290 G. R. DE BEER 



The hypophysial cavity is now distinct with the cells arranged 

 radially around it. There is as yet no distinction betweeli the 

 cells on opposite sides of the cavity, the differentiation into 

 pars anterior and pars intermedia has not yet appeared. On 

 the side of the brain there is no pars nervosa, though the 

 infundibular cavity and the recessus saccularis are already 

 very distinct. 



Summary. 



1. The hypophysis of Petromyzon arises as a solid ingrowth. 



2. The depression in which the hypophysis and olfactory 

 organs come to lie is formed by the great expansion of the upper 

 lip in conjunction with the sides of the anterior surface of 

 the head. 



3. The beginning of the histological differentiation of the 

 glandular elements takes place from a solid strand before the 

 appearance of any cavity. 



4. The hypophysial cavity arises late as a split in the thickness 

 of the hypophysis, and afterwards extends in both directions 

 communicating forwards with the external depression. 



5. The homologue of the pars tuberalis is probably to be 

 found in the ' Uebergangsteil ' of Stendell, a chromophobe 

 portion situated between the pars anterior and pars intermedia. 



6. The hypophysis of Amia is derived from the ectoderm, 

 thus agreeing wdth all other known forms. 



7. It arises outside the stomodaeum on the anterior surface 

 of the head. 



8. It is a solid ingrowth which loses all connexion with its 

 point of origin, and within which the hypophysial cavity makes 

 its appearance at a later stage. 



9. Outside and in front of the stomodaeum is probably the 

 primitive position of origin of the hypophysis, without con- 

 nexion with either mouth or nose. 



10. In Selachians and Amniotes where the fore-brain is early 

 very large and the cranial flexure pronounced, the hypophysis 

 arises further posteriorly and so is included in the hollow of the 

 stomodaeum. 



