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TRANSVERSE SECTION THROUGH THE BRAIN oF A RABBIT OF
Five Crentimerres. (After Mihalkovices.)


The section passes through nearly the posterior border of the
septum lucidum, immediately in front of the foramen of Monro.
hms. cerebral hemispheres ; cal. corpus callosum; amm. cornu
ammonis (hippocampus major); ems. superior commissure
of the eornua ammonis ; spé. septum lucidum ; frx 2. anterior
pillars of the fornix ; ema. anterior commissure ; érm. lamina
terminalis; str. corpus striatum; @/. nucleus lenticularis
of corpus striatum; vr 1. lateral ventricle; vir 3, third
ventricle ; zpl. slit between cerebral hemispheres.
380 DEVELOPMENT OF ORGANS IN MAMMALIA. (CHAP.
The outer wall of the hemispheres gradually thick-
ens, while the inner wall becomes thinner. In the
latter, two curved folds, projecting towards the interior
of the lateral ventricle, become formed. These folds
extend from the foramen of Monro along nearly the
whole of what afterwards becomes the descending cornu
of the lateral ventricle. The upper fold becomes the
hippocampus major (cornu ammonis) (Figs. 123 amm,
124 and 125 h, and 126 am).
The wall of the lower fold becomes very thin, and a
vascular plexus, derived from the connective-tissue
septum between the hemispheres, and similar to that of
the roof of the third ventricle, is formed outside it. It
constitutes a fold projecting into the cavity of the
lateral ventricle, and together with the vascular con-
nective tissue in it gives rise to the choroid plexus of
the lateral ventricle (Figs. 124 and 125 pl).
