Igo COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
cerned differ considerably in structure and relations. The olfactory 
epithelium is always restricted to one or two patches at the anterior end 
of the head and differs from the taste buds in histological structure. 
Both sensory and supporting cells of the olfactory organs are variously 
constituted. The supporting cells are the stouter, some being ciliated, 
some muciparous at their free ends. The sense cells (fig. 176, C) are 
thread-like or rod-like, being greatly expanded around the spherical 
nucleus, while the basal end of each contracts to a nerve fibre which 
extends back to the olfactory tract (p. 168), where the dendrites, inter- 
lacing with those of the olfactory lobe, form the glomeruli. In the 
higher vertebrates a third kind of cells, the basal cells; occur at the base 
of the olfactory epithelium. 
The olfactory epithelium arises as part 
of the surface ectoderm of the top of the 
head, but with growth it changes its position. 
For protection it sinks beneath the surface as 
an olfactory sac, connected with the external 
world by (usually) a pair of openings, the ex- 
ternal nares. The growth of the dorsal side 
of the head carries the nares toward the tip 
of the snout and, in the elasmobranchs, to 
the ventral side of the head. 
The accessory parts of the olfactory 
Hit, die Neaoea gr Lee the skeletal nasal capsules (p. 
cecilian (Epicrium), after Sara- 62), which are always present; in the tetra- 
Tuten aa dee . podous forms glands to keep the epithelium 
lateral cavity; mp, middle pas- moist, and the organ of Jacobson. The in- 
sage; os, olfactory sac. é P 
volution of the nasal sacs necessitates some 
mechanism for bringing the external medium (water or air) to the sen- 
sory cells. These will be described in connection with the several 
groups below. The organ of Jacobson is a kind of accessory 
olfactory organ, first appearing in the amphibia, supplied by the first 
and fifth nerves and apparently serving to test the character of the food 
while in the mouth. The position of the organ near the internal 
nostrils lends probability to this view of the function. 
'* The cyclostomes differ markedly from the other vertebrates in their olfactory 
organs. The unpaired area of olfactory epithelium develops in the region of the 
anterior neuropore (p. 12) and becomes involved with the involution for the 
hypophysis (fig. 190) so that there is but a single external opening, serving for both 
