The Development of Scyphomedusse. 243 



some cases they only represent an aggregation of yolk granules 

 and the coagulated liquid often found in the cle.avage cavity, and 

 are not cellular. 



From the observations on Cyanea and Aurelia it is clear that 

 at least in some cases there is no sign of an ingression of cells, or 

 delamination, leading to entoderm formation, while in all cases 

 invagination is the chief, if not the only process. 



It was mentioned earlier, and is shown in figs. 33, 34, that the 

 blastopore begins its closure inside and proceeds outward. The 

 result is the complete separation of the entoderm layer before the 

 lips of the blastopore have fused in the ectoderm. Since the gas- 

 trulation has been by invagination the primitive entoderm cells 

 are already in a definite layer, and further changes in the two layers 

 are only differentiations of the layers already present. The en- 

 toderm layer sometimes obliterates the cleavage cavity and lies 

 closely against the ectoderm at a very early period even before 

 gastrulation is completed (fig. 31) . More often, however, the cleav- 

 age cavity is not filled till much later, and even after the embryo 

 has begun to elongate into the planula there may be some space 

 between the two layers (fig. 35, 36). When this elongation begins 

 the cells of the ectoderm are long and slender and the nuclei 

 are at the extreme outer ends (fig. 35), the cytoplasm being mostly 

 limited to a very thin layer at the outer ends of the cells. The rest 

 of the cell is packed with yolk bodies. There are some shorter cells 

 which are limited to the deeper part of the layer. The entoderm 

 cells are elongate, but relatively broad and rather few in number. 

 Continued division results in the formation of long, very narrow, 

 ectoderm cells (fig. 36) and, since the yolk is largely used up, the 

 cytoplasm is more uniformly distributed and the nuclei are in the 

 centre of the cells. The entoderm is still filled with yolk. The 

 condition shown in this figure is one often met with and is 

 evidence of the more rapid division of the ectoderm cells, thus 

 pulling away from the entoderm layer. In some cases the two 

 layers develop equally fast and remain closely joined at all times. 

 From this condition, shown in fig. 36, the completed planula 

 (fig. 37) is easily derived. The ectoderm uses up all its yolk, 

 the cells, now completely and uniformly filled with cytoplasm, 



