270 SCOTT. ' [Vol. I. 



common with paired hypoblastic diverticula from the alimentary- 

 canal provided with segmental nerves and arterial arches, with 

 cartilages and muscles. In the case of the aborted first pair of 

 gill-clefts in the lamprey, which I first identified, and which, as 

 Dohrn has shown, give rise to the ciliated grooves, all the acces- 

 sory structures of gill-clefts are present at one time or another 

 in the course of development. But it may be replied, that, 

 admitting the facts in the case of the Teleosts to be as Dohrn 

 believes them, and as Ryder (30) is also inclined to believe, 

 this group may have retained the primitive mode of formation 

 of the hypophysis, while other vertebrates have acquired a 

 secondary mode, just as the pineal eye best shows its primitive 

 character in the comparatively highly organized Lacertilia. 

 Now, the two cases are not at all parallel : the pineal eye has 

 been retained in one group, and simply degenerated in the 

 other — a degeneration which might easily occur independently 

 in many groups. With regard to the hypophysis, the question 

 may thus be stated: Is it probable that the primary mode of 

 development should be retained in one highly specialized type, 

 while an entirely new method should be independently assumed 

 by all other groups, higher and lower? Here there is no case of 

 simple degeneracy, but of a transformation occurring independ- 

 ently in widely separated groups, and following essentially the 

 same method in all. 



My own view of the relations of the pituitary body in Petro- 

 myzon has been explained in another place (35)» and need 

 only be referred to here. I believe the primary mode of devel- 

 opment to be that exhibited by Avibly stoma and Bombmator, 

 namely, a median epiblastic involution from the surface of the 

 head to the infundibulum, which has either no connection, or 

 a later or secondary connection, with either the mouth or the 

 olfactory organs. In the Selachians, a connection between the 

 stomodaeum and the pituitary rudiment is necessarily pro- 

 duced by the immense size of the fore-brain and the great ex- 

 tent of the cranial flexure. In Petromyzon, the coalescence of 

 the olfactory organs in the median line of course involves a 

 coincidence of the nasal with the pituitary involution, which 

 is thus seen to be secondary in the same sense as the connec- 

 tion with the stomodasum in the higher groups is secondary. 

 If, as Balfour has maintained, the origin of the involution from 



