HYPOPHYSIS OF PETROMYZON AND AMIA 275 



The connexion with the exterior may be regarded as a delayed 

 appearance of Rathke's pocket. The adaptive nature of this 

 connexion in Myxine where the hypophysial cavity also com- 

 municates with the gut suggests that the hypophysial cavity is 

 secondarily modified, as does also the size of the hypophysial 

 cavity in these forms. The fact that it has lost all connexions 

 with the glandular portions also lends support to this view. 

 For whatever the primitive function of the hypophysis may have 

 been, it was an ectodermal organ sunk beneath the skin and the 

 hypophysial cavity must be supposed to have served to keep 

 the organ in communication with the exterior. In vertebrates 

 above Cyclostomes where a hypophysial cavity exists it is in 

 association with the glandular portions of the organ, though 

 functionless since the latter have adopted the method of internal 

 secretion. In Cyclostomes the cavity is separated from the 

 glands in the adult by a good thickness of connective tissue, 

 although in earlier stages it was in contact with them and they 

 even formed its dorsal wall. It is this fact which enables one to 

 beheve that the hypophysial cavity of Petromyzon is homo- 

 logous with that of other forms, a homology which would be 

 difficult to establish from the adult. But even here Petro- 

 myzon is peculiar. In vertebrates typically the hypophysial 

 cavity separates the pars anterior from the pars intermedia. 

 In Petromyzon, as we have seen, the glands differentiate before 

 the appearance of a cavity and the pars anterior and pars 

 intermedia lie in a straight line with the cavity (when it does 

 form) beneath them. Starting from this condition, i.e. the 

 hypophj^sis arising from the front and growing backwards, and 

 the hypophysial cavity horizontal with the glands on the brain 

 side (dorsal), the pars nervosa of the pituitary very slightly 

 developed and not projecting ventrally ; the conditions obtain- 

 ing in higher vertebrates can be derived when the following 

 changes are taken into account. 



(i) The hypophysis grows up from beneath and meets the 

 brain at right angles. The consequence of this is that that 

 portion of the roof of the primitive hypophysial cavity in 

 connexion with the pars anterior which was dorsal and 



