264 



G. R. DE BEER 



of the upper lip. The relations of the apex to the optic chiasma 

 confirm this. 



The solid ingrowth of the hypophysis extending by itself 

 from the region of the optic chiasma to the notochord does so 

 not by the deepening of the hypophysis depression but by its 

 own lengthening. 



It is usually stated that the monorhinal condition of the 



Ui 



P e t r o m y z o n p 1 a n e r i . Embryo thirty-two days old. 



lamprey is due to the connexion of the hypophysis with the 

 olfactory organs which thus communicate with the exterior 

 by means of a single (hypophysial) aperture. It appears to me 

 to be easier to regard the agent as the upper lip in conjunction 

 with the sides of the anterior surface of the head, which by their 

 expansion cause both organs to be situated in the same 

 secondary depression. 



The hypophysis of Petromyzon then belongs to the class of 



