280 G, R. DE BEER 



led to the view that this is its primitive position (Stendell 

 1914a). 



Eecently, however, attention has been drawn to those cases 

 where the hypophysis arises outside the stomodaeum and just 

 anterior to it. In this connexion the works of Gotte (1915) 

 and Atwell (1919) for Amphibia, of Eeighard and Mast (1908) 

 and the writer for Amia, of Wells (unpublished) for Clupeus 

 may be mentioned. In these cases the hypophysis has no 

 connexion with either mouth or nose, and this position Scott 

 (1883) believes to be primitive. Where the hypophysis arises 

 within the stomodaeum, i.e. in Selachians and Amniotes, 

 there is a great development of the fore-brain and cranial 

 flexure ; this rotation of the anterior part of the head causes 

 the ventral elements of the head to lie relatively farther back 

 and accentuates the stomodaeal invagination. This I believe 

 to be the cause of the hypophysis being situated in the stomo- 

 daeum in these forms, but the difference between the two types 

 is more apparent than real. In addition there is the fact that 

 the hypophysis is ectodermal tissue which must get into contact 

 with the infundibulum. Where the fore-brain is large and the 

 cranial flexure pronounced this can most conveniently be done 

 through the stomodaeum. But primitively the fore-brain 

 cannot have been large and there was less cranial flexure. This 

 condition is preserved in the Teleosts and Amphibia, and here 

 the typical position for the hypophysis to arise is outside and 

 dorsal to the stomodaeum. Text-fig. 23 is a diagrammatic 

 comparison between Squalus, Amia, and Petromyzon showing 

 the modifications brought about by the fore-brain and the 

 cranial flexure. 



Not only are the ventral elements of the head of Squalus 

 pushed backwards, but the dorsal elements are pulled forward 

 owing to the outer side of the curvature of the cranial flexure 

 being dorsal. So the dorsal nerve-roots lead backwards to their 

 respective gill arches whereas in Amia they incline forwards. 

 The anterior position of the heart in Amia is partly due to the 

 embryo being flattened out on the yolk. 



Support is lent to the view that this is the primitive position 



