THIRD PERIOD OF DEVELOPMENT. 77 



the mesoblastic somites and the cells which form the 

 archenteron. 



We will now gain a closer acquaintance with the 

 processes which fall under this section, while we take 

 into consideration the results yielded by the improved 

 methods of investigation. 



FOKMATIOX OF THE MESOBLAST FOLDS AND 

 MESOBLASTIC SOMITES. 



We will first explain more accurately the formation 

 of the mesoblastic somites. 



If we examine stages in which the dorsal surface 

 begins to sink in, that is to say, stages which are some- 

 what older than that represented in Fig. 33, upon 

 artificially prepared transverse sections, we can then 

 obtain a closer knowledge respecting the state of the 

 mesoblast folds. 



We see such a transverse section represented in Fig. 

 72. The material of the still very flat mesoblast folds 

 passes indeed directly over into the hypoblast plate. 

 The boundary line however of the mesoblast folds is 

 in the direction of the median plane already sharply 

 defined. On the outer surface of the hypoblast two 

 distinct furrows are to be found, which form the 



