Fig. bl .—Astacvs flnviatilis. — Diograminatic sections of cnihryos ; partly after Reichenbaoli, parlly 

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

 the invagination of the blastoderm to constitute the hypol)last or rudiment of the mid-j;ut has 

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

 section of an ovum, in which the rudiments ot 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 just hatched, in longitudinal section ; a, anus ; hi. blastoderm ; bp, blasto- 

 pore ; e, eye ; ep.h., epiVjlast ; /f/, fore-gut ; /|7', its oesophageal, and /f/*. its gastric portion; 

 h, heart ; hg, hind-gut ; n>, mouth ; mg, hypoblast, archenteron, or mid-gnt ; r, yelk. The 

 dotted portions in D and E represent the nervous system. 



