Chap. VI.] GENERAL EMBRYOLOGY. 93 



divided by the notochord into two separate sheets, the mesdblastic 

 plates, (m. p.) passing round and meeting below the digestive &* 

 cavity (mesenteron, mn.). 



We have yet to note some further changes in the mesoblastic 

 plates. (1.) Each splits 1 into two layers, of which the inner layer 

 adjoining the mesenteron is called the visceral or splanchnic layer 

 (sp. I.), while the outer, that adjoining the epiblast, is the parietal 

 or somatic layer. The spaqe between them is the body- cavity 

 (b. c.). (2) On each side of the neural canal and the noto- 

 chord longitudinal masses of the mesoblast are separated off 

 from the mesoblastic plates. These median dorsal parts of the 

 mesoblastic plates are now known as vertebral plates (v.p.), while 

 the remainder of the mesoblastic plates are now termed lateral 

 plates. In the vertebral plates the cleavage of the mesoblast 

 and resulting, body-cavity are eventually obliterated. (3.) The 

 vertebral plates become segmented by transverse constrictions 

 into a series of more or less cubical bodies (for which in the 

 chick see Fig. 37, m. s.), the mesoblastic somites. (4.) In the lateral 

 plates the cleavage of the mesoblast is obliterated only at the 

 dorsal end, near the vertebral plates. Below this the body- 

 cavity is persistent. The splanchnic layer eventually forms the 

 muscular coat of the alimentary canal, while the somatic layer 

 gives rise to the endothelial lining of the body-walls. 



The blastopore has probably by this time closed, so that 

 the mesenteron has neither anterior nor posterior openings to 

 the exterior, though posteriorly it may open into the neural 

 canal, by a passage known as the neurenteric canal (n. e. c. in 

 frog, Fig. 36, in.). External openings are formed, often com- 

 paratively late in embryonic life, by the formation of external 

 depressions which meet hollow outgrowths from the mesenteron. 

 The anterior depression, that for the mouth, is called the stomo- 

 dceum ; the posterior depression, that for the vent, is called the 

 proctodceum. The former is always in vertebrates a new invagi- 

 nation; but the latter may be in some types (perhaps in the 



1 The cleavage of the mesoblast begins before the mesoblastic plates meet 

 below the mesenteron. The two processes are kept separate in the figures and 

 description for the sake of clearness. 



