HYPOPHYSIS CEREBRI 313 



this medullary layer from the posterior lobe. Silvestrini also 

 locates the active substance in the medullary layer ; Salvioli and 

 Carraro, on the contrary, believe that it is contained in the pars 

 nervosa. 



It should be mentioned that Livon held the view that the 

 internal secretion of the hypophysis passes into the brain by way 

 of the nerve fibres in the peduncle. 



HISTORY OF DEVELOPMENT. 



Since the. earliest investigations by Rathke in 1838, the 

 development of the hypophysis has formed one of the most 

 widely discussed problems in the department of embryology. 

 Even to-day, the views held concerning it are very divergent. 



The subject of the earliest discussion, in which, in addition 

 to Rathke, Reichert, Luschka, Kolliker, Fr. Miiller, His, Maclay, 

 and Duvry were concerned, was the supposed relationship between 

 the hypophysis and the epithelium of the buccal cavity, Rathke's 

 pouch more particularly. Mikalkovics's experiments with mam- 

 mals, Goette's experiments with amphibia, and Dohrn's experi- 

 ments with ammoccetes, appear to show that the hypophysis is 

 exclusively ectodermal in origin. This view was shared by the 

 vast majority of embryologists and finds expression in the text- 

 books of the subject. 



The development of the hypophysis is described by Hert\vig 

 as follows : In the embryo of a rabbit, 6 mm. in length, at a 

 developmental stage when the cephalic flexure and the stomadseum 

 have already made their appearance, the site of the future hypo- 

 physis is situated anteriorly and dorsally of the spot where the 

 anterior end of the chorda dorsalis immediately approaches the 

 insertion of the pharyngeal membrane. After the perforation of 

 the pharyngeal membrane (in the chick on the fourth day, in 

 the human embryo in the fourth month), a groove is formed, 

 shallow at first, but gradually becoming deeper, which is known 

 as Rathke's pouch or the hypophysis pouch. This groove, which 

 develops in the direction of the funnel-shaped, depending base 

 of the thalamencephalon, becomes clothed with several layers of 

 cylindrical cells, and eventually forms a longish pouch (embryo 

 rabbit, 12 mm.). 



At a later stage, this pouch is almost completely divided from 

 the primitive mouth cavity by the development of the carti- 

 laginous base of the skull, a narrow opening only being left. 

 At the same time, the posterior wall of the pouch has reached the 

 infundibulum and has been pushed backwards by it into the 

 cavity of the pouch (embryo rabbit, 20 mm.). A little later, the 

 lumen becomes obliterated and the pouch forms a solid cell 

 column. In Selachians, the pouch persists throughout life as a 

 hollow canal, one end of which penetrates the cartilaginous base 



