THIRD PERIOD OF DEVELOPMENT. 93 



appear to have an active change of form, especially in 

 the formation of the dorsal furrow, which stands in 

 close mechanical connexion with the beginning of the 

 mesoblast folds. 



Differences in the energies of growth come before 

 us in far more manifold fashion, and stand in close 

 relation with all the more important processes of 

 development. We shall see that in the separation of 

 the neural plate, and in the arching of the same, 

 the differences between its energy of growth and that 

 of the neighbouring epiblast play an important part. 

 We shall further recognise that, although the first foun- 

 dation of the mesoblast folds was introduced through an 

 active change of form, yet later the energy of growth 

 of the mesoblast folds, which is greater in comparison 

 with the neighbouring parts, causes their further for- 

 mation and their division into mesoblastic somites. 



The energy of growth is in general at first greater 

 in the anterior region of the body, and keeps equal 

 pace with the differentiation which advances towards 

 the posterior end. 



We will now go somewhat more closely into the 

 details in the mechanical part of the development 

 processes. 



