86 THE AMPHIOXUS. 



a little further forward than the mesoblast folds. 

 Along the dorsal ridges, which form the sides of 

 the dorsal furrow, the median plate, 1 which consti- 

 tutes the base of the dorsal furrow, is now divided 

 off from the lateral epiblast by a sharp separa- 

 tion inside it. This separation is chiefly to be seen 

 in sections, and is especially remarkable for the fact 

 that a discontinuity occurs in the arrangement of the 

 cells (Fig. 72). The cells begin to move from the 

 side over the neural plate. 



"We can too in Fig. 36, in the posterior region of 

 the body, view the edges growing forward towards 

 the median line as wavy border lines running upon 

 the surface. In the living object, where this super- 

 ficial arching layer lies right on the neural plate, 

 nothing is to be noticed of this. In the objects 

 treated with reagents the wavy boundary of the 

 arching layer comes most prominently to view, when 

 it has separated itself somewhat from the neural 

 plate through the action of the reagents. 



This arching begins on the sides of the gastrula- 

 mouth, which lies at the posterior end of the medullary 

 plate. The gastrula-mouth is now bridged over 'by 



1 Neural plate. 



