THIRD PERIOD OF DEVELOPMENT. 105 



at the expense of this dorsal fold of the archen- 

 teron ; that is to say, out of the material of that 

 median dorsal hypo blast plate, which in the forma- 

 tion of the mesoblast folds and mesoblastic somites 

 obtained its separation between them. This hypoblast 

 plate, however, as already mentioned, is not entirely 

 used up in the formation of the notochord, but the 

 lateral cells of the latter become connected with the 

 ventral portion of the hypoblast. Now, since this 

 connexion takes place before the notochord becomes 

 split off, it is difficult to give this proof with precision 

 for the region of the anterior mesoblastic somites. 

 For this purpose the exact comparison of the draw- 

 ings of numerous series of sections is necessary. 

 This is easier in the later shortened development of 

 the succeeding mesoblastic somites. This we can 

 quite well recognise in Fig. 101 and Fig. 102 (cp. 

 also Figs. 118 and 141), that the middle part of the 

 hypoblast plate belongs to the foundation of the 

 notochord. 



We will now proceed to view the changes which 

 the foundation of the notochord in general undergoes 

 in the region of one of the anterior mesoblastic 

 somites. 



