THIRD PERIOD OF DEVELOPMENT. 95 



embryo, and in the foldings which are continually 

 forming more sharply in the internal parts, processes 

 of growth and movement in the hypoblast are the 

 preponderating cause. 



If we consider more closely the details of the 

 mechanical processes, it is apparent that quite various 

 formations are brought into connexion through one 

 and the same mechanical process. We shall thus 

 recognise that the appearance of the dorsal furrow 

 not only introduces the formation of the neural canal, 

 but stands in just as intimate a relation to the forma- 

 tion of the mesoblast folds. Thus we can also see, on 

 a consideration of the series of sections, that those two 

 longitudinal furrows which form the median bound- 

 ing of the mesoblast folds and play an important part 

 in the sinking of the dorsal surface stand in just as 

 intimate relation to the shrinking of the neural canal 

 as to the formation of the mesoblast folds. 



The formation of the mesoblast folds is now to be 

 referred to a more important surface extension of the 

 hypoblast in the posterior region. 



The formation too of the mesoblastic somites may be 

 referred to mechanical processes in the hypoblast. 

 We see that the formation of the first mesoblastic 



