CRUSTACEA. 513 



time becomes closed. Some of the nuclei now pass to the 

 periphery of the yolk mass ; the cells appertaining to them 

 gradually become distinct and assume 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 proctodaeum and mesenteron become 

 flattened, and in the neighbourhood of these cells a lumen 



FlG. 239. TWO STAGES IN THE DEVELOPMENT OF PAL^MON SEEN IN SECTION. 



(After Bobretzky.) 



A. Gastrula stage. 



B. Longitudinal section through a late stage, hy. hypoblast ; sg. supra-resopha- 

 geal ganglion ; vg. ventral nerve cord ; hd. proctodseum ; st. stomodseum. 



first appears. The stomodaeum and proctodaeum are formed as 

 in Astacus. Fig. 239 B shews the relative positions of the 

 proctodaeum, stomodaeum, and mesenteron. Although the 

 process of formation of the hypoblast and mesenteron is 

 essentially the same in Astacus and Palaemon, yet the differences 

 between these two forms are very interesting, in that the yolk is 

 external to the mesenteron in Astacus, but enclosed within it in 

 Palaemon. This difference in the position of the yolk is rendered 

 possible by the fact that the invaginated hypoblast cells in 

 Palaemon do not, at first, form a continuous layer enclosing a 

 central cavity, while they do so in Astacus. 



The mesoblast appears to be formed of cells budded off 

 from the anterior wall of the archenteron (Astacus, fig. 237 D), 

 B. II. 33 



