HYPOPHYSIS OF PETROMYZON AND AMIA 259 



Dohrn (1883), Gotte (1883), Haller (1896), von Kupffer (1893), 

 Scott (1888), Stendell (1914), Sterzi (1904), and Woerdemann 

 (1914). (To Sterzi's work I have unfortunately been unable to 

 gain access.) It is described as an invagination giving rise to 

 a cavity into which in the adult the olfactory organs open and 

 which extends posteriorly for a considerable distance between 

 the brain and the gut. 



In those cases where the hypophysis arises within the stomo- 

 daeum (Selachians and Aniniotes) it may develop either as 

 a hollow invagination (Rathke's pocket) or as a solid ingrowth 

 of a mass of cells within which the hypophysial cavity may make 

 its appearance later. Where it arises outside the stomodaeum 

 as in Amia (Eeighard and Mast 1908 ; the writer, see part ii 

 of this paper) or in Amphibia (Gotte 1875, Atwell 1919) 

 no invagination occurs, but the rudiment is a solid plate pushing 

 in from the ectoderm. Now the hypophysis of Petromyzon 

 certainly arises outside the stomodaeum, but it is situated 

 next to the region which enlarges enormously during develop- 

 ment, viz. the upper lip. It was my first object to decide 

 whether the so-called invagination is due to the growth of the 

 parts composing its walls or to actual invagination, or to both 

 causes. Pending decision on this point I shall call it the 

 hypophysis depression. 



The Hypophysis Depression. 



In the earliest stage at which any rudiment of the future 

 organ can be discerned (Text-fig. 1) a thickening of ectoderm 

 on the antero-ventral surface of the head marks the region 

 where the hypophysis and olfactory organs will develop. On 

 the supposed primitive nature of the connexion between these 

 organs I have little to say except that I see no reason for 

 regarding it as anything more than a topographical one, 

 two organs both developing from the ectoderm at the same time 

 and in the same restricted region, viz. the antero-ventral 

 surface of the head of necessity arising together. There is at 

 this stage (just before hatching) no trace of invagination. The 

 stomodaeum is indicated and is separated from the hypophysis 



