THIRD PERIOD OF DEVELOPMENT. 103 



notochord appears in the region of the mesoblastic 

 somites shut off from the bounding surface of the 

 archenteron, and lengthened towards the front into 

 the anterior end of the body (Fig. 50). 



On a consideration of the whole embryo one would 

 be inclined to hold the view, that the notochord in the 

 region of the mesoblastic somites is formed from the 

 hypoblast, and then, through a secondary outgrowth 

 of this foundation, extends into the anterior end of 

 the body (Figs. 48, 50, 54). On consideration of the 

 transverse sections we shall recognise that this is not 

 the case, but that the anterior end of the notochord is 

 formed in this position out of the hypoblast in front 

 of the mesoblastic somites. 



The most important method for the examination of 

 the notochord formation is the study of the transverse 

 sections. 



We must here revert to the consideration of the 

 transverse sections of the early part of development, 

 in order to take notice of some conditions which are 

 preparatory for the formation of the notochord. It 

 can indeed be already recognised in the stage with 

 three mesoblastic somites ^Figs. 83, 84) that the dorsal 

 median portion of the alimentary canal which lies 



