258 G. R. DE BEER 



research that there are a few points which deserve further 

 attention. 



The main outhnes of the structure and development of the 

 hypophysis of Petromyzon have been known since the work of 

 Dohrn, Gotte, and Scott, and the problems of the hypophysis 

 then presented themselves, but they have been answered in 

 many contradictory ways by them and the many other 

 observers who have devoted their attention to them. Shortly, 

 the main problems are the following : 



(1) What is the primitive position of the hypophysis, and has 



it any primitive relations Avith the mouth or the nose 

 or any other organs ? 



(2) What is the primitive method of its development given 



the great diversity which exists in the vertebrates, and 

 especially the great difference presented by Cyclo- 

 stomes as compared with almost all other vertebrates ? 



In this paper I am concerned with these two questions and 

 am led to discuss a few more, such as the relations of the hypo- 

 physial cavity to the glandular portions of the pituitary body 

 and the possible presence of one of these portions in Petromyzon 

 corresponding to the pars tuberalis of higher vertebrates. 



I wish to express my thanks to Mr. Huxlej- and Dr. Hogben, 

 not only for stimulating my curiosity in some of these questions 

 but also for useful criticism. To Professor Goodrich also my 

 gratitude is due for helpful advice. 



Methods call for no special comment ; as complete a series 

 of embryos as possible was studied, cut into serial sections in 

 the transverse, sagittal, and horizontal planes. Stains such 

 as picronigrosin and Licbtgriin were useful in detecting 

 boundaries and limiting membranes where such were obscure. 

 The material of Petromyzon p 1 a n e r i was collected during 

 a stay at Naples, and some stages were supplemented by the 

 kindness of Professor Goodrich. The work was done in the 

 Department of Zoology and Comparative Anatomy at Oxford. 



The origin of the hypophysis in Petromyzon from the anterior 

 surface of the head and close to the rudiment of the olfactory 

 organ is familiar in every text-book, after the researches of 



