Fic. 57.—Astacus fluviatilis.—Diagrammatic sections of embryos ; partly after Reichenbach, partly 
original (x20), 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 the stage represented in fig. 58, 4.) 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; ¢, eye; ep.b., epiblast ; fg, fore-gut; fg!, its esophageal, and fg?, 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, 
