FOURTH PERIOD OF DEVELOPMENT. 141 



in the neighbourhood of the neurenteric canal (Fig. 

 142). The growth of the notochord at the posterior 

 end follows, owing to an abstriction of the fold 

 formation of the dorsal wall which continually pro- 

 gresses backwards. Just as however the undiffer- 

 entiated mesoblast folds from which the foundation 

 of numerous mesoblastic somites is still to proceed, 

 are completely abstracted from the hypoblast as 

 separate formations at the end of this period of 

 development, so in the same way, at the conclusion 

 of the embryonal development, follows the abstric- 

 tion of the undiffereiitiated notochord fold from the 

 archenteron. This now completely isolated foundation 

 forms the material at whose expense the notochord 

 later on also continues to grow at the posterior end 

 during the formation of new myotomes. Mesoblast 

 and notochord are thus further increased by new 

 formations, which, as before, originate at the expense 

 of the undifferentiated foundations of the posterior 

 end of the body, except that these undifferentiated 

 parts are no longer connected with the hypoblast, 

 but are completely separated. 



In the anterior end of the body the notochord 

 had already become almost completely separated 



