288 SCOTT. [Vol. L 



thickening imperfectly divides the cavity into lateral chambers 

 (Fig. II, PI. IX, and Fig. 35. PI- X). In larvae of 43 mm. 

 in length this median thickening of the epithelium cells has 

 become very much thinner than that which lines the chambers, 

 though its character as a septum is now very much more clearly 

 indicated. This septum is for the most part made up of a 

 mass of connective tissue covered by a thin layer of columnar 

 epithelium, like that which lines the anterior non-olfactory 

 parts of the nasal chambers. The two chambers are now 

 deeper and more distinct; the sensory epithelium is thickened 

 and is confined to the posterior and external walls of the 

 chambers. 



Shortly after the shifting of the nasal opening from the 

 ventral to the dorsal surface of the head, a ridge-like elevation 

 of the skin, including both dermis and epidermis, is formed 

 around the opening. This ridge rapidly grows in height and 

 forms a freely-projecting funnel, at the bottom of which lies 

 the nasal pore. The funnel is a conspicuous object in sections 

 of advanced larvae through this region of the head. The nasal 

 organ proper, then, of the Ammoccete may be described as a 

 pair of imperfectly separated epithelial chambers, continued 

 into a short and narrow canal which ends blindly, though 

 probably still connected with the hypophysis. Langerhans' 

 figure of a section through the olfactory organ of the Ammo- 

 ccete represents a stage more advanced than any that I have 

 been able to find in a larva; it was probably taken from a 

 specimen just at the beginning of the metamorphosis into the 

 adult animal. At no time is there any indication of "smell 

 buds," such as Blaue (9) has described in the Teleosts and 

 Amphibia, but, as in the Selachians, the sensory cells extend 

 throughout the olfactory organ proper (which does not include 

 the anterior wall of the chambers), and seem to have entirely 

 displaced the intervening indifferent epiblast cells. 



In the adult lamprey the olfactory organ undergoes a great 

 increase in size and complexity ; radiating septa covered with 

 columnar ciliated epithelium (Fig. 22, PI. X) extend from the 

 circumference towards the centre, each septum being supplied 

 with fibres from the olfactory ganglia. At the same time the 

 naso-palatal canal, which throughout larval life remained ex- 

 ceedingly small and almost rudimentary, now becomes greatly 



