[Illustration: FIG. 58.—_Astacus fluviatilis._—Surface views of the earlier stages in the development of the embryo, from the appearance of the blastopore (A) to the assumption of the nauplius form (F) (after Reichenbach, × about 23). _bp_, blastopore; _c_, carapace; _fg_, fore-gut involution; _h_, heart; _hg_, hind-gut involution; _lb_, labrum; _mg_, medullary groove; _o_, optic pit; _p_, endodermal plug partly filling up the blastopore; _pc_, procephalic processes; _ta_, abdominal elevation; _2_, antennules; _3_, antennæ; _4_, mandibles.] By this invagination a most important step has been taken in the development of the crayfish. For, though the pouch is nothing but an ingrowth of part of the blastoderm, the cells of which its wall is composed {211} henceforward exhibit different tendencies from those which are possessed by the rest of the blastoderm. In fact, it is the primitive alimentary apparatus or _archenteron_, and its wall is termed the _hypoblast_. The rest of the blastoderm, on the contrary, is the primitive epidermis, and receives the name of _epiblast_. If the food-yelk were away, and the archenteron enlarged until the hypoblast came in contact with the epiblast, the entire body would be a double-walled sac, containing an alimentary cavity, with a single external aperture. This is the _gastrula_ condition of the embryo; and some animals, such as the common fresh-water polype, are little more than permanent _gastrulæ_. Although the gastrula has not the slightest resemblance to a crayfish, yet, as soon as the hypoblast and the epiblast are thus differentiated, the foundations of some of the most important systems of organs of the future crustacean are laid. The hypoblast will give rise to the epithelial lining of the mid-gut; the epiblast (which answers to the ectoderm in the adult) to the epithelia of the fore-gut and hind-gut, to the epidermis, and to the central nervous system. The mesodermal structures, that is to say the connective tissue, the muscles, the heart and vessels, and the reproductive organs, which lie between the ectoderm and the endoderm, are not derived directly from either the epiblast or the hypoblast, but have a _quasi_-independent origin, from a mass of cells which first makes its {212} appearance in the neighbourhood of the blastopore, between the hypoblast and the epiblast, though they are probably derived from the former. From this region they gradually spread, first over the sternal, and then on to the tergal aspect of the embryo, and constitute the _mesoblast_.