EPIBLAST, MESOBLAST, AND HYPOBLAST. 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 archen- 
teron, and its wall is termed the hypoblast. The rest of 
the blastoderm, on the contrary, is the primitive epider- 
mis, 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 ali- 
mentary 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 gastrule. 
Although the gastrula has not the slightest resem- 
blance 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 con- 
nective tissue, the muscles, the heart and vessels, and 
the reproductive organs, which lie between the ectoderm 
and the endoderm, aie 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 appear- 
P2 
