128 



CRUSTACEA. 



(Nos. 64 and 65). Its primitive character is shown by the fact 

 that the cells of the gastrula-invagination retain their original con- 

 nection, so that the archenteric vesicle persists as such until its 

 transformation into the mid-gut, even although the abundant food- 

 yolk here already somewhat modifies the development. 



In Astacus, the blastoderm arises through purely superficial 

 cleavage (MomN, No. 61). After the cleavage-nuclei have become 

 arranged at the surface of the egg, the food-yolk which represents- 

 the separate cleavage-cells breaks up into the so-called primary or 

 Rathke's yolk-pyramids (Fig. 55, p. 114), a spherical central mass 



remaining unaf- 

 fected by this 

 process of cleavage 

 (cf. above, p. 109). 

 After the separa- 

 tion of the blasto- 

 derm-cells from 

 the food-yolk and 

 the complete de- 

 velopment of the 

 blastoderm, the 

 primary yolk- 

 pyramids again 

 fuse. At this 

 stage, the rudi- 

 ment of the em- 

 bryo is percep- 

 tible on the ven- 

 tral side of the 

 egg as a thicken- 

 ing of the blasto- 

 derm, in which the rudiment of the future germ-band is visible. 

 There are originally five distinct thickenings (Fig. 60) in connection 

 with this rudiment, viz. the two optic lobes (K), the two thoraco- 

 aMominal rudiments (TA), and an unpaired thickening behind 

 these, the entoderm-disc (JES), which in the next stage, by invagi- 

 nation, yields the archenteric vesicle (Fig. 61). The invagination 

 of this disc is inaugurated by the appearance of a crescentic furrow 

 enclosing its anterior edge ; this soon becomes a circular furrow 

 by the development of a posterior portion. In consequence of this, 

 the central portion of the entoderm-disc, which has sunk below 



FIG. 60. Astacus fluviatilis, part of the surface of an egg, with 

 embryo beginning to form (after REICHENBACH, from LANG'S 

 Text-book). BM, formative zone of the mesoderm ; ES, ento- 

 derm-disc ; K, cephalic lobes with the rudiment of the eyes ; 

 TA, thoraco-abdominal rudiments. 



