No. 2.] DEVELOPMENT OF PETROMVZON: 287 



2. The Olfactory Organ. — The nasal epithelium appears 

 very early, in embryos of 17-18 days, and is formed by the 

 increase in height of the epiblastic cells situated along the 

 median line of the ventral surface of the head. The sensory 

 epithelium speedily divides into two imperfectly distinguish- 

 able layers of spindle-shaped cells (Fig. 2, PI. VIII), the 

 superficial one being ciliated, as Shipley has shown. At this 

 stage the olfactory organ forms a thick mass, which projects 

 inwards beyond the level of the skin and lies in close contact 

 with the fore-brain ; posteriorly it is continuous with the pit 

 of columnar epithelium, which will later give rise to the pitui- 

 tary body. The organ is from the first single and median in 

 position, and even the median fold, which eventually indicates a 

 division into lateral halves, does not appear until a much later 

 stage. The next alteration of importance is the shifting of the 

 opening of the nasal pit from the ventral to the dorsal surface 

 of the head. This, as already indicated, is brought about 

 partly by the rotation of the head, which accompanies the cor- 

 rection of the cranial flexure, so that the nasal epithelium pre- 

 sents forwards instead of downwards, as before (Fig. 3, PI. VIII ; 

 Fig. 12, PI. IX). As far as the position of the olfactory organ 

 itself in relation to the nervous axis is concerned, it does not 

 materially change from this time onwards ; but this process of 

 itself would cause the organ to remain at the anterior end of 

 the head, as in Fig. 3. The great extension and rotation of the 

 upper lip changes the position of the nasal opening very essen- 

 tially; by advancing the anterior wall of the pit, it restricts the 

 opening to a small pore, and brings the pore to the top of the 

 head, a considerable distance behind the front end of the lip. 

 It is thus evident that the very peculiar and exceptional situa- 

 tion of the olfactory organ in the Cyclostomata has reference 

 rather to the position of the opening than to that of the 

 olfactory epithelium, except in so far as the latter is single and 

 median. It is likewise clear that the opening of the nasal 

 canal upon the dorsal surface of the head is a necessary result 

 of the mode of development of the upper lip and the forma- 

 tion of the suctorial disc. 



Throughout larval life the olfactory organ remains very 

 simple and unfinished in character. The first essential change 

 appears in larvae of about 10 mm. in length, where a median 



