HYPOPHYSIS OF PETROMYZON AND AMIA 285 



The line of demarcation between it and the underlying endoderm 

 is not easy to see, but with an oil-immersion objective traces 

 of the original basement membrane of the endoderm can be 

 observed. More striking is the difference in yolk content, for 

 whereas the hypophysis (since such I beheve these ectoderm 

 cells to be) is practically devoid of it, the endoderm cells both 

 of the roof of the fore-gut and of the adhesive organ contain 



Text-fig. 26. 



Ado 



Amia calva. Young embryo (coiled round the yolk) showing 

 the rudiments of the adhesive organ and of the hypophysis. 



numerous large granules. At the same time the brain appears 

 to be pressing down on the hypophysis and squeezing it against 

 the endodermic roof of the fore-gut. 



In the next stage (Text-figs. 29 and 30) this pressure has 

 increased, for not only is the hypophysis less thick but it has 

 impressed its convex dorsal and ventral surfaces into the 

 corresponding concavities of the brain and gut -roof. The 

 floor of the brain which in previous stages was bent dorsally 

 in the region of the recessus opticus here continues flat for 

 a further distance forwards. It looks as if the brain and the 



