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Development during the fetal phase

Over the course of the fetal phase the neopallium experiences considerable expansion at the expense of the archipallium and palaeopallium.
In this, the hemispheres gradually grow around the diencephalon. Through the strong proliferation of the amount of striate bodies, a thickening in the connection region between the telencephalon and diencephalon occurs so that these two parts merge into one another over large areas. The portion of the meninges that then still lie between the two brain sections atrophy and a fusion of the hemisphere floors with the thalamus takes place.
The broad connection zone permits the entry of nerve fibers that connect the hemispheres with the other segments of the nervous system.  The white matter in the stem region appears as the internal capsule.

Definition

Lamina affixa: adhesion location between ependymal layer of the ventral segment of the hemisphere vesicles and the dorsal area of the thalamus. This region then becomes the floor of the central part of the lateral ventricles between the stria terminalis and the taenia choroidea.

Fig. 93 - Cross section through the prosencephalon at around the 11th week
media/module22/v9c_develtelencephB_w11.gif

1
Location where the diencephalon is covered by the expansion
of the cerebral hemispheres
2
IIIrd ventricle
3
Hypothalamus
4
Thalamus
5
Pallidum
6a
Body of the caudate nucleus
6b
Tail of the caudate nucleus
7a
Choroid plexus in the lateral ventriclel
7b
Choroid plexus in the IIIrd ventricle
8
Pallium
9
Lateral ventricle
10a
Cortex of the dorsal hippocampus
10b
Cortex of the ventral hippocampus
15
Inferior horn of the lateral ventricle
17
Choroid fissure

Legend
Fig. 93

As can be recognized in this cross section caudal to the interventricular foramen (see sectional plane B) at around the 11th week the hemisphere vesicles grow together with the diencephalon.

Note the transition from the hemispheres to the roof of the diencephalon (choroid fissure). Through the horseshoe-shaped expansion of the hemispheres certain structures near the ventricle are relocated into the temporal lobes.

Fig. 94 - Cross section through the prosencephalon in the fetal phase
media/module22/v9c_develtelencephB_w12.gif

1
Fusion region between the diencephalon and the telencephalon
2
IIIrd ventricle
3
Hypothalamus
4
Thalamus
5
Pallidum
6a
Body of the caudate nucleus
6b
Tail of the caudate nucleus
6c
Putamen
6c
Claustrum
7a
Choroid plexus in the lateral ventriclel
7b
Choroid plexus in the IIIrd ventricle
8
Pallium
9
Lateral ventricle
10a
Cortex of the dorsal hippocampus
10b
Cortex of the ventral hippocampus
15
Temporal horn of the lateral ventricle
16
Lamina affixa
17
Choroid fissure

Legend
Fig. 94

At a still later time, a cross section caudal to the interventricular foramen (see sectional plane B) reveals the considerable expansion of the neopallium as well as the adhesion between the hemisphere floor and the roof of the diencephalons in the form of the lamina affixa. The already extensive differentiated striate bodies are forced to the side.

Fig. 95 - Cross section through the prosencephalon in stage 23 (ca. 56 days)
media/module22/v9c_develtelencephA_w11.gif

1
Fusion region between the diencephalon and the telencephalon
2
IIIrd ventricle
3
Hypothalamus
4
Thalamus
5
Pallidum (medial striatum)
6a
Body of the caudate nucleus
6b
Tail of the caudate nucleus
6c
Putamen
6d
Claustrum
7
Choroid plexus of the lateral ventricle
8
Pallium
9
Lateral ventricle
10a
Cortex of the dorsal hippocampus
10b
Cortex of the ventral hippocampus
11
Insular cortex
12
Amygdala
13
Corpus callosum
14
Fornix
15
Temporal horn of the lateral ventricle
16
Capsula interna

Legend
Fig. 95

Cross section through the prosencephalon at the level of the interventricular foramen (see sectional plane A). In this advanced stage the diencephalon, through the massive expansion of the hemispheres, is extensively grown over (dotted green line). Through the horseshoe-shaped growth of the hemispheres certain structures near the ventricles are relocated together into the temporal lobes. With the formation of the lateral sulcus, the insular cortex is slowly covered over in the depths.

Out of the lateral neostriatum, lying on the floor, arise the caudate nucleus and the putamen in the course of the further development. In contrast, out of the medially lying palaeostriatum of the diencephalon emerges the globus pallidus (in short: pallidum). Macroscopically this, together with the putamen, is summarized as the lentiform (or lenticular) nucleus. The amygdala or archistriatum finally develops ventrally to the lentiform nucleus and apparently derives from the diencephalon and the telencephalon.