HYPOPHYSIS OF PETROMYZON AND AMIA 279 



upper lip. In the hypophysis of Gnathostomes he divides the 

 hypophysial cavity into three parts ; the most posterior he 

 terms Rathke's pocket and separated from it by a constriction 

 he distinguishes the remainder of the cavity as ' Mittelraum ' 

 and ' Vorraum '. Applying these homologies to Petromyzon 

 he gets the external hypophysis depression to correspond with 

 the ' Vorraum ', the glandular portion of the hypophysis with 

 Rathke's pocket, and the upper lip with that region situated 

 just behind Rathke's pocket in Gnathostomes. The consequence 

 of this is that the olfactory organs and mouths of Cyclostomes 

 and Gnathostomes are not homologous. Such conclusions are 

 unacceptable. The recognition of the secondary nature of 

 monorhiny, the relations of the olfactory organs to the olfactory 

 nerves and brain and those of the trigeminal and facial nerves 

 to the mouth are against his view. Besides, it does not appear 

 that the acceptance of his divisions of the hypophysial cavity 

 facilitates their interpretation. 



We are left then with the view that the hypophysial cavity 

 of Petromyzon is homologous with that of other forms but 

 differs from them by secondary modifications. 



The Primitive Position of the Hypophysis. 



The most striking feature about the hypophysis of Petro- 

 myzon at first sight is the fact that it has nothing to do with 

 the mouth, but communicates with the exterior on the dorsal 

 side of the head. The latter feature is, as previously stated, 

 due to the great expansion of the upper lip with the front of 

 the head, so that when a stage before the upper lip has developed 

 (Text-fig. 2) is considered, the hypophysis faces ventrally. 

 But even then it has no connexion with the stomodaeum. The 

 limits of the stomodaeum are not easy to define since they are 

 not marked by any structural peculiarity ; but they may be 

 taken as being the points where the concave curve of the 

 invagination changes and becomes convex. The fact that in 

 the majority of vertebrates, viz. Selachians and Amniotes, 

 the hypophysis develops from within the stomodaeum has 



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