289 
but rarely described. LEE describes it in reptiles and concludes that 
this double arrangement is a primary condition and does not, as has 
been suggested, have reference to the differentiation of JACOBSON’S 
organ from the olfactory chamber. As already described, the main 
olfactory in Acanthias is made up of two chief divisions (a and 8, 
Figs. 1—14) which are in turn composed of several smaller bundles 
of fibres. In embryonic stages the separation between the two main 
divisions is very distinct; there are two clusters of glomeruli in the 
lobe with an intervening clear space (Figs. 13—14). In the adult 
condition, owing to the crowding together of the bundles, the double 
arrangement is not so well defined. It should be understood that 
this double nature of the olfactory as far as described, in other ani- 
mals, corresponds to the divisions (a) and (6) of the main olfactory 
in Acanthias and not to the median (mv) and lateral elements. Miss 
Anna NEIGLICK has traced in my laboratory the early development 
of the olfactory of several amphibia (Necturus, Frog, Toad, Amblystoma 
etc.) The elements (a) and (6) have in all cases been identified but 
not the median element. 
Especial interest attaches to the nerve that has been designated 
median nerve (nv) in this paper. On account of its close relation 
with the fibres of the main olfactory and to the nasal membrane, it 
is best for the present, to refer it to the olfactory system and, perhaps, 
to designate it “accessory olfactory”. We need to know its central 
and peripheral terminations and whether it is represented in other 
animals before saying much about its homology. 
It must not be confused with the thalamic nerve discovered by Miss 
Piatt 91 and Froriep 791, and whose history was worked out by Horr- 
MANN ’97. That nerve is between the mid-brain and thalamencephalon. 
The two exist simultaneously but the thalamic nerve is transitory. 
Although this nerve has not been previously described in Sela- 
chians its central portion was shown in a Fig. of Galeus canis by 
FritscH*) in 1878. It is represented in his Fig. 6 as cut off on both 
sides of the brain, extending a little in front of the fore-brain, but 
having no connection with the olfactory organ. It is designated by 
FRITSCH supernumerary nerve (“überzähliger Nerv”). Dr. O. S. STRONG 
informs me that be has noticed this nerve for the past two or three 
years in the brain. of the adult skate. 
In histological features it resembles the nerve described by 
Pinkus 2) in Protopterus and by Aruıs?) in Amia. There are rounded 
1) Untersuch. über den feineren Bau des Fischgehirns, 1878. 
2) Anat. Anz., Bd. 9, 1894. 
3) lic. 
Anat, Anz. XVI. Aufsätze. 19 
