No. 2.] DEVELOPMENT OF PETROMYZON: 269 



weiter gegen den Grund der sogenannten Mundbucht ge- 

 schoben, wo sie dann auch allmahlich, wie so viele andere 

 ursprlingliche Einstlilpungcn der Haut oder Ausstiilpungen des 

 Darmes abgeschniirt ward, urn durch Wachsthums-Verschieden- 

 heiten ganzlich dislocirt zu werden und in Bcziehungen zu treten 

 die ihr urspriinglich fremd vvaren, — bei der anderen Gruppe 

 aber wurde sie mitsammt der Nase auf den Riicken liber und 

 vor das Gehirn geschoben, biisste aber nicht ihre selbstandige 

 Mundung ein." 



The chief fact upon which Dohrn relies in support of this 

 hypothesis is derived from his observations on the Teleosts (11), 

 that the hypophysis is formed from a pair of hypoblastic 

 diverticula toward the infundibulum. But, aside from the very 

 great obscurity in the process and uncertainty as to the origin 

 of the cells in question, which Dohrn very candidly admits, the 

 Teleosts form such an exceedingly specialized group, and their 

 developmental history, as at present interpreted, deviates in so 

 many important respects from that of other vertebrates, both 

 higher and lower, that morphological generalizations should be 

 made from their structure only with the greatest caution. In the 

 present case the facts as stated by Dohrn are opposed by the 

 process observed in other groups, where the hypophysis is of 

 unmistakably epiblastic origin. Even the paired condition 

 of the pituitary rudiment in the Teleosts, upon which Dohrn 

 lays such stress, is by no means clear from his figures. It 

 would more accurately be described as a continuous mass of 

 cells somewhat thinned in the median line on account of the 

 close approximation of the infundibulum to the fore-gut. 



In Pctramyzon Dohrn agrees with Gotte in regarding the en- 

 tire nasal canal as belonging to the hypophysis, though without 

 any cogent reason for such a view. I can see no better ground 

 for accepting this hypothesis than in the case of the higher 

 vertebrates for regarding the entire stomodaeum as a part of the 

 hypophysis. One assumption is quite as reasonable as the other. 

 Dohrn's view that the hypophysis represents a pair of coalesced 

 gill-clefts, I regard as altogether untenable, if the facts of em- 

 bryology are to be allowed any weight. A median epiblastic 

 involution from the surface of the head, with no special nerve 

 supply, no remnants of skeletal or muscular structures, and no 

 arterial arch at any stage of its development, has very little in 



