546 The Dipnoan Brain. [June 
In a shark, Scymnus, as figured by T. J. Parker (New Zealand 
Inst. Trans., xv. 1882; Nature, December 30, 1886), each lateral | 
portion of the peosencepliai, between the unpaired part and the 
olfactory lobe, presents a fusiform and nearly symmetrical dilata- 
tion. In 1876 the writer called attention (Am. Four. Sci., xii. 105) 
to the fact that in the lamprey the only part which can be regarded 
as a cerebral hemisphere lies /a¢evad of the olfactory lobe (caudad 
when the paired portions are at a right angle with the meson); 
also that in most sharks and rays the “ hemisphere” projects on 
the opposite side of the olfactory crus, and usually unites with 
its platetrope (lateral homologue). In Dipnoi the cerebral out- 
growth is ventral. In either of these directions in which what 
may be regarded as the special organ of the mind is projected 
among these low or generalized forms there would seem to be 
mechanical obstacles to any considerable expansion; but dor- 
sally there is opportunity for comparatively unlimited extension, 
and it is in this direction that the hemispheres begin to develop 
in the Amphibia and attain such enormous growth in Birds and 
Mammals. This revolution, so to speak, of the hemisphere 
about the olfactory axis accords with other considerations which 
have led Spitzka and the writer independently to consider the 
prevailing idea that the olfactory lobes are mere appendages of 
the cerebrum as nearly the reverse of the trut 
EXPLANATION OF FIGURES. 
The figures are tracings from photographs, with slight modifications. The pro- 
portions and leading features are therefore accurate. ere are some points of dif- 
ference between the two brains, and some conditions which may be artificial; these 
will be referred to in connection with the several figures. Attention is called to 
the fact that, to one familiar with the brains of Amphibia, Reptiles, na the lower 
Mammals, the cephalic portion (prosencephal) of the Ceratodus brain looks more 
natural when held upside-down ; this is due to the unusual relative Sras of the 
olfactory tract and the cerebral hemis sphere. 
The following apply to the three fig The dotted areas represent cut or arti- 
ficial surfaces; the heavy line, forming the ental margin of the cut surfaces, repre- 
sents the endyma, which lines the cavities and is reflected over the plexes. The 
pa (esually lighter) which forms the ectal margin of the cut surfaces, and the out- 
parts not cut, represents the a, from which vessels are given off to form 
‘he. plexes. At some places—e.g., the metaplex, or membranous roof of the meta- 
cele, and the dorso-caudal wall of the saccular conarium—the parietes consist only, 
or ame sa of the m and the endyma; at other places—e. Ss the greater 
of the 
oe 
y reien oani tuent the parietes is very thin. 
f of the mesocele gion and the parts caudad 
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