OF THE MARSUPIALIA AND MONOTREMATA. 
637 
Now the corpus callosum breaks through just at the limit between these two divi- 
sions, and by its further growth backwards, the upper division comes to lie on its outer 
surface and is converted into the stria alba Lancisi and stria obtecta of the corpus cal- 
losum, and into the fascia dentata of the hippocampus major; whilst the inferior or 
inner arch, with its longitudinal fibres, forms the fornix and septum ( scheideivand ). 
The fornix is thus, as was known to Arnold and Retzius, a transformation of the upper 
margin of the transverse fissure. The lower margin of the fissure is formed into the 
taenia semicircularis or stria cornea, which, as is well known, is connected at each end 
with the extremities of the fornix. It will be seen from the preceding observations that 
the anterior perpendicular part of the fornix is originally united with the corresponding 
part of the other side, and the body of the fornix developes itself out of the uppermost 
part of this spot, adjoining the primitive corpus callosum. Lower down the parts sepa- 
rate and then resolve themselves into the columnae fornicis, or anterior crura, and the two 
halves of the septum lucidum, the ventricle of which is thus no primitive formation. In 
this part also originates, not by growing together from opposite sides, but by histological 
differentiation, the anterior commissure (F), which is evident a short time before the 
corpus callosum. The septum lucidum and body of the fornix, in the beginning very 
small, gradually increase in extent with the development of the corpus callosum. 
According to Schmidt, the opinion formerly entertained that the genu of the corpus 
callosum was the part first formed, and that the hinder part developed afterwards, is not 
correct. The rudimentary corpus callosum on its first appearance already contains the 
elements of all its subsequent parts, as from the very first, fibres radiate from it into the 
hinder and middle, as well as the anterior lobes, and the intimate connexion of the 
former with the posterior crura of the fornix can already be recognized. It increases, 
with the rest of the hemisphere, chiefly in longitudinal extent, spreading both backwards 
and forwards from the point of its first appearance, but principally in the former direc- 
tion. The curved part in front, called the genu, is not formed until the end of the fifth 
month, and about a month later, the thickening and extension of the hinder end over 
the corpora quadrigemina gives the permanent form to this part of the brain. 
I will next proceed to trace the modifications of the parts of the brain above indicated, 
in certain of the placental mammalia. The preparations from which the figures are 
taken were all made in the same manner as that adopted in the case of the human brain, 
viz., (I.) a vertical longitudinal section in the middle line, exhibiting the inner surface 
of a single (the right) hemisphere, the thalamus opticus and crus having been removed 
so as to show clearly the whole surface with the parts forming the upper boundary of 
the ventricular aperture; (II.) a vertical transverse section through the middle of the 
anterior commissure. 
The Sheep. — In the longitudinal section of the sheep’s brain (Plate XXXVII. fig. 1), 
the elongated narrow corpus callosum (B) is seen lying in a line nearly horizontal, or 
corresponding with the long axis of the hemisphere ; slightly concave in the middle 
