268 ZELENY. [VOL. II. 



2. Those who have claimed an epiblastic origin for the 

 hypophysis are Mihalkovics ('77), Aves and Mammalia ; Balfour 

 ('78), Elasmobranchii ; Orr ('87), Lacerta ; Lundsborg ('94), 

 Salmonidae ; Dean ('96), Amia ; Haller ('96), Vertebrata in 

 general ; Hoffmann ('96), Elasmobranchii ; Melchers ('99), 

 Lacertilia, Of these Orr ('87), although he describes the 

 hypophysis as of epiblastic origin and so figures it in his 

 sections, nevertheless considers it probable that hypoblast 

 cells may take some part in its development. 



3. Those who have claimed that the hypophysis is partly of 

 epiblastic and partly of hypoblastic origin are Kupffer ('93), 

 Acipenser and Ammocoetes ; Valenti ('95), Amphibia (Bufo) ; 

 Nussbaum ('96), Mammalia ; and considered probable by Orr 

 ('87), Lacerta. 



It is of special interest to note that even this partial list 

 gives each of the three views a large number of the groups 

 of vertebrates upon which to base its general character. 



Material and Methods. 



The material upon which the following observations are 

 based was obtained in Minnesota during the summers of 1898 

 and 1899. The embryos of Aspidonectes are from Grey 

 Cloud Island in the Mississippi River below St. Paul, and 

 those of Chrysemys and Chelydra are from the neighborhood 

 of Hutchinson. The fixing fluid used was Gilson's mercuro- 

 nitric mixture. The embryos were stained in toto in haema- 

 calcium or in paracarmine. The series of sagittal sections 

 were found to be the most helpful in determining the cell- 

 layer from which the hypophysis is derived, and the following 

 descriptions are taken entirely from such sections. 



Description of Stages. 



The following stages will be figured and described : 

 Stage A. The hypophysial evagination has not yet 



appeared. 



Stage B. The hypophysial evagination is very evident and 



the pharyngeal membrane has not yet been broken. 



