DEVELOPMENT OF ELASMOBRANCH FISHES. 219 



gi-adually upwards to the dorsal side of the chorda, and persists there 

 for a long time in the fonn of a large thin-walled vesicle. 



The last part of the description beginning at "The con- 

 nection of" does not hold good for any of the genera which 

 I have had an opportunity of investigating, as will appear from 

 the sequeL 



In a previous section^ the history of the alimentary tract 

 was comj^leted up to stage G. 



In stage H the point where the anus will (at a very much 

 later period) appear, becomes marked out by the alimentary 

 tract sending down a papilliform process towards the skin. 

 This is shewn in PI. vi. figs. H and /, an. 



That part of the alimentary tract which is situated behind 

 this point may, for convenience, be called the postanal section. 

 During stage H the postanal section begins to develope a 

 terminal dilatation or vesicle, connected with the remainder 

 of the canal by a narrower stalk. The relation in diameter 

 between the vesicle and the stalk may be gathered by a com- 

 parison of fig. 3a and 35, PL x. The diameter of the vesicle 

 represented in section in PI. x. fig. 3, is 0*328 Mm. 



The walls both of the vesicle and stalk are formed of a 

 fairly columnar epithelium. The vesicle communicates in front 

 by a narrow passage (PI. x. fig. 3a) with the neural canal, and 

 behind is continued into two horns (PI. x. fig. 2, al.) cor- 

 responding with the two caudal swellings spoken of above 

 (p. 73). Where the canal is continued into these two horns, 

 its walls lose their distinctness of outline, and become con- 

 tinuous with the adjacent mesoblast. 



In the succeeding stages up to K the tail grows longer and 

 longer, and with it grows the postanal section of the alimen- 

 tary tract, without however altering in any of its essential 

 characters. 



Its features at stage K are illustrated by an optical section 

 of the tail of an embryo (PL xvii. fig. 5) and by a series of 

 transverse sections through the tail of another embryo in PL 

 XVII. fig. 6a, 66, Qcy 6d. In the optical section there is seen 

 a terminal vesicle {alv) opening into the neural canal, and con- 



1 p. 87 et seq. 



15—2 



