i6o THE AMPHIOXUS. 



This period of development, which thus comprises 

 the stages from the first perforation of the mouth 

 opening up to that of the anus, occupies one and a 

 half to two clays, which should be added to the 

 previously stated duration of the embryonal develop- 

 ment. 



FORMATION OF THE BODY APERTURES. 



We will now take into consideration the origin of 

 the above-named apertures, which is described as 

 characteristic of this period. Kowalevsky states that 

 the formation of the mouth follows somewhat earlier 

 than that of the first gill-slit. I myself saw these two 

 openings make their appearance simultaneously as 

 very fine slits. 



The formations introducing the perforation of the 

 mouth and first gill-slit have already been described. 



The mouth appears as a very fine opening, which 

 lies in the middle of the disc-shaped thickening of the 

 hypoblast, and places the alimentary canal in com- 

 munication with the exterior. This fine opening 

 lies, as has already been mentioned, in the region of 

 the first somite. During the following days this 

 opening enlarges very slowly. This enlargement 



