So THE AMPHIOXUS. 



In the transverse sections it can also be seen how 

 the series of cells of the mesoblastic somites pass 

 at once medianwards and no longer continuously into 

 the series of cells of the middle hypoblast plate 

 (Fig. 80). Now too in the arrangement of the cells 

 the median separation of the mesoblastic somites is 

 far more sharply expressed. Finally, in the trans- 

 verse sections too, the lateral separation of the 

 mesoblastic somites is correlated also to the arrange- 

 ment of the cells. 



In the stage with three mesoblastic somites it can 

 be seen on transverse sections that in the region of 

 the anterior mesoblastic somites a universal sharper 

 definition of their cell material has made its ap- 

 pearance towards the hypoblast plate (Fig. 81). 

 Most sharp is the differentiation of the mesoblastic 

 somite towards the median hypoblast plate, which 

 we saw was earliest introduced. There, what was 

 at first a shallow indentation becomes a sharp-edged 

 furrow, along which is to be seen a distinct differen- 

 tiation of the series of cells of the mesoblast from 

 those of the archenteron. 



We will now acquaint ourselves with the alterations 

 of the mesoblastic somites of the next stages, in 



