But it is equally true that a tadpole is very
different from any known fish.
In like manner, the brain of a human foetus, at the fifth month,
may correctly be said to be, not only the brain of an ape, but
that of an Arctopithecine or marmoset-like ape; for its
hemispheres, with their great posterior lobster, and with no
sulci but the sylvian and the calcarine, present the
characteristics found only in the group of the Arctopithecine
Primates. But it is equally true, as Gratiolet remarks, that, in
its widely open sylvian fissure, it differs from the brain of any
actual marmoset. No doubt it would be much more similar to the
brain of an advanced foetus of a marmoset. But we know nothing
whatever of the development of the brain in the marmosets. In
the Platyrrhini proper, the only observation with which I am
acquainted is due to Pansch, who found in the brain of a foetal
Cebus Apella, in addition to the sylvian fissure and the deep
calcarine fissure, only a very shallow antero-temporal fissure
(scissure parallele of Gratiolet).
Now this fact, taken together with the circumstance that the
antero-temporal sulcus is present in such Platyrrhini as the
Saimiri, which present mere traces of sulci on the anterior half
of the exterior of the cerebral hemispheres, or none at all,
undoubtedly, so far as it goes, affords fair evidence in favour
of Gratiolet's hypothesis, that the posterior sulci appear before
the anterior, in the brains of the Platyrrhini.
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