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

 original (x20). A. An ovum in which the blastoderm is just formed. B. An ovum in wliieli 

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

 taken place. (This nearly answers to 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 just hatched, in longitudinal section ; a, anus ; bl. blastoderm ; bp, blasto- 

 pore ; e, eye; ep.b., epiblast ; fg, fore-gut ; fg 1 , its oesophageal, and /p*. its gastric portion; 

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

 dotted portions in D and E represent the nervous system. 



