130 



CRUSTACEA. 



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

 and protoplasmic portions of the cells, increase in number, separate 

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

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

 Avhile the secondary yolk-pyramids break up and form a mass of 

 yolk now situated within the archenteron, which is later re-absorbed 

 {cf., for the development of the mid-gut, \). 174). 



A remarkable arrangement of the rudiment of the mid -gut in the egg of 

 Astacus, wliich was not noticed by Bobretzky and Reichenbach, and there- 

 fore must be regarded as of only occasional occui'rence, has been brought to 



light by ScHiM- 



•^ KEWITSCH (No. 



„ "» *"' / 66). The latter 



found within the 

 cavity of the 

 mid-gut a second 

 vesicle consisting 

 of cells. This 

 inner vesicle, in 

 the stage ■which 

 precedes the clos- 

 ing of tlie gas- 

 trula - mouth, is 

 divided off by a 

 process of delami- 

 nation from the 

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

 compares it with those inner cells of the germ of the mid-gut gland in 

 Palaemon, which do not rise to the surface to form the epithelium, but 

 eventually break up in the interior of the gut {cf. below, p. 131). 



The most characteristic feature in the development of the mid-gut 

 of Astacus is that the food-yolk, which originally lies outside the 

 gastrula-vesicle and fills the cleavage-cavity, is later absorbed by 

 the wall of the entoderm-vesicle, and finally, in the development of 

 the mid-gut, reaches its lumen. All other Decapoda that have been 

 investigated differ from Astacus in the fact that the .cells of the 

 entoderm-vesicle lose their epithelial character, and become scattered 

 through the yolk as wandering cells, and only at a later stage again 

 unite at the surface of the yolk to form the epithelium of the 

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

 disappears ; the hepatic rudiment is solid as long as the entoderm- 

 cells are distributed in the yolk, and the lumen of the mid-gut 

 arises only when, later, its contained yolk becomes broken up and 

 absorbed. 



Fig. 62.— Median longitudinal section throngli the gastrula stage of 

 Astacus flvviatilis (after Reichenbach). d, food-yolk; ec, ecto- 

 derm ; eh, entoderm-mound ; en, entoderm ; m, secondary meso- 

 derm ; mes, mesoderm. 



