No. I.] AMPHIBIAN BRAIN STUDIES. ; 59 
bands, and their relation to the constrictions of thé brain, sug- 
gest at this period a striking homology between them.” (See 
Plate XVI., Fig. 62, F.) This does not include the cerebellum, 
the tracts of which he has not followed. It is true that the 
fibres beneath the fore-brain branch from the lateral longitudinal 
band in much the same manner as do those passing to the re- 
gion of the other commissures; but I cannot at present adopt 
his view that they represent the anterior commissure, because 
the development of this commissure, as I have found it in the 
Amphibia and Mammalia, indicates that it is strictly commis- 
sural, and not the decussation of a longitudinal tract, as must 
be inferred if its fibres pass directly into such a tract. Immedi- 
ately beneath the anterior commissure in the Amphibian fore- 
brain, I have observed fibres decussating from longitudinal bun- 
dles to the opposite hemisphere (op. cit., Fig. 11, Plate XIV.), 
which probably represent those attributed to the anterior com- 
missures by Orr (op. cit., p. 346). 
Upon these grounds, pro and con, the hypothesis may be 
restated as originally: that the early constriction of 
tieworaim roof which gives rise to the sfour wesi- 
cles is for the accommodation of three nerve-fibre 
tacts decussating dorsally, viz, the superionvand 
posterior commissures and the cerebellum, which 
in their primitive condition have a serial homology. 
If in some of the lower vertebrates, ¢.g., the Uvodela, the 
cerebellum is intersegmental, and in other vertebrates, lower 
and higher, it becomes equivalent to the other segments, it 
merely accords with what we may conjecture as to the prob- 
able evolution of the encephalic segments, that they were not 
primary features of the vertebrate brain, but were defined 
secondarily, with the concentration of certain groups of func- 
tional centres at certain points. The neuromeres are probably 
remnants of the true primitive segmentation. 
II. THE CRANIAL NERVES. 
EXIT FROM THE BRAIN. 
I. The olfactory nerves arise from the lateral or infero-lateral 
portion of the olfactory lobes and are distributed in the usual 
manner in the olfactory sac, as shown in the figures of Proteus. 
They vary little in size throughout the Amphibia. 
