266 SCOTT. \yoh. I. 



region the lumen of the canal disappears and the solid, pro- 

 liferating mass of cells broadens into a lens-shaped body, 

 which embraces the free end of the infundibulum, the upper 

 surface being concave and the lower convex. The portion of 

 the canal immediately in front of the pituitary rudiment thins 

 out, and apparently develops fibres which still retain a con- 

 nection with the posterior end of the nasal canal proper. This 

 connection persists for a considerable period, and, so far as I 

 have been able to observe, it does not appear to be severed at 

 all during larval life. In the larva of 53 mm. in length (Fig. 

 17, PI. IX), the hypophysis consists of a broad mass of closely 

 crowded epithelium cells, which, as yet, show no division into 

 follicles. It is no longer in contact with the roof of the mouth, 

 as was the case in the earlier stages (Fig. 4, PI. VIII), but sep- 

 arated from it by a thick layer of connective tissue developed 

 from the mesoblast. At a later stage this connective tissue 

 apparently sends trabeculse into the substance of the pituitary 

 body and break it up into a series of solid follicles, as has been 

 described by W. Miiller for the adult (25), At the time of 

 metamorphosis, as is well known, the nasal canal becomes 

 greatly enlarged, both in length and breadth, extending down- 

 wards and backwards between the brain and the pharynx. 

 The hypophysis (Fig. 21, PI. IX) now lies imbedded in the 

 upper wall of the canal, and in contact with the infundibulum. 



The morphological signification of the pituitary body is a 

 question of great difficulty, as to which the most diverse opin- 

 ions have been held. The study of its development in Petro- 

 inyzon would seem to bring us at least one step nearer to the 

 solution of the problem. In most vertebrates this body is de- 

 rived from a diverticulum of the stomodaeum, agreeing with 

 the process in Pctromyzon only in so far as it is of epiblastic 

 origin. The mode of formation in the lamprey is apparently 

 so divergent from that seen in the higher vertebrates, that they 

 seem to have very little in common, and on that ground 

 Balfour declined to accept my account. However, when exam- 

 ined in the light of comparative embryology, much of this 

 apparent divergence disappears, and with the possible excep- 

 tion of the Teleosts, the mode of formation is seen to be funda- 

 mentally the same throughout the vertebrate series. 



The old hypothesis that the conario-hypophysial tract repre- 



