THIRD PERIOD OF DEVELOPMENT. 91 



neural canal, answering to the metameric type, 

 advance from in front, where are the older mesoblastic 

 somites, backwards to the region of the younger 

 ones, while the arching of the neural plate 

 followed in a reverse direction. However, in the 

 later stages, this differentiation of the neural canal, 

 which advances from in front backwards, is obliterated 

 by the mechanical part of the development, and can- 

 not be so clearly seen. 



REMARKS UPON THE MECHANICAL PART OF THE 

 DEVELOPMENT PROCESSES. 



"We will now make some remarks about the mechani- 

 cal part of the development processes just observed, 

 in so far as they force themselves upon our notice in 

 the close observation of the embryos and their series 

 of sections. This perhaps may not be without use 

 for the theoretical considerations to follow later ; for 

 it may perhaps be concluded from the great sim- 

 plicity of the mechanical processes that the whole 

 development is very simple and short. 



In that stage of the development which we have 

 just described there took place a series of processes 

 which may be referred to foldings, stretchings, and 



