HYPOPHYSIS OF PETROMYZON AND AMIA 



261 



rudiment of the hypophysis. The region of ectoderm between 

 it and the stomodaeum is no longer flat but convex, being the 

 beginning of the expansion of the upper lip. Is the dent the 

 beginning of a true invagination or is it due to the expansion 

 of the upper Hp ? The latter I believe to be the true interpreta- 

 tion. It bears no resemblance to the normal appearance of 

 Eathke's pocket or any invagination, as may be seen by com- 



Text-fig. 2. 



Petromyzon planeri. Embryo just hatched, beghining 

 of hypophysis depression. 



paring it with the stomodaeum or the olfactory organ. An 

 invagination presents one concave curve ; the hypophysis 

 depression is made up of two convex curves meeting at an apex. 

 There is a solid inpushing of cells at this stage although the 

 depression is a mere dimple, whereas in those cases where 

 the hypophysis arises by invagination it is sunk beneath the 

 surface by means of that invagination which must be almost 

 as deep (see Text-fig. 23 a). 



The upper lip continues to increase in size, and the hypo- 

 physis depression comes to assume the form of a cleft which 

 deepens and the upper and lower sides of which become more 

 and more parallel as the upper lip enlarges. At the same time 

 the sides of the upper lip grow up with the sides of the anterior 



