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Dk. Cunningham—Surface Anatomy of the Primate Cerebrum. 13

has told us so, and Schmidt has figured them at this stage; but they do not
attain, even on this aspect of the hemisphere, a high degree of development
until the ninth or the tenth week. Again, we must not forget that if we
are to regard the " vordere Bogenfurche" of His as a transitory fissure, we
must consider that the process actually begins on the inner surface of the
hemisphere as early as the fifth week. As we shall see further on, however,
this fissure hardly belongs to the same series as the others, seeing that it
presents an altogether different direction.

On the outer surface of the hemisphere the formation of the transitory
fissures is delayed. It is doubtful if they ever show on this aspect before
the tenth week. In most cases they attain their maximum degree of
development between the third and fourth months, and this holds good
for both surfaces of the hemisphere.

A certain amount of latitude must also be allowed in reckoning their
time of disappearance; but as a rule it will be found that they become
obliterated between the periods at which the fornix and corpus callosum
become developed; in other words, somewhere towards the end of the
fourth or the beginning of the fifth month. From this general statement,
however, we must make an exception in favour of two fissures which appear
on the outer aspect of the occipital lobe, and whose history I purpose
discussing at some length further on.

Transitory Fissures on the Medial Wall of the Hemisphere.—As a general
rule, the transitory fissures on the mesial wall of the hemisphere are very
definite in their relations, although they vary greatly in number. (PI. I.,
figs. 5 and 12; PI. in., figs. 1 to 6). They consist of a series of furrows
which radiate in a stellate manner from the fissura arcuata (Bogenfurche)
towards the free border of the hemisphere. At first they appear as a
number of notches or indentations in the upper boundary of the fissura
arcuata. These are shallow and broad at the base, and they give to the
medial cerebral wall a wrinkled or crumpled appearance. At this stage
they might very readily be mistaken for depressions brought about by the
shrinkage of a hardening reagent. After a while they deepen and lengthen
out, and they may extend as a continuous series all the way round from the
frontal pole in front to the extremity of the temporal pole below. The