The 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 outgrowth from the latter, and the two have a common external opening, the naso-pituitary aperture (Fig. 224). Later the extraordinary 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 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-cranial fontanelle, it widens out into a spacious cul-de-sac and terminates on the dorsal side of the pharynx, beneath the anterior end of the notochord. In _Myxine_ the pituitary involution ends by opening into the pharynx. {392}[Illustration: FIG. 225.—Side view of the brain of _Petromyzon_, with the olfactory organ and the pituitary caecum in section. _cblm_, Cerebellum; _crb.h_, cerebral hemisphere; _dien_, thalamencephalon; _f_, fold in the nasal tube; _gl_, nasal glands; _inf_, infundibulum; _l.gn.hb_, left ganglion habenulae; _med.obl_, medulla oblongata; _na.ap_, naso-pituitary aperture; _n.ch_, notochord; _Nv^1-nv^{10}_, cranial nerves; _Nv^{12}_, first ventral spinal nerve; _olf.cp_, olfactory capsule; _olf.l_, olfactory lobe; _olf.m.m_, olfactory mucous membrane; _opt.l_, optic lobe; _pn_, pineal body; _pn′_, inferior pineal body; _pn.e_, parietal eye; _pty.b_, pituitary body; _pty.p_, pituitary cul-de-sac; _sp_, median septum of the olfactory sac; _sp^1_, first dorsal spinal nerve. (From Parker and Haswell, after Ahlborn and Kaenische.)]