Houser, The Neurones of a Selachian. 161 
The pallium has its neurones grouped, without arrangement 
into layers, chiefly in the pallial eminences. Three varieties of 
neurones are to be distinguished. The pallium receives impres- 
sions radiated from the adjacent olfactory nuclei; it therefore 
anticipates the olfactory connections of the reptilian and higher 
brains. The tractus pallii, arising from the pallial neurones, is 
regarded as giving the olfactory sense a quite direct connection 
with posterior regions. 
2. Conclusion. 
To one who has read this far, it must be evident that 
there is a most remarkable structural similarity between the 
brain of Mustelus and the brains of higher vertebrates. The 
results obtained by me do not bear out the conclusions of 
SZCZAWINSKA ('98) relative to the very low plane occupied by 
the selachian neurones; see Section II, 2. The neurones of 
Mustelus are, of course, simpler in their external morphology, 
and their architectural relations are of a far less complicated 
order, yet it is none the less true that they ‘anticipate the con- 
ditions found in higher vertebrates in all important particulars. 
Such a fact is certainly the more remarkable when the great 
differences in the scale of general organology are taken into 
account. The fact can only be interpreted to mean that the 
nervous system of the primitive vertebrate had its essentials of 
organization well defined before the divergence of the several 
phyla occurred. A brain of the type presented by the selachian 
of to-day has not become sufficiently specialized during the 
lapse of time entirely to mask the ancestral characteristics. The 
brain of one of the higher vertebrates embodies many modifi- 
cations of the original plan, wrought in the course of its grad- 
ual evolution. These alterations may even become more con- 
spicuous than the primary structures upon which they have 
been superposed, and it is only through comparisons with a 
less differentiated condition that we can hope to distinguish the 
new from the old. 
A comparative study of the several brain-segments of Mus- 
telus is productive of some results that might not have been 
