XIV 



SENSE-ORGANS 



391 



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-pitvutary 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 



I pt.s. 1 

 ol.o. u.lp. lip. 



Fig. 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. Ir, Brain ; 

 in, infundibulum ; Up, lower lip ; ms, niesenteron ; 'w, notochord ; ol.o, olfactory 

 organ ; pn, pineal body ; pt.s, pituitary sac ; st, stomodaeum ; ilI^i, 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- 

 cranial 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 Myxine the pituitary involution 

 ends by opening into the pharynx. The apparently monorhinal 



