EPIBLAST, MESOBLAST, AND HVrOBLAST. 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 hyiwhlast. The rest of 

 the blastoderm, on the contrary, is the primitive epider- 

 mis, and receives the name of eplhlast. If the food- 

 yellc were away, and the archenteron enlarged until the 

 hj'poblast 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 polj'pe, are little more 

 thaji permanent cjastndce. 



Although the gastrula has not the sliglitest 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 sj^stems of organs of the 

 future crustacean are laid. The hyi)oblast will give rise 

 to the epithelial lining of the mid-gut ; the epiblast 

 (which answers to the ectoderm in the adult) to tlie 

 epithelia of the fore-gut and hind-gut, to the epidermis, 

 and to the central nervous system. 



The mesodei'mal 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, ai'e not derived directly from either 

 the epiblast or the hypoblast, but have a g'jwsi-indejiendent 

 oi'igin, from a mass of cells which first makes its ajipear- 



r 2 



