158 THE AMPHIOXUS. 



THE EXTERIOR EPITHELIUM. 



We have still to mention the function of the 

 exterior epithelium, and the formation of the caudal 

 fin which originates from it. 



The epiblast epithelium becomes continually 

 thinner. Its cells are at the conclusion of the 

 embryonal period very thin, extended, and flattened. 

 It is only in a few places that an exception can 

 be seen. We must here particularly mention the 

 anterior end of the body where the thick epithelial 

 layer seems to form a sort of tactile organ. 



Finally, at the posterior end the cells grow out to 

 an extraordinary height, while they here introduce the 

 formation of an epithelial caudal fin. This primary 

 caudal fin, which is only a provisional formation, does 

 not originate as a fold, but is a pectinate elevation of 

 the epithelium. These epithelial cells contain, as a 

 general rule, numerous fine black pigment granules. 



All the cells of the exterior epithelium, even those 

 which compose the caudal fin, carry each one of them 

 a long cilium, by means of which the embiyo moves 

 itself slowly forward, although it is now capable of 

 really strong muscular activity, which, however, does 

 not appear except on special provocation. 



