DECAPODA. 



129 





the surface, projects for a time in the form of a small mound into 

 the lumen of the archenteric vesicle (entodermal mound, Fig. 62, eh). 

 Even before the process of invagination begins, active proliferation 

 of cells takes place at the anterior edge of the entoderm-disc (Fig. 

 60, BM) ; in this way a number of cells are produced which shift 

 below the blastoderm (Figs. 61 and 62, mes). These form the 

 first rudiment of the mesoderm, which thus, in Astacus, has its 

 origin at a definite point on the anterior margin of the blastopore, 

 where the ectoderm passes into the entoderm. 



After the invagination of the gastrula is completed, the blastopore 

 closes at a point correspond- 

 ing to the most posterior 

 portion of the embryonic 

 rudiment. According to 

 REICHENBACH, this lies 

 somewhat behind the spot 

 at which the ectodermal 

 proctodaeal invagination 

 will arise later. 



The archenteric vesicle 

 which results from the in- 

 vagination is at first small 

 in relation to the size of 

 the egg. Its cells increase 

 in size later by absorbing 

 food-yolk (Fig. 63 A, en), 

 which is deposited in each 



. J FIG. 61. Longitudinal section through the gastrula 



individual entoderm-Cell in stage of Astacus fluviatilis (after RRICHENBACH, 



such a manner that the from HATSCHEK>S Text-look). D, food-yolk; m, 



mesoderm ; P, blastopore } * indicates the spot at 

 nucleus and the principal which the anterior end of the body develops. 



mass of protoplasm come 



to lie on the outer surface of the archenteric vesicle. This absorp- 

 tion of food-yolk takes place most actively at the dorsal and lateral 

 parts of the archenteric vesicle, the ventral wall, which is in closer 

 contact with the other embryonic rudiments, taking a smaller share 

 in this process (Fig. 63). Finally, the entire mass of food-yolk is 

 absorbed by the entoderm -cells. These latter consequently swell 

 into exceedingly large and columnar elements, arranged radially, 

 and form the so-called secondary yolk -pyramids. This archenteric 

 vesicle, the surface of which, in later stages, becomes divided into 

 lobes, gives rise to the definitive mid-gut and liver (mid-gut gland) 



