104 THE AMPHIOXUS. 



between the mesoblast folds shrinks to a flat channel, 

 whose concavity is turned towards the lumen of the 

 alimentary canal. This channel deepens considerably 

 in the stages with four and five mesoblastic somites 

 (Figs. 88, 94), and the result is finally in the stage 

 with six mesoblastic somites (Fig. 97) the formation of 

 a sharply marked fold, first in the region of the most 

 anterior mesoblastic somites, and gradually, too, in 

 that of the posterior ones. Further back, in the region 

 of the unseparated mesoblast fold, the notochord fold 

 gradually flattens (Figs. 91, 92). The lumen of the 

 channel becomes in the process of folding a narrow 

 slit, so that the internal ends of the cells of the right 

 and left half of the fold finally touch one another (Figs. 

 98, 101). 



At the time when this folding is sharply marked, 

 the slits which connect the cavity of the meso- 

 blastic somite with the lumen of the archenteron 

 are already closed, and are only indicated through 

 the sharper separation of the cells in this place. The 

 formation of the notochord is thus first anticipated 

 at a time in which the mesoblastic somites are already 

 completely separated from the hypoblast. 



The notochord is now in the following stages formed 



