Fig. 57.— Astaciis JlnvmtiU!^. — Dingrammatic KectioTis of embryos ; partly after Relchenbacli, partly 

 original (X 20). A. An ovum in which the blastoderm is just formed. B. An OTura in which 

 the invagination of the blastodenn to constitute the hypoblast or rudiment of the mid-^t has 

 taken place. (This nearly answers tn the stage represented in fig. 58, A.) C. A longitudinal 

 section of an ovum, in which the rudiments of the abdomen, of the hind-gut, and of the fore- 

 gut have appeared. (This nearly answers to the stage represented in fig. 58, E.) D. A similar 

 section of an embryo in nearly the same stage of development as that represented in C, fig. 59. 

 E. An embryo )ust hatched, in longitudinal section ; a, anus ; 1)1. blastoderm ; bp, blasto- 

 pore ; e, eye ; ep,b,, epiblast ; f<j, fore-gut ; fg^, its oesophageal, and fp^, its gastric portion ; 

 ft, heart: Jiq, hind-gut; t», mouth; mg, hypoblast, archenteron, or mid-gut; v, yelk. The 

 dotted portions in D and E represent the nervous system. 



