[Illustration: FIG. 57.—_Astacus fluviatilis._—Diagrammatic sections of embryos; partly after Reichenbach, partly original (× 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 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_, blastopore; _e_, eye; _ep. b._, epiblast; _fg_, fore-gut; _fg_^1, its œsophageal, and _fg_^2, 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.] Thus, at this period, the body of the developing crayfish is nothing but a spherical bag, the thin walls of which are composed of a single layer of nucleated cells, while its cavity is filled with food-yelk. The first modification {209} which is effected in the vesicular blastoderm manifests itself on that face of it which is turned towards the pedicle of the egg. Here the layer of cells becomes thickened throughout an oval area about 1‐25th of an inch in diameter. Hence, when the egg is viewed by reflected light, a whitish patch of corresponding form and size appears in this region. This may be termed the _germinal disk_. Its long axis corresponds with that of the future crayfish. Next, a depression (fig. 58, A, _bp_) appears in the hinder third of the germinal disk, in consequence of this part of the blastoderm growing inwards, and thus giving rise to a small wide-mouthed pouch, which projects into the food-yelk with which the cavity of the blastoderm is filled (fig. 57, B, _mg_). As this infolding, or invagination of the blastoderm, goes on, the pouch thus produced increases, while its external opening, termed the _blastopore_ (fig. 57, B, and 58, A–E, _bp_), diminishes in size. Thus the body of the embryo crayfish, from being a simple bag becomes a double bag, such as might be produced by pushing in the wall of an incompletely distended india-rubber ball with the finger. And, in this case, if the interior of the bag contained porridge, the latter would very fairly represent the food-yelk.