparable with that shown between 

 the embryos of figure 21. 



a. The einhrijo at the close of gastrulation. The picture presented 

 by the embryo at the close of gastrulation is shown in median 

 frontal section by figure 23 which was drawn from an Ehrlich's 

 haematoxylin-eosin preparation. Sections of seven primary 

 entoderm cells occupy the greater part of the young worm, each 

 lying inside an amoeboid secondary entoderm cell. No clear 

 regions have yet appeared in the interior of the former; the yolk 

 globules are evenly distributed. 



A similar condition exists in the ectoderm, although a slight 

 advance is evidenced by the tendency of the small globules to 

 fuse into larger masses. The posterior cells are so engorged 

 that their membranes are stretched almost to the point of 

 rupture. On the other hand the anterior ectoderm cells are 

 flattened and contain only a few yolk granules. It is probable 

 that certain large nuclei just beneath the ectoderm in this 

 region are soon to be added to it as parts of new cells. 



The formative mass from which are to be derived all the de- 

 finitive organs lies toward the anterior end of the embryo. 

 Two lines of cells, however, are pushing posteriorly beneath 

 the ectoderm and the primary entoderm. Certain of these are 

 undergoing mitosis, while a few differ from the majority in possess- 

 ing lighter cytoplasm and definite boundaries. These are re- 

 garded as secondary entoderm cells which did not migrate out- 

 ward to ingest the yolk -laden cells, but were reserved to take a 

 part in the formation of the intestine. 



It is impossible in the section from which figure 23 was drawn 

 to detect cell membranes between the smaller nuclei which 

 characterize the greater portion of the undifferentiated mass. 

 One might well regard it as a syncytium were it not that the 

 membranes are discernible in figures 18 and 21 which represent 

 slightly younger embryos, and in figure 24 of one somewhat 



