16 Scientific Proceedings, Royal Dublin Society. 



evaginations of the gill and mouth-clefts. It arises in form of a 

 pair of more or less distinct endodermal evaginations, considerably 

 in front of those forming the mouth. 



Yarious suppositions have been advanced by zoologists as to 

 what the hypophysis represented. The primary idea was to 

 connect it in some way with the ciliated pit in tunicates, and it 

 was for some time looked upon as a sensory organ. Another sup- 

 position was that it was a glandular structure. Dohrn is opposed 

 to either of these views. He regards the hypophysis as a pair of 

 praeoral gill-clefts, arrested in their development in such a manner 

 as not to reach the surface of the skin. 



In concluding his second Paper Dohrn makes reference to 

 Hatschek's results on the development of Amphioxus. According 

 to that author, the hypophysis owes its origin to two endodermal 

 evaginations lying in front of the mouth. One of these is con- 

 stricted off completely from the endoderm, and breaking through 

 the ectoderm forms a ciliated pouch, which to all appearances 

 might be supposed to be ectodermal in its origin. The second 

 evagination remains for a longer time connected with the endoderm, 

 but its ultimate fate had then not been made known. Dohrn 

 suggests that these two evaginations might be homologous with 

 the ciliated pit of tunicates on the one hand, and the hypophysis 

 of vertebrates on the other. 



III. — The Hypophysis in Petromyzon. 1 



One of the great features in the development of Petromyzon is 

 the presence of a stomodaeum, as has already a been pointed out by 

 Scott ; but Dohrn leaves the consideration of this structure for a 

 later study. In front of the stomodaeum (mo.), and close to it, we 

 find another invagination (hy.), which is preceded by a third (na.). 

 The latter is the nasal organ, between which and the mouth lies 

 the second, the rudimentary hypophysial invagination. (See fig. i.) 



However, instead of being drawn within the mouth, as in other 

 vertebrates, the powerful development of the upper lip (ul.) not only 

 causes the hypophysis to be removed close to the nasal organ, but 

 the latter itself is pushed along until it reaches the dorsal surface. 



1 Ibid., vol. iv. 1883, pp. 172-189. 



