448 DEVELOPMENT OF ELASMOBRANCH FISHES. 



furrow appears at the border of the germinal disc and is continued from the 

 upper to the lower side. By the closing of the groove there is formed the 

 medullary canal above, while the part of the groove on the under surface 

 directed below is chiefly converted into the hind end of the alimentary 

 tract. The connection of the two tubes in Acanthias persists till the for- 

 mation of the anus, and the part of the nervous tube which lies under the 

 chorda passes gradually upwards to the dorsal side of the chorda, and per- 

 sists there for a long time in the form 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 1 the history of the alimentary tract was 

 completed 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. 8, 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 be- 

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

 parison of figs. 30 and 3^, PI. n. The diameter of the vesicle 

 represented in section in PI. n, fig. 3, is O'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. n, fig. $a) with the neural canal, and 

 behind is continued into two horns (PI. 11, fig. 2, al.) cor- 

 responding with the two caudal swellings spoken of above 

 (p. 288). 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. 



1 P- 33 et sec l- 



