96 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



the primitive involution. If it was not for this peculiarity of the 

 vertebrate anus we would naturally suppose, from the disap- 

 pearance of a considerable portion of the alimentary canal lying 

 behind its present termination, that in the adult the alimentary 

 canal once extended much farther back than at present, and 

 that the anus we now find was only a secondary anus, and not 

 the primitive one. It is perhaps possible that this hinder portion 

 of the alimentary canal is a result of the combined growth of 

 the tail and the persisting continuity (at the end of the body) of 

 the epiblast with the hypoblast. 



Whichever view is correct, it may be well to mention, in 

 order to shew that the difficulty about the anus of Rusconi is 

 no mere visionary one, that Gotte (" Untersuchung iiber die 

 Entwickelung der Bombinator igneus," ArcJiiv. fur Micro. 

 Anat., vol. v. 1869) has also described the disappearance of the 

 hind portion of the alimentary canal in Batrachians, a rudiment 

 (according to him) remaining in the shape of a lymphatic trunk. 



It is, perhaps, possible that we have a further remnant of 

 this " hind portion " of the alimentary canal amongst the higher 

 vertebrates in the " allantois." 



Organs developed from the Digestive Canal. 



In reference to the development of the liver, pancreas, &c., 

 as far as my observations have at present gone, the Dog-fish 

 presents no features of peculiar interest. The liver is developed 

 as in the Bird, and independently of the yolk. 



There are, however, two organs derived from the hypoblast 

 which deserve more attention. Immediately under the noto- 

 chord, and in contact with it (vide PL 3, fig. 10; 4, 11 and I2,;r), 

 a small roundish (in section) mass of cells is to be seen in most 

 of the sections. 



Its mode of development is shewn in fig. 10, x. That section 

 shows a mass of cells becoming pinched off from the top of the 

 alimentary canal. By this process of pinching ofif from the 

 alimentary canal a small rod-like body close under the noto- 

 chord is formed. It persists till after the appearance of the 

 external gills, but later than that I have not hitherto succeeded 

 in finding any trace of it 



