1 56 THE AMPHIOXUS, 



Kowalevsky lias given us no information respecting 

 the relation of position of mouth, and gill-slit, with 

 reference to the segments. 



I will now myself describe the processes belonging 

 to the end of this period of development, which are 

 preparatory of the formation of the mouth and first 

 gill-slit. 



In the region of the first two segments, the anterior 

 blind end of the alimentary canal is thickened in a 

 club-like manner, whereby the body appears raised in 

 this region. The raising of this section of the alimen- 

 tary canal is connected with the formation of mouth 

 and first gill-slit. 



The formation of the mouth is introduced by a 

 disc-like thickening of the epiblast. The latter con- 

 sists here of somewhat high cells, in contrast to the 

 other and larger flat cells. 



This thickening of the epiblast lies on the left side 

 of the body, corresponding more or less to the middle 

 of the side wall of the alimentary canal. It is also in 

 immediate contact with the hypoblast, since here the 

 mesoblast does not grow forward so far ventralwards 

 (Figs. 132, 133). 



While the formation of the mouth opening princi- 



