THIRD PERIOD OF DEVELOPMENT. 115 



In the stages, with five or six mesoblastic somites, 

 there follows in the anterior region of the body the 

 complete closing of the mesoblastic somite cavities. 



After the closing, too, of the slit communicating 

 beween the mesoblastic somite and the archenteron 

 canal, still, for some time its place may be traced in 

 the arrangement of the cells at that point, in conse- 

 quence of a feebly expressed discontinuity in the cells 

 of the hypoblast. It can then be seen (Fig. 101) that 

 the hypoblast slit is moved somewhat dorsally, and 

 this is in connexion with the folding of the notochord. 



If we observe the transverse section of a fully 

 separated mesoblastic somite, we find that it possesses 

 a more or less triangular form (Figs. 97-100). Three 

 sides can be distinguished in it, that is, a base 1 which 

 is formed of those cells which constitute the edges 

 of the communicating slit, and which are in contact 

 with the archenteron ; an inner side, 2 which is in con- 

 tact with the notochord and the nervous system ; and 

 an outer side, 3 which is in contact with the outer 

 boundary. 



The base 1 of the triangle furnishes the fibrous layer 

 of the alimentary canal. 



1 Visceral. 2 Notocliordal. 3 Parietal. 



