512 



FORMATION OF THE LAYERS. 



and is finally obliterated. Very shortly after this occurrence 

 there is formed, slightly in front of the point where the last trace 

 of the blastopore was observable, a fresh epiblastic invagination, 

 which gives rise to the proctodaeum, and the opening of which 

 remains as the definite anus. The proctodaeum (fig. 238 A, kg) 

 is very soon placed in communication with the mesenteron (mg). 

 The stomodaeum (fg) is formed during the same stage as the 

 proctodaeum. It gives rise to the oesophagus and stomach. 

 The hypoblast cells which form the wall of the archenteron 

 grow with remarkable rapidity at the expense of the yolk ; the 

 spherules of which they absorb and digest in an amoeba-like 

 fashion by means of their pseudopodia. They become longer 

 and longer, and finally, after ab- 

 sorbing the whole yolk, acquire 

 a form almost exactly similar to 

 that of the yolk pyramids dur- 

 ing segmentation (fig. 238 B). 

 They enclose the cavity of the 

 mesenteron, and their nuclei 

 and protoplasm are situated ex- 

 ternally. The cells of the me- 

 senteron close to its junction 

 with the proctodaeum differ 

 from those elsewhere in being 

 nearly flat. 



In Palaemon (Bobretzky) 

 the primitive invagination (fig. 

 239 A) has far smaller dimen- 

 sions than in Astacus, and ap- 

 pears 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 

 hypoblast in which the outlines 

 of the individual cells would 

 seem at first not to be distinct, 



FlG. 238. TWO LONGITUDINAL SEC- 

 TIONS OF THE EMBRYO OF ASTACUS. 

 (From Parker ; after Bobretzky.) 



A. Nauplius stage. B. Stage after 

 the hypoblast cells have absorbed the 

 food yolk. The ventral surface is turned 

 upwards, fg. stomodaeum ; hg. procto- 

 daeum ; an. anus; in. mouth; ing. me- 

 senteron ; abd, abdomen ; h. heart. 



The blastopore in the mean- 



