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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 5

advanced brain the callosal fissure. Certain observers speak very decidedly
upon this point. Schwalbe* remarks : " The ' Bogenfurche ' in its upper
part becomes the upper boundary of the corpus callosum." Schmidtf gives
expression to a similar view but considers that a portion of the fore-part of
the fissure u gradually disappears as the hemisphere wall increases in thickness
." Mihalkovics^ states that, "during the formation of the corpus callosum,
it becomes shallower ; and as the upper surface of the corpus callosum extends
at a later period up to this furrow, the latter is retained in that deep incision
which lies between the corpus callosum and the gyrus fornicatus." The
beautiful drawings which accompany Mihalkovics' monograph can hardly be
said to bear out this statement. In figs. 24 and 25 the shallow anterior
end of the arcuate fissure is seen ending at a higher level than the corpus
callosum, whilst there is not a trace of the u vordere Bogenfurche" represented
. Hertwig employs equally decided language in regard to this point.
He says : " The callosal fissure .... constitutes then the anterior part of
the arcuate fissure, whilst the posterior part is termed later the great
hippocampal fissure."

A study of the specimens which I have collected has forced me to
adopt a different view in regard to the fate of the fore-part of the fissura
arcuata. I have not been able to satisfy myself that any part of it is retained
as the callosal sulcus. I believe that it is transitory, and that it is
gradually obliterated during the time that the corpus callosum assumes
shape. A brain which is figured in Plate i (fig. 12) first led me to doubt
the generally accepted view in regard to this question. It was obtained
from a foetus, which had apparently reached a stage of development corresponding
to the first half of the fourth month. There is not a trace of the
corpus callosum, but the fornix is fully formed in connexion with the lower
edge of the arcus marginalis, although the two sides have not as yet
adhered to form the central body part. The fissura arcuata is in three
separate portions—a hinder part corresponding in position with the future

* Lclirbuch der Neurologie, p. 256.

f "Bcitriige zur Eutwicldungsgeschiclite des Gehirns," Zeitsch. f. Wiss. Zool., 1862, Bd. sii.,
pp. 54, 55.

I Kiitwioklungsgefcliichte des Geliirns, p. 148, vide pi. in., figs. 24, 25.