Fro, yJ.Astacvs fluviatilis. Diagrammatic sections of embryos ; partly after Reichenbaeh, partly 

 wnginal ( 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 the 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 appeared. (This nearly answers to the stage represented in fi<r. 53, 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 ; ftp, blasto- 

 pore ; e, eye ; ep.b. , epiblast ; fg, fore-gut ; /f/ 1 , its oesophageal, and /(/ 2 , its gastric portion; 

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

 dotted portions in D and E represent the nervous system. 



