Chap, xiv.] DEVELOPMENT. 529 



the Coelenterata into two layers, the epidermic and the 

 nervous ; the former gives rise to a structureless 

 cuticle, or undergoes hardening and becomes chitinous, 

 horny, or calcified, thereby giving rise to the various 

 forms of external skeletons ; or its cells may 

 undergo invagination, and give rise to the various 

 forms of tegumentary glands, of which the mammary 

 are the most specialised. In the region of the mouth 

 the epiblast very ordinarily folds inwards to form the 

 stomodaeum, and in that of the anus to form the 

 proctodaeum. 



The nervous layer undergoes one of two kinds of 

 modifications ; its cells either become thickened along 

 certain tracts, which in the more primitive forms 

 remain permanently connected with the epidermic 

 layer, or, as in all the Chordata, the nervous layer is 

 grooved along its middle line, and, giving rise to the 

 medullary canal, becomes separated from the epider- 

 mis. (See page 416.) Other parts of the layer become 

 modified to form the organs of sense. 



The mesoblast, the mode of development of which 

 varies greatly in different forms, arises in such primi- 

 tive forms as Peripatus or Amphioxus in the form of 

 paired outgrowths of the enteron ; in the segmented 

 Metazoa, such as the earthworm, the insect, or Am- 

 phioxus, the mesoblast on either side becomes divided 

 into a series of cubical masses, the mesofolastic 

 somites. Where the mesoblast arises from the ente- 

 ron, the somites are from the first hollow within ; where, 

 as in the earthworm, the somites arise from special cells, 

 and are at first solid, a cavity is gradually developed 

 within them ; and the cavities of either side becoming 

 in time continuous with one another, give rise to the 

 general body cavity (ccelorn). The outermost portion 

 of the mesoblast comes into contact with the epiblast, 

 and gives rise to the dermis and the muscles of the 

 body wall ; the innermost layer comes into relation 

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