ISO THE AMPHIOXUS, 



tinuous series of developments. The appearances are 

 there very clear and convincing, and we can easily 

 get a view of all the transitions. 



After the formation of the ninth mesoblastic somite, 

 the two diverticula, which are still in open communi- 

 cation with the mesenteron, begin to assume an 

 unsymmetrical form. That which is on the right 

 side enlarges, its cells flatten in the manner of an 

 endothelium, and it moves more in an anterior direc- 

 tion. Through the extension of this diverticulum the 

 alimentary canal is pushed back from the anterior end 

 of the body. The diverticulum on the left side is at 

 the same time moved somewhat backwards. 



In the next stages, during the trunk-like growth of 

 the anterior end, the diverticulum on the left side is 

 completely abstricted from the mesenteron. That 

 on the right side, which has extended still more, 

 still continues by means of a small opening in open 

 communication with the anterior end of- the mesen- 

 teron (Figs. 54, 55). This opening may thus be 

 observed at a time when the endothelium-like change 

 of the cells has already made a very considerable 

 advance, and when the varied formation of the 

 evaginations of the mesenteron, which were originally 



