392 FISHES CHAP. 
condition of the Cyclostomes is probably a secondary acquisition. 
At the earliest embryonic stage at which any trace of an olfactory 
organ is apparent, there is a median thickening of the epidermis, 
possibly a vestige of some older sensory organ comparable, it may 
be, to the so-called olfactory organ of Amphioxus; on each side 
of it there is a lateral thickening, the rudiments of the paired 
organs! The three thickenings, or “ plakodes,” then sink inwards 
to form an olfactory pit. The partial subdivision of the adult 
organ by a vertical septum, and the presence of two olfactory 
70.€ t.gn.Ab 
lolfl crbh 
Cpl | ( dien 
: y | TH, 
/ Li if Y]} rch 
eee oi OU) a Wy 
Hy WM, 
wa 0.5 4-8 
Fic, 225.—Side view of the brain of Petromyzon, with the olfactory organ and the 
pituitary caecum in section. cb/m, Cerebellum ; crb.h, cerebral hemisphere ; dien. 
thalamencephalon ; f, fold in the nasal tube ; g/, nasal glands ; inf, infundibulum ; 
l.gn.hb, left ganglion habenulae ; med.ob/, medulla oblongata ; na.ap, naso-pituitary 
aperture ; .ch, notochord ; Nv!-nv!®, cranial nerves; Vv!*, first ventral spinal 
nerve ; olf.cp, olfactory capsule; olf/, olfactory lobe ; olf.m.m, olfactory mucous 
membrane ; opt./, optic lobe ; px, pineal body ; px’, 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’, first dorsal spinal nerve. (From Parker and Haswell, after 
Ahlborn and Kaenische. ) 
nerves, point to the same conclusion.” All Fishes possess olfactory 
organs which are obviously paired. In Elasmobranchs and 
Dipnoi they retain their primitive ventral position. Many 
Sharks and Dog-Fishes possess an oro-nasal groove leading from 
each olfactory organ to the corresponding angle of the mouth. 
The Dipnoi proceed a stage farther, and, by the conversion of 
the grooves into short canals, the olfactory pits communicate 
with the mouth by true internal nostrils, as in the higher 
! Kupffer, Stud. vergl. Entwickl. d. Kopfes d. Kraniaten, iii. 1895, p. 6. 
* In Bdellostoma the olfactory organ arises as a pair of outgrowths from the 
pituitary involution (Bashford Dean, Kupffer’s Festschrift, Jena, 1899, p. 269). 
