No. 2.] DEVELOPMENT OF PETROMYZOISr. 267 



sents the invertebrate mouth and gullet is now entirely aban- 

 doned, especially since Spencer has conclusively shown the 

 nature of the pineal gland. Gotte, as we have already seen, 

 believes that the hypophysis of the higher vertebrates is the 

 representative of the nasal canal (Nasenrachengang) of the 

 Cyclostomata, a view which is mainly supported by the fact 

 that in Bombinator a connection between the pituitary involu- 

 tion and the nasal pits occurs. But, as appears from Gotte's 

 figures and descriptions, this connection is only by slight 

 superficial folds which also connect the hypophysis with 

 the mouth. In other Amphibia there is no such connection ; 

 in the frog and newt, according to Miss Johnson and Miss 

 Sheldon (19), the stomodaeum arises as a solid ingrowth of the 

 deeper layer of the epiblast, of which the posterior portion 

 fuses with the archenteron, while the free anterior portion 

 gives rise to the pituitary body. Dr. Orr, who is working in 

 this laboratory upon the development of Ambly stoma, kindly 

 allows me to make use of the results which he has obtained. 

 Here a solid epiblastic cord grows in from the front of the head 

 and reaches the infundibulum before the formation of the 

 mouth, and there is no connection at any time of the pituitary 

 involution with either the olfactory pits or the mouth. There 

 is no good reason for considering the nasal canal of the Cyclo- 

 stomata as essentially different from the canals of other 

 vertebrates. In the Cyclostomata, it is true, this canal is 

 median and unpaired; but, as we shall see later, the single 

 olfactory organ of this group has almost certainly been pro- 

 duced by a coalescence of paired organs, and, if this is true, 

 the canals would also coalesce in the same manner. Otherwise 

 the only peculiarity in the nasal canal of the lamprey is in its 

 great size ; this implies nothing new, but rather the exten- 

 sion of parts already present. This extension, as J. Miiller 

 showed long ago, has been conditioned by the unusual situation 

 of the nasal opening. There is no more reason for regarding 

 the entire canal of the Cyclostomata as belonging to the 

 hypophysis than in the case of the higher vertebrates for re- 

 garding the entire stomodaeum as belonging to that body. 



Dohrn (12) has independently reached Gotte's conclusion 

 as to the nature of the canal in Pctromyzon, and further regards 

 this canal as produced by the coalescence in the median line 



