Fig. 57.— Astacvs flnviatilis. — Diagrammatic sections of embryos ; partly after Reichenbach, partly 

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

 the invagination of the blastoderm to constitute the hypoblast or rudiment of the mid-gut has 

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

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

 gut have aitpeared. (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 just hatched, in longitudinal section ; o, anus ; hi. blastoderm ; bp, blasto- 

 pore ; e, eye; ep.b., epiVjlast ; /gr, fore-gut ; /(/i, its oesophageal, and fn^, its gastric portion; 

 h, heart ; hq, hind-gut ; w, mouth ; mg, hypoblast, archenteron, or mid-gut ; v, yelk. The 

 dotted portions in D and E represent the nervous system. 



