DEVELOPMENT OF PARAVORTEX GEMELLIPARA 507 



6. Ingestion of the yolk-gorged primary entoderm cells by those 

 of the secondary entoderm 



A remarkable event now happens. Some of the derivatives 

 of the secondary entoderm cells pointed out in figures 14 and 16 

 become amoeboid and make their way around and between those 

 of the primary entoderm until they come to lie, in some cases, 

 next to the free yolk. Three of these are visible in each embryo 

 of figure 18. Though the nuclei are always prominent, it is 

 only in well-preserved and favorably stained material that the 

 cytoplasm can easily be distinguished. In such cases there 

 can be no question of a syncytium here. Each cell is a unit. 



Another amoeba-like character is now manifested by these 

 cells; each primary entoderm cell is ingested by a secondary 

 entoderm cell. Referring again to figure 21 we find that the 

 section passed through the nuclei of several secondary entoderm 

 cells, while the cytoplasm of each stretches in a thin sheet over 

 the surface of the primary entoderm cell which is being engulfed. 

 There is in this embryo no certainty that the cytoplasm has yet 

 completely surrounded the yolk-filled cells. That this occurs 

 later will become clear. It is evident that the nuclei of the 

 primary entoderm cells do not entirely disappear until after 

 ingestion. 



7. Gastrulation and absorption of yolk by the ectoderm cells 



If by the process of gastrulation is meant the infolding or 

 overgrowth of the entoderm by the ectoderm, then a true gas- 

 trulation occurs in Paravortex gemellipara. Bresslau ('04) in 

 describing the development of Mesostomum thought the term 

 inapplicable. It is used in this paper as expressing the process 

 at least homologous with gastrulation. 



The first step consists in the differentiation of the ectoderm. 

 The outermost cells of the mes-ectodermic mass at two points 

 (or perhaps in a ring) on the anterior surface of the embryo now 

 become flattened. Between these two points lies a pair of cells 

 which are to give rise to the lining epithelium of the pharynx 



