39 2 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. 1 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 



fity-P 



FIG. 225. Side view of the brain of Petromyzon, with the olfactory organ and the 

 pituitary caecum in section. Mm, Cerebellum ; crb.h, cerebral hemisphere ; dien. 

 thalamencephalon ; f, fold in the nasal tube ; r/t, nasal glands ; inf, infundibulum ; 

 l.gn.hb, left ganglion habennlae ; medjbl, medulla oblongata : na.ap, naso-pituitary 

 aperture ; n.ch, notochord ; 2?v l -nv w , cranial nerves ; Nv l ", first ventral spinal 

 nerve; olf.cp, olfactory capsule; olf.l, olfactory lobe; olf.m.m, olfactory mucous 

 membrane; opt. I, 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\ first dorsal spinal nerve. (From Parker and Haswell, after 

 Ahlborn and Kaenische. ) 



nerves, point to the same conclusion. 2 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 



1 Kupffer, Stud, vergl. Entwickl. d. Kopfcs d. Kraniaten, iii. 1895, p. 6. 



2 In Bdellostoma the olfactory organ arises as a pair of outgrowths from the- 

 pituitary involution (Bashford Dean, Kupffer's Festschrift, Jena, 1899, p. 269). 



