DEVELOPMENT OF PARTS DERIVED FROM FORE-BRAIN. = 555 
and behind the large foramen of Monro. This is the choroidal fissure, and the fold of the 
cerebral wall, which is thus thrust into the cavity, remains thin and entirely epithelial. 
After a time mesoblastic tissue from the great longitudinal fissure finds its way into the 
choroidal fissure and occupies the interval between ‘the two thin layers which form the fold, 
This mesoblastic tissue forms the choroid plexus of the lateral ventricle, and in the early 
stages of the hemisphere it is so voluminous that it fills up the relatively large cavity of 
the lateral ventricle. 
Development of the gyri and sulci.—From the beginning of the third month of 
development to about the end of the fourth month the surface of the cerebral hemisphere 
may present a number of linear and radially disposed fissures both upon its outer and its 
mesial aspects. These are produced by deep infoldings of the thin wall of the cerebral 
vesicle, and, with one or two exceptions, they entirely disappear when the fifth month of 
development is reached. At this period the cerebral surface again becomes smooth. 
They are therefore termed the transitory fissures. Only one permanent fissure, viz. the 
hippocampal, is with any degree of certainty lineally connected with one of these transitory 
fissures. Two, as a rule, occupy the ground which is later on held by the parieto-occipit al 
and the calearine fissures, of which they may be regarded as the precursors, but it is 
likely that in most cases they are obliterated before their permanent successors appear 
upon the stage. 
At the time when the transitory fissures make their appearance, and during the whole 
period that they exist, it is important to note that the outer surface of the growing brain 
is applied closely to the deep surface of the enclosing cranial capsule. It therefore appears 
likely that the infolding of the cerebral wall, which produces the transitory fissures, is caused 
by a growth antagonism between the cranium and the enclosed cerebral vesicle, The 
cerebral vesicle, growing more rapidly than the cranium, has to accommodate itself to the 
restricted space in w hich it is placed by the infolding of its thin wall. 
Tn an important memoir by Hochstetter the existence of these transitory fissures in the brain 
of the living foetus is denied. The arguments advanced in favour of this view are very striking, 
but they do not explain all the circumstances of the case. 
After the occipital lobe is fully formed and the fifth month is reached, all growth 
antagonism between the cranial capsule and the enclosed cerebral vesicle ceases. The 
surface of the cerebral hemisphere becomes smooth, and the cranium, growing for a time 
more rapidly than the brain, leaves a relatively wide space between the cerebral surface 
and the surrounding cranial envelope. This is occupied by sodden subarachnoid tissue, 
and when this stage is reached (in the latter part of the fifth month) the permanent 
fissures and gyri ‘begin ‘to make their appearance. The incomplete sulci owe their 
origin to the upheaval of the cerebral cortex on either side of the appearing fissures, 
and the gyri which bound them are formed as the result of an exuberance of surface 
growth in localised areas. Owing to the wide interval between the cranial wall and the 
surface of the cerebral hemisphere, the particular surface areas which grow and foreshadow 
the future gyri suffer no restriction, and they take the form of rounded eminences which 
rise from the general surface level of the cerebral hemisphere. As growth goes on, how- 
ever, the brain gradually assumes a bulk more nearly in accord with the cavity of the 
cranium, and the space for extension becomes more limited. Finally, about the beginning 
of the eighth month, the gyral elevations come into close contact with the cr anial walle 
and a second period of erowth antagonism between the brain and its enclosing capsule is 
entered upon. As a re sult of this the eyri are pressed together, the fissures assume more 
definite shape, and the ordinary convolutionary forms make their appearance. So 
intimate, indeed, is the contact between the cerebral hemisphere and the skull capsule 
that the gyri, to some extent, produce an imprint on the deep aspect of the cranial 
bones. 
As already explained, the complete fissures are produced by an infolding of the wall 
of the cerebral vesicle. One of these is a transitory fissure, viz. the external perpendicular 
fissure of Bischoff. This differs from the earlier transitory fissures described above in so 
far that it does not come into view until the end of the fifth month, when the latter have 
completely disappeared (p. 523). 
Cerebral commissures.—The development of the cerebral commissures is sur- 
rounded with much difficulty, and cannot be said at the present moment to be fully under- 
stood. It would seem that the corpus callosum, the anterior commissure, and the fornix 
take origin in the thickened upper part of the lamina terminalis. The triangular interval 
which is left between these commissures is occupied by the septum lucidum, the precise 
nature of which is somewhat doubtful, Probably it is formed from the lamina terminalis, 
m Bei “- ies 
