DEVELOPMENT OF PARAVORTEX GEMELLIPARA 505 



In the slightly older embryos represented in figures 17 and 

 18 the secondary entoderm cells are still actively dividing, but 

 most of them are larger than the anterior mes-ectoderm cells. 



5. Absorption of yolk by the primary entoderm 



Toward the end of the morula stage a remarkable process 

 of nutrition is inaugurated. As far as has been determined 

 from the hterature no closely similar process has been described 

 as occurring in the nourishing of any embryo. The primary 

 entoderm cells, which were seen in figures 14 and 16 already 

 to have but a loose connection with the main embryonic mass, 

 now applj^ themselves like amoebae to the surface of the yolk 

 which surrounds the posterior half of the embryo. The firm- 

 ness of their attachment is evident in figure 17; while the em- 

 bryos have shrunken away, most of the yolk entoderm cells 

 remain closely in contact with the yolk. 



It is the function of these cells to absorb a large part of the 

 yolk mass brought into the capsule inside the vitelline cells. 

 The membranes of the latter have all disintegrated, leaving 

 the yolk flakes and granules free in the capsule. Inspection 

 of the living capsule shows the particles in vigorous Brownian 

 movement. There is, therefore, no obstacle to the action of 

 the entoderm cells. They are shown in figure 17 plainly acquir- 

 ing the yolk particles. Their nuclei are large, distinct, and 

 according to the rule in active cells throughout this animal, 

 markedly lobulated. 



The nature of the yolk granules suffers little change during 

 the process, for those inside the entoderm cells have the same 

 form and color as those lying just outside the cell membrane. 

 It is to be noted that all exist as spherules; no flakes such as 

 constitute a considerable portion of the free yolk are at first 

 seen in the entoderm. 



The gradual increase in size of the entoderm cells, while in- 

 gesting the yolk on the median side of the embryo, at the same 

 time forces the latter outward until that portion of it which 

 from now on is to be recognized as the anterior end comes to 



