HYPOPHYSIS OF PETROMYZON AND AMIA 265 



those which develop by a soHd inpushing. A diagrammatic 

 representation of the growth of the upper lip and formation 

 of the hypophysis depression is given in Text-fig. 6. 



The Formation of the Hypophysial Cavity. 



The solid ingrowth lies between the dorsal wall of the gut 

 and the floor of the brain, separated from them by well-marked 

 membranes. In the stage represented in Text-fig. 7 no glandular 



Text-fio. 6. 



_..;5 



Petromyzon plane ri. Diagram showing the expansion of 

 the upper lip and formation of the hypophysis depression. 



differentiation has as yet occurred. The first indication of the 

 histological differentiation which will produce the future 

 pituitary body is to be found in the floor of the brain. Just 

 dorsal to the posterior portion of the hypophysis a thickening 

 of the brain-floor occurs composed of neuroglia (Text-fig. 8), 

 thus foreshadowing the pars nervosa. There is no trace of 

 hypophysial cavity. 



In a later stage (35 mm., Text-fig. 9, and imder higher magni- 

 fication in Text-fig. 10) the hypophysis is beginning to undergo 

 its histological differentiation (changes which will result in the 

 formation of the glandular elements which it contributes to the 

 pituitary body). Beneath the optic chiasma the strand of tissue 



T 2 



