Fig. 260. Transverse section through the brain of a sheep’s embryo of 2.7 cm. in length. (From Kölliker.)
The section passes through the level of the foramen of Munro.
st. corpus striatum; m. foramen of Munro; t. third ventricle; pl. choroid plexus of lateral ventricle; f. falx cerebri; th. anterior part of optic thalamus; ch. optic chiasma; o. optic nerve; c. fibres of the cerebral peduncles; h. cornu ammonis; p. pharynx; sa. presphenoid bone; a. orbitosphenoid bone; s. points to part of the roof of the brain at the junction between the roof of the third ventricle and the lamina terminalis; l. lateral ventricle.
The corpora striata are united at their posterior border with the optic thalami. In the later stages of development the area of contact between these two pairs of ganglia increases to an immense extent ([fig. 261]), and the boundary between them becomes somewhat obscure, so that the sharp distinction which exists in the embryo between the thalamencephalon and cerebral hemispheres becomes lost. This change is usually (Mihalkovics, Kölliker) attributed to a fusion between the corpora striata and optic thalami, but it has recently been attributed by Schwalbe (No. [349]), with more probability, to a growth of the original surface of contact, and an accompanying change in the relations of the parts.
The outer wall of the hemispheres gradually thickens, 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 Munro 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. 259] amm, [260] and [261] h, and [262] am). When the rudiment of the descending cornu has become transformed into a simple process of the lateral ventricle the hippocampus major forms a prominence upon its floor.
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Fig. 261. Transverse section through the brain of a sheep’s embryo of 2.7 cm. in length. (From Kölliker.)
The section is taken a short distance behind the section represented in fig. 260, and passes through the posterior part of the hemispheres and the third ventricle.
st. corpus striatum; th. optic thalamus; to. optic tract; t. third ventricle; d. roof of third ventricle; c. fibres of cerebral peduncles; c´. divergence of these fibres into the walls of the hemispheres; e. lateral ventricle with choroid plexus pl; h cornu ammonis; f. primitive falx; am. alisphenoid; a. orbitosphenoid; sa. presphenoid; p. pharynx; mk. Meckel’s cartilage.
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 far into the cavity of the lateral ventricle, and together with the vascular connective tissue in it gives rise to the choroid plexus of the lateral ventricle ([figs. 260] and [261] pl).