ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 353 



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 disappear 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 appear- 

 ance of the external gills — there being slight differences 

 probably in this respect in the different species. 



In this history there are two points of especial interest : 



(1) The terminal vesicle. 



(2) The disappearance of a large and well-developed por- 

 tion of the alimentary canal. 



The interest in the terminal vesicle lies in the possibility 

 of its being some rudimentary structure. 



In Osseous fishes Kuppfer has described the very early 

 appearance of a vesicle near the tail end, which he doubtfully 

 speaks of as the " allantois.^' The figure he gives of it in 

 his earlier paper (' Archiv. fiir Micro. Anat.' vol. ii, pi. xxiv, 

 fig. 2) bears a very strong resemblance to my figures of this 

 vesicle at the time when the hind end of the alimentary 

 canal is commencing to disappear ; and I feel fairly confident 

 that it is the same structure as I have found in the Dog-fish: 

 but until the relations of the Kuppfer's vesicle to the ali- 

 mentary canal are known, any comparison between it and the 

 terminal vesicle in the Dog-fish must be to a certain extent 

 guess-work. 



I have, however, been quite unsuccessful in finding any 

 other vesicular structure which can possibly correspond to 

 the so-called allantoic vesicle of Osseous fish. 



The disappearance of a large portion of the alimentary 

 canal behind the anus is very peculiar. In order, however, 

 to understand the whole difficulties of the case I shall be 

 obliged to speak of the relations of the anus of the Dog-fish 

 to the anus of Rusconi in the Lamprey, &c. 



In those vertebrates whose alimentary canal is formed by 

 an involution, the anus of Rusconi represents the opening of 

 this involution, and therefore the point where the alimentary 

 canal primitively communicates with the exterior. When, 

 however, the " anus of Rusconi " becomes closed, the wall of 

 the alimentary canal still remains at that point in close 

 juxtaposition to the surface, and the new and final anus is 



