388 Proceedings of Royal Soeiety of Edinburgh. [may 7, 
4. Hemispheres formed ; Fusion of Septa Lucida and Marginal 
Arches more extensive.^ hut still incomplete. — (a) Eusion limited to 
septa lucida. (Arrest of development at end of fourth month.) 
Anterior commissure and. knee of corpus callosum present. Fornix 
present, hut its psalterium absent (Case XXI.). (6) Union of 
septa lucida complete ; hut in marginal arches limited more or less 
to anterior part. Corpus callosum present anteriorly, hut generally 
thin (as in lower mammalia). Splenium absent or thin. Psalterium 
of fornix present, if fusion has extended sufficiently far back. 
Cases XVI., XYIL, XVIII., XIX., XX. 
The destination of the septum lucidum and marginal arch in 
Series 3 (and in those cases of Series 2 in which they have been 
developed) remains to be examined. We have seen that these 
structures lie between the (embryonic) fissure hippocampi and the 
fissura choroidea, and that the fornix is developed along their 
inferior margin. If, now, we find a structure having the same 
relation or position to the fissura choroidea, the fornix, and the 
fissura hippocampi, we may fairly conclude that it represents the 
septum lucidum and marginal arch. There seems little difficulty 
in identifying the area {spt) in my case (fig. 2), and in Onufrowicz 
(sp^, fig. 16), Kaufmann (fig. 28), Anton (fig. 13), Eichler (fig. 11), 
and Knox (fig. 12), as the septum lucidum. 
The marginal arch presents greater difficulty. Onufrowicz and 
Kaufmann consider that the fibres occupying its position belong to 
the system of fronto-occipital association fibres, and pass to the 
outer side of the posterior cornu of the lateral ventricle into the 
tapetum — a structure usually held to be composed of callosal 
fibres; that they are, in fact, the fibres of the cingulum of Burdach, 
no longer concealed by the fibres of the corpus callosum. This 
view I consider to be untenable, for the following reasons : — 
The cingulum lies in the substance of the gyrus fornicatus, separated 
by part of its grey matter from the corpus callosum (see Meynert, 
Psychiatry,, p. 40, and fig. 18). The structure under consideration, 
however, is separated by a fissure from the gyrus fornicatus. In my 
case, its fibres certainly do not pass into the so-called tapetum, but 
seem rather to end in the investment of the cornu ammonis posteriorly 
(at least in their greatest part). And lastly, it does not become pro- 
minent in a brain in which the corpus callosum has atrophied (see 
