336 Proceedings of Royal Society of Edinhurgli. [may 7, 
the lateral outgrowth from the lower margins of the falx cerebri of 
the choroidea superior (velum interpositum) with its fringe of 
vessels, the choroid plexus. (See fig. ‘^?>,falx, tel.chor,, and^fec/ior.) 
The cerebral wall covering this plexus becomes gradually reduced to 
the layer of epithelium, w^hich forms its investment in the adult. 
The two fissures include between them a portion of the cortex (fig. 
31, r.h. and fasc. dent.\ which, from its position and form, is termed 
the convolution of the marginal arch (the randbogen of German 
authors). This convolution is continuous in front with that part 
of the cortex {syt) which forms the septum lucidum, and pos- 
teriorly it passes into the gyrus uncinatus. Along with the septum 
lucidum it becomes the seat of the following series of important 
changes : — 
About the middle of the third month of intra-uterine life the 
triangular areas of the cortex which correspond to the two septa 
lucida {s'pt) become fused together, and unite along their margins 
(thus including the cavity of the fifth ventricle between them). 
In the beginning of the fourth month the lower borders of the 
fused septa lucida, and of the as yet ununited marginal arches, 
become differentiated into the anterior pillars, body, and fimbria 
(and commissure?) of the fornix (fig. 31). About the same time 
(probably at a slightly later date) the anterior commissure appears 
in the lower angle of the septa lucida. Towards the end of the 
fourth month, along the anterior and upper periphery of the septa 
lucida, the rostrum and knee of the corpus callosum (fig. 32, cal) 
are developed. During this month, also, the two marginal arches 
become gradually united as far back as the posterior extremity of 
the optic thalamus. 
During the fifth and sixth months the fused portion of the 
marginal arches becomes gradually differentiated from before back- 
wards into the corpus callosum. With the exception of this and 
of a small portion of grey matter, the induseum griseum, and the 
nervus Lancisii {Inc. fig. 32) above, and of the fornix below the 
corpus callosum, the whole of this part of the marginal arch becomes 
modified into callosal fibres. In many mammalia the upper portion 
of the arch becomes callosal, while the underlying part becomes 
cornu ammonis, which thus extends much farther forwards than 
in man. The fusion of septa lucida and marginal arches necessarily 
