No. 2.] DEVELOPMENT OF PETROMYZOAT. 265 



dreitheilige vorderen Sinnesplatte (Anlage des Hirnanhangs 

 und der Geruchsplatten) mit dem unpaaren Nasenrachengang 

 nicht ohne weiteres von der Hand zu weisen ware." 



The earliest stage that I have observed in the development 

 of the pituitary body is in an embryo of seventeen days (Hy. 

 Fig. I, PI. VIII), where a slight ingrowth of epiblastic cells is 

 seen immediately in advance of the upper lip, the nasal epi- 

 thelium not having as yet become differentiated from the gen- 

 eral epiblast. Dohrn's Fig. i represents a stage which I have 

 not succeeded in finding, but I altogether agree with his state- 

 ment that the hypophysis is formed quite independently of 

 the olfactory epithelium. In the following stage (18 days, 

 Fig. 2), a considerable change is observable; the olfactory 

 epithelium is now well developed and passes posteriorly into 

 a canal, which is a common involution for the nasal canal and 

 the pituitary body. This involution is now much deeper than 

 in the former stage, it having lengthened pari passu with the 

 growth of the fore-brain ; but of still greater moment in pro- 

 ducing this effect is the lengthening of the upper lip, already 

 described, v/hich deepens the canal by extending its inferior 

 wall. The end of the involution has in this stage reached the 

 infundibulum, with which it is in close contact. In larvae 

 which have just come out of the ^^g (4.8 mm.) the involution 

 is very much longer (Fig. 4), its lower end still remaining in 

 contact with the infundibulum. This increase in length has 

 been entirely conditioned by the growth of the upper lip and 

 its commencing rotation, together with the correction of the 

 cranial flexure, which now begins to be apparent. As can be 

 seen in Fig. 4, these changes in the lip have very much con- 

 tracted the entrance to the nasal pit (compare Fig. 2). The 

 prolongation of cells to the infundibulum is shown in trans- 

 verse sections to be a canal with a very small lumen and 

 thicker dorsal than ventral walls. The involution is of nearly 

 uniform size throughout, though gradually tapering as we pass 

 backwards. In later stages the further increase in size of the 

 lip, its rotation so as to point directly forwards, together with 

 the growth of the hemispheres and olfactory lobes, produce a 

 corresponding increase in the length of the canal, and the 

 formation of the hypophysis proper begins by a transformation 

 of the mass of cells which underlie the infundibulum. In this 



