DECAPODA. 



131 



In Palaemon, for instance, according to Bobretzky (No. 41), a 

 small gastrula- 



invagination t fut 



(Fig. 64 A). 4 ** 



develops at a 

 time when the 

 blastoderm is 

 not completely 

 formed, i.e., 

 when the blas- 

 toderm-cells 

 have not sepa- 

 rated from their 

 yolk - pyramids 

 over the whole 

 •circumference 

 of the egg. 

 The cells of 

 this gastrula- 

 invagination, 

 after the closing 

 of the blasto- 

 pore, lose their 

 ■epithelial char- 

 acter (Fig. 64 

 B). From the 

 lateral walls of 

 the entoderm- 

 vesicle (ins), 

 elements arise 

 which, at a 

 later stage, 

 lying on the 

 germ -band, 

 represent the 

 mesoderm, 

 while the ento- 



ilerm-cells arising from the floor of the vesicle (en) pass into the 

 yolk, traversing it like wandering cells, and increasing in number 

 within it. Each of these entoderm-cells swallows in an amoeba-like 

 manner the surrounding food-yolk, so that the whole of the yolk 



Fig. 63.— Median longitudinal section through two embryos of 

 Astacus fluviatilis (after Reichenbach). A, section through the 

 Nauplius stage. B, through the stage in which the rudiments of 

 the ambulatory limbs have formed, d, food-yolk ; dp, secondary 

 yolk-pyramids ; ec, ectoderm ; en, entoderm ; ep, entoderm plate ; 

 g, rudiment of the ventral chain of ganglia ; h, rudiment of the 

 heart ; M, hind-gut ; m, mesoderm ; md, mid-gut ; og, supra-oeso- 

 phageal ganglion ; sp, splanchnic layer of the mesoderm ; vd, fore- 

 gut ; t, thoracoabdominal rudiment. 



