94 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



The anus is a rather late formation, but its position becomes 

 very early marked out by the hypoblast of the digestive canal 

 approaching at that point close to the surface, whilst receding 

 to some little distance from it on either side. The portion of 

 the digestive tract I propose at present dealing with is that 

 between this point, which I will call, for the sake of brevity, the 

 anus and the hind end of the body. This portion of the canal 

 is at first very short ; it is elliptical in section, and of rather a 

 larger bore than the remainder of the canal. Its diameter be- 

 comes, however, slightly less as it approaches the tail, dilating 

 again somewhat at its extreme end. It is lined by a markedly 

 columnar epithelium. Though at first very short, its length 

 increases with the growth of the tail, but at the same time its 

 calibre continually becomes smaller as compared with the re- 

 remainder of the alimentary canal. 



It commences to become smaller, first of all, near, though 

 not quite, at its extreme hind end, and thus becomes of a conical 

 shape; the base of the cone being just behind the anus, while 

 the apex of the cone is situated a short distance from the hind 

 end of the embryo. The extreme hind end, however, at the 

 same time does not diminish in size, and becomes relatively 

 (if not also absolutely) much larger in diameter than it was 

 at first, as compared with the remainder of the digestive canal. 

 It becomes, in fact, a vesicle or vesicular dilatation at the end 

 of a conical canal. 



Just before the appearance of the external gills this part of 

 the digestive canal commences to atrophy. It begins to do so 

 close to the terminal vesicle, which, however, still remains as 

 or more conspicuous than it was before. The lumen of the 

 canal becomes smaller and smaller, and finally it becomes a 

 solid string of cells, and these also soon become indistinguishable 

 and not a trace of the canal is left. 



Almost the whole of it has disappeared before the vesicle 

 begins to atrophy, but very shortly after all trace of the rest 

 of the canal has vanished the terminal vesicle also vanishes. 

 This occurs just about the time or shortly after the appearance 

 of the external gills there being slight differences probably in 

 this respect in the different species. 



In this history there arc two points of especial interest : 



