130 



CRUSTACEA. 



of Astacus. The outer portions of the entoderm-cells, i.e., 

 and protoplasmic portions of the cells, increase in numbe 

 from their inner yolk-bearing portions, the yolk-pyramids 

 into contact with each other, thus forming the mid-gut 

 while the secondary yolk-pyramids break up and forr 

 yolk now situated within the archenteron, which is late 

 (cf. t for the development of the mid-gut, p. 174). 



A remarkable arrangement of the rudiment of the mid -gut 

 Astacus, which was not noticed by BOBRETZKY and REICHENB 

 fore must be regarded as of only occasional occurrence, has 



1 



p 



FIG. 62. Median longitudinal section through the gastrula stage of 



Astacus fluviatilis (after REICHENBACH). d, food-yolk; ec, ecto- t 



derm ; eh, entoderm-mound ; en, entoderm ; m, secondary ineso- d 



derm ; mes, mesoderm. p 



n 



entoderm-cells ; in later stages it seems to be absorbed, 

 compares it with those inner cells of the germ of the mi 

 Palaemon, which do not rise to the surface to form the e 

 eventually break up in the interior of the gut (cf. below, p. 



The most characteristic feature in the development 

 of Astacus is that the food-yolk, which originally 

 gastrula-vesicle and fills the cleavage-cavity, is lal 

 the wall of the entoderm-vesicle, and finally, in th( 

 the mid-gut, reaches its lumen. All other Decapod, 

 investigated differ from Astacus in the fact that 

 entoderm-vesicle lose their epithelial character, and 

 through the yolk as wandering cells, and only at a 

 unite at the surface of the yolk to form the ef 

 mid-gut. In these forms, the lumen of the original 

 disappears; the hepatic rudiment is solid as long s 

 cells are distributed in the yolk, and the lumen 

 arises only when, later, its contained yolk becomes , 

 absorbed. 



