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 suppose, 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 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 Götte ("Untersuchung über die Entwicklung der Bombinator igneus," _Archiv. für 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 notochord, and in contact with it (vide Pl. 3, fig. 10; Pl. 4, figs. 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 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 off from the alimentary canal a small rod-like body close under the notochord 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. It was first seen by Götte (_loc. cit._) in the Batrachians, and he gave a correct account of its development, and added that it became the thoracic duct. I have not myself worked out the later stages in the development of this body with sufficient care to be in a position to judge of the correctness of Götte's statements as to its final fate. If it is true that it becomes the thoracic duct it is very remarkable, and ought to throw some light upon the homologies of the lymphatic system.