DEVELOPMENT OF THE ELASMOBRANCH FISHES. 95 



(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 Kupffer 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 (ArcJdv. filr 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 com- 

 mencing 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 Kupffer's vesicle to the alimentary 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 under- 

 stand 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 formed at or close to that 

 point. In the Dog-fish, although the anus of Rusconi is not 

 present, still, during the closing of the alimentary canal, the point 

 which would correspond with this becomes marked out by the 

 alimentary canal there approaching the surface, and it is at this 

 point that the involution to form the true anus subsequently 

 appears. 



The anus in the Dog-fish has thus, more than a mere secon- 

 dary significance. It corresponds with the point of closing of 



