426 



FORMATION OF THE LAYERS. 



rapidity at the expense of the yolk ; the spherules of which they 

 absorb and digest in an amceba-like fashion by means of their pseudo- 

 podia. They become longer and 

 longer, and finally, after absorbing 

 the whole yolk, acquire a form 

 almost exactly similar to that of 

 the yolk pyramids during segmen- 

 tation (fig. 238 B). They enclose 

 the cavity of the mesenteron, and 

 their nuclei and protoplasm are 

 situated externally. The cells of 

 the mesenteron close to its junction 

 with the proctodseum differ from 

 those elsewhere in being nearly 

 flat. 



In Palsemon (Bobretzky) the 

 primitive invaginatiou (fig. 239 A) 

 has far smaller dimensions than in 

 Astacus, and appears before the 

 blastoderm cells have separated 

 from the yolk pyramids. The cells 

 which are situated at the bottom 

 of it pass into the yolk, increase 

 in number, and absorb the whole 

 yolk, forming a solid mass of hypo- 

 blast in which the outlines of the 

 individual cells would seem at first 



B 



FIG. 238. Two LONGITUDINAL SEC- 

 TIONS OF THE EMBRYO OF ASTACUS. (From 



Parker; after Bobretzky.) 



A, Nauplius stage. B. Stage after not to be distinct. The blastopore 

 the hypoblast cells have absorbed the 

 food yolk. The ventral surface is turned 



upwards. 



stomodanini ; luj. procto- 



in the mean time becomes closed. 

 Some of the nuclei now pass to 



dasum; an. anus; m. mouth; mg. me- the periphery of the yolk mass; 

 seuteron; abd. abdomen; h. heart. t h e ce l} s appertaining to them 



gradually become distinct and as- 

 sume a pyramidal form (fig. 239 B, hy), the inner ends of the 

 cells losing themselves in a central mass of yolk, in the interior of 

 which nuclei are at first present but soon disappear. The mesenteron 

 thus becomes constituted of a layer of pyramidal cells which merge 

 into a central mass of yolk. Some of the hypoblast cells adjoining 

 the junction of the proctodseum and mesenteron become flattened, 

 and in the neighbourhood of these cells a lumen first appears. The 

 stomodseum and proctodseum are formed as in Astacus. Fig. 239 B 

 shews the relative positions of the proctodseum, stomodseum, 

 and mesenteron. Although the process of formation of the hypoblast 

 and mesenteron is essentially the same in Astacus and Palsemon, 

 yet the differences between these two forms are very interesting, 

 in that the yolk is external to the mesenteron in Astacus, but en- 

 closed within it in Palsemon. This difference in the position of the 

 yolk is rendered possible by the fact that the invaginated hypoblast 



