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Dr. Cunningham—Stir/ace Anatomy of the Primate Cerebrum. 79

the fourth month the elongation and backward bending of the fossa is more
marked, and now the area is surrounded by a high projecting boundary of
mantle-wall which limits it in the most definite manner. But soon another
change in the outline becomes manifest. The simple arch-like form is lost.
In the process of backward bending the anterior boundary is not affected
equally throughout its whole length. Its upper two-thirds from this stage on
alone participates in the bending, so that it tends to assume more and more

I. II. III. IV. Y. VI.

VII. VIII. IX. X.

Fig. 24.—Diagram representing the different outlines assumed by the Sylvian depression in course
of its development. Each outline has been taken from an actual specimen, and enlarged.

a horizontal inclination, while the lower third of this wall maintains its
original direction, and descends in a more or less vertical direction. The
two portions of the original anterior wall meet each other in a very wide
open rounded-angle, and the outline of the Sylvian field becomes somewhat
triangular. This change then is brought about by the elongation of the
original anterior boundary wall, and the bending back of its upper two-
thirds. About the beginning of the fifth month the triangular form of the