xIV } SENSE-ORGANS 3901 
Cyclostomata are unique amongst Craniates in the apparently 
unpaired condition of the olfactory organ, and in its remarkable 
relation to the pituitary involution. In the embryo Lamprey 
the median and ventral olfactory pit is carried inwards with the 
pituitary invagination, so that the former appears as a dorsal out- 
growth from the latter, and the two have a common external 
opening, the naso-pituitary aperture (Fig. 224). Later the ex- 
traordinary forward growth of the upper lip to form the roof 
of the buccal funnel has the effect of shifting the naso-pituitary 
involution and its aperture to a final position on the dorsal side 
of the head. It is due to this dorsal displacement that, as we 
Fic. 224.—Two stages in the development of the olfactory organ and the pituitary 
involution in Petromyzon. A is the earlier, B a much later stage. 67, Brain ; 
in, infundibulum ; /./p, lower lip ; ms, mesenteron ; 7, notochord ; o/.0, olfactory 
organ ; pn, pineal body ; pt.s, pituitary sac ; st, stomodaeum ; w./p, upper lip. 
(From Parker and Haswell, after Dohrn.) 
shall see, the pituitary caecum reaches the ventral surface of the 
brain by perforating the basis cranii from above, instead of from 
below as in all other Craniates. The pituitary body is pinched 
off from the dorsal side of the naso-pituitary involution. In 
the adult. Lamprey the olfactory organ appears as a round sac 
divided by a median septum into two lateral chambers (Fig. 
225), the lining epithelium of which is raised into prominent 
ridges. Behind the sac the pituitary involution is prolonged 
backwards beneath the brain, and, after traversing the basi- 
eranial fontanelle, it widens out into a spacious cul-de-sac and 
terminates on the dorsal side of the pharynx, beneath the an- 
terior end of the notochord. In Mywxine the pituitary involution 
ends by opening into the pharynx. The apparently monorhinal 
