354 F. M. BALFOUR. 



formed at or close to that point. In the Dog-fish^ although 

 the anus of Ruscoui 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 

 secondary significance. It corresponds with the point of 

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

 peculiarity of the vertebrate anus we would naturally sup- 

 pose, from the disappearance 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 epi- 

 blast with the hypoblast. 



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

 order to show that the difficulty about the anus of Rusconi 

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

 die Entwickelung der Bombinator igneus," 'Archiv. fiir 

 Micro. Anat.,' vol. v, 1869) has also described the disappear- 

 ance of the hind portion of the alimentary canal in Batra- 

 chians, 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 de- 

 veloped 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 PI. XIV, fig, 10; XV, 11 

 and 12, x), a small roundish (in section) mass of cells is to 

 be seen in most of the sections. 



Its mode of development is shown 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 off 

 from tlie alimentary canal a small rod-like body close under 

 the notochord is formed. It persists till after the appearance 



