HYPOPHYSIS OF PETROMYZON AND AMIA 281 



of the hypophysis by Goodrich's (1917) suggestion that the 

 hypophysis is represented in Amphioxus by the deep groove 

 and depression known as the pre-oral pit in the larva and wheel 

 organ in the adult. This, as its name implies, is anterior to the 

 mouth. 



There remains the question as to whether an invagination 

 (Rathke's pocket) or a solid ingrowth is the more primitive 

 method of formation of the hypophysis. It is dangerous in 

 a point of this kind to attempt to induce phylogeny from 

 ontogeny, for the mode of development obeys embryonic 

 conditions. I should suggest that the hypophysis of the 

 primitive vertebrate was an invagination as is the pre-oral pit 

 of Amphioxus, but that wdien the combination with the 

 infundibulum forming the pituitary body w^as evolved, the 

 mode of development became influenced by the distance which 

 the ectodermal tissue has to travel. 



So in Petromyzon or Amia or Amphibia the ingrowth is 

 solid, in Selachians and Amniotes it tends to be hollow. 



The Pars Tuberalis. 



To the description of the main glandular elements of the 

 pituitary body of Petromyzon I have little to add. The pars 

 anterior is made up of two portions, an anterior lobe composed 

 of chromophil cells, and a posterior of chromophobe. This 

 latter portion is termed by Stendell (1914a) the ' Uebergangs- 

 teil ', by Gentes ' the middle lobe '. Stendell (1913) regards it 

 as morphologically part of the pars anterior (' Hauptlappen ') ; 

 Sterzi as part of the pars intermedia. It is seen in the ammo- 

 coete of Petromyzon fluviatilis in longitudinal section 

 in Text-fig. 17, where it occupies the region between the pars 

 anterior and the pars intermedia. The outer sides of this 

 ' Uebergangsteil ' lap back round the pars intermedia on each 

 side, as seen in transverse section in Text-lig. 15 and in hori- 

 zontal in Text-fig. 24. Text-fig. 25 is a reconstructed view 

 of the whole organ from the ventral surface. This fact was 

 also observed by Woerdeman (1914). 



Recently attention has been paid to the pars tuberalis as 



u 2 



