96 THE AMPHIOXUS. 



somite is introduced by a flattening cf the hypoblast 

 cells in front of the mesoblastic somite region. There- 

 with an extension of this layer is necessarily occasioned r 

 by which the cells are pushed backwards, and through 

 this pressure the transverse folding is caused, which 

 leads to separation of the mesoblastic somite. 



The formation of the succeeding mesoblastic somites, 

 I would be inclined to refer to a pressure of the cells 

 of the mesoblast folds in the longitudinal axis similar 

 to that in the formation of the first mesoblastic somite. 

 The embryos elongate especially during the forma- 

 tion of the next mesoblastic somites. This elongation 

 appears now in the neighbourhood of the mesoblastic 

 somite region of the mesoblast folds a more produc- 

 tive one than in the neighbouring parts of the embryo. 



The preponderating elongation in the neighbourhood 

 of the mesoblastic somites, owing to which the foldings 

 are caused, as well as the elongation of the embryos in 

 general, appears to be brought about partly through 

 change of form in the cells, partly through growth 

 occasioned by using up the yolk corpuscles. 



The change of form in the cells may be recognised 

 in Figs. 43-47, as also in the series of sections. ~VVe see 

 indeed that the cells of the mesoblast folds become 



