FOURTH PERIOD OF DEVELOPMENT. 155 



THE ALIMENTARY CANAL. 



The alimentary canal begins to ciliate in this period 

 of development on its inner surface, as on every 

 cell a cilium is formed. The formation of the mouth 

 and of the first gill-slit foundation is thus prepared. 



Kowalevsky thus writes : " On the right side more 

 or less, on the place where the diverticulum of the 

 alimentary canal ends in front, the walls of the 

 interior canal (alimentary canal) and of the body 

 coalesce on a little space. There arises first, from the 

 thickening of the tissue, a somewhat dark spot. In 

 the middle of this an opening is soon formed origina- 

 ting from the separation of the cells which have 

 coalesced. The opening so formed is surrounded by 

 edges raised like walls. This aperture is the opening 

 of the mouth." 



He sa} r s, further : " Soon after the formation of the 

 mouth, it may be noticed that at the lower edge the 

 wall of the alimentary canal coalesces with that of the 

 body, and soon there is here seen an opening ; this is 

 the first gill-slit. This newly arisen opening does not 

 remain long in this place, but moves to one side of 

 the body, that, namely, opposite to the opening of the 

 mouth." 



