[ 191 ]

THE DEVELOPMENT OF THE HUMAN INTERNALCAPSULE AND LENTIFORM NUCLEUS

By W. HEWITTDepartment of Anatomy, St Thomas's Hospital

Medical School, London, S.E. 1

INTRODUCTION

The head of the human caudate nucleus develops in the floor of the lateral ventriclefrom three rostro-caudally aligned elevations called medial, lateral and intermediatestriatal elevations, successively appearing in that order. Caudally these elevationsunite and this part gives rise to the tail of the caudate nucleus and amygdaloid body(Hewitt, 1958).Accounts of the development of the internal capsule (Arey, 1954; Frazer, 1931;

Hamilton, Boyd & Mossman, 1952; Harrison, 1959; Hochstetter, 1929; Johnston,Davies & Davies, 1958; Patten, 1946; and Streeter, 1912) describe its formationfrom fibres passing to and from the cerebral cortex through a mass of cells, usuallycalled the corpus striatum, in the basal part of the hemisphere. This cellular mass,perhaps better termed Basal Ganglia, forms the substance of the striatal elevations.All but two of the authors to whom reference has been made consider or assume thismass to consist of a homogeneous collection of cells. The exceptions (Hochstetter,1929, makes no mention in the text of the fact that his illustrations clearly revealthe cells arranged in curved laminae and Hamilton, Boyd & Mossman, 1952)illustrate heavy condensations of cells deep to the surface of the two striatal eleva-tions which these authors in common with all the others mentioned, except one(Harrison, 1959) who describes the three, give as the total number of elevationscontributing to the caudate nucleus. All these authors describe how the passageof the internal capsule through the cells of the basal ganglia mass subdivides theminto those on its outer side which become the lentiform nucleus and those on theinner side of the internal capsule forming the substance of the caudate nucleus.None of these descriptions fully explains the formation of the parts of the lentiformnucleus, gives adequate details of the stages in the development of the internalcapsule or explains the route it follows through the basal ganglia.

MATERIALS

Serial sections, stained with haematoxylin and eosin, of whole embryos andfoetuses or their cerebral hemispheres were used. Transverse sections of 7-5 and25 mm. C.R. length embryos were studied together with a 15 mm. C.R. length embryowhich had been orientated so that coronal sections through its cerebral hemisphereswere made. Coronal sections of one cerebral hemisphere from embryos and foetusesof 85, 70 and 135 mm. C.R. length were also examined. Except for the sections ofthe 135 mm. foetus which were 20,,t thick all the other sections were lo0e thick. Wax-

192 W. Hewittplate reconstructions of the 7 5, 15, 35 and 135 mm. material were also availableand these were frequently used to develop a three-dimensional understanding of thesections under examination.

RESULTS

In the 7-5 mm. embryo (PI. 1, fig. 1) the basal ganglia presented as a single ridge inthe side wall of each cerebral vesicle. This ridge becomes the medial striatal elevationin later stages of development (PI. 1, fig. 1, M.S.E.). The wall of the cerebral vesicleconsisted of a mantle zone lined on its ventricular surface with an ependymal zone;no marginal zone was visible. The substance of the striatal elevation consisted of athickening of the mantle zone, composed of loosely arranged cells, continuous withthe thinner mantle zone over the rest of the vesicles. The surface of the elevationwas covered with the ependymal zone which was much the same thickness as thatlining the remainder of the vesicles. There was no sign of an internal capsule.

In the 15 mm. embryo (PI. 1, fig. 2) the developing head of the caudate nucleuswas composed of two contiguous elevations, the medial and lateral striatal elevations(P1. 1, fig. 2, M.S.E., L.S.E.). The medial striatal elevation derived from that seenin the side wall of the cerebral vesicle of the 7*5 mm. embryo now lay in the floor ofthe hemisphere on the medial side of the second or lateral striatal elevation. Deepto the ependymal zone covering the ventricular surface of these masses was themantle zone which was considerably thickened to form the substance of the basalganglia as in the 7-5 mm. embryo. No marginal zone was visible in the region underexamination. In the centre of the mantle zone mass, opposite the large interventri-cular foramen, deep to the surface of the caudal end of the developing head of thecaudate nucleus, was a stratified lamina of somewhat more densely packed cells.This lamina, which will be referred to later as the inner or medial lamina of cells,was approximately one-fifth of a millimetre in cranio-caudal extent and was cup-shaped with its mouth directed backwards and medially towards the side wall ofthe diencephalon (PI. 1, fig. 2, M.L.). Within the concavity of this lamina was asmall bundle of fibres (P1. 1, fig. 2, I.C.) which, when followed caudally, passedfrom the mouth of the lamina to the side wall of the diencephalon through the cere-bral peduncle. This bundle of fibres constituted the early formation of the internalcapsule.In the 25 mm. embryo (PI. 1, fig. 3) apart from the pallium which now possessed

a thin cortical layer of densely arranged cells extending down over the upper part ofthe outer surface of the basal ganglia, these otherwise consisted of the same com-ponents as those of the 15 mm. embryo. The internal capsule was thicker thanpreviously observed and principally consisted of parallel upper and lower com-ponents directed to the diencephalon but not extending to the pallium. Betweenthese two components was a third, less obvious, intermediate set of fibres inter-spersed with cells reaching as far as the pallium (PI. 1, fig. 3, I.C.). The upper com-ponent of the internal capsule extended between a small, dense, round condensationof cells in the mantle zone and the diencephalon (PI. 1, fig. 3, M.L.). This mass, ormedial lamina, was deep to the medial striatal elevation on and behind the plane ofthe interventricular foramen and its cells were almost continuous with the thickependymal zone on the ventricular surface of the elevation (PI. 1, fig. 3, M.S.E.).

The human internal capsule and lentiform, nucleus 193This lamina had the same cup shape and bore the same relations to the upper partof the internal capsule as the single lamina and internal capsule bore to one anotherin the 15 mm. embryo and were considered equivalent.The lower component of the internal capsule emerged just above the lower edge of

a second curved lateral lamina of cells lying outside the medial lamina (P1. 1, fig. 3,L.L.). This lamina was deep to the lateral striatal elevation and when traced upwardsits cells became continuous with those deep to the ependymal zone on the surface ofthe striatal elevation (P1. 1, fig. 3, L.S.E.). The curvatures of the two laminae werethe same and were disposed along the circumference of concentric circles, exceptbelow where the lateral lamina joined a thicker dense mass of cells in the lowest partof the basal ganglia forming a triangular mass the apex of which extended mediallybelow the medial lamina. It was to the upper surface of this mass close to its apexthat the lower component of the internal capsule was attached. The intermediatecomponent of the internal capsule extended to the pallium and consisted of finefibres interspersed with cells of the lower edge of the medial lamina and centre of thelateral lamina through which the fibres passed. Thus the intermediate portion of theinternal capsule, although not completely interrupting the continuity of the cells inthe laminae, was beginning to trace out the path which other fibres would followin order to complete the adult appearance of the internal capsule. Below and lateralto the internal capsule the triangular mass of mantle zone cells already mentionedhad the form of the adult lentiform nucleus in coronal section (P1. 1, fig. 8). Thus itwas apparent that the developing internal capsule was beginning to split the cellsof the two laminate into those joining the caudate nucleus on the ventricular sideand those forming the lentiform nucleus on the lower and outer side. Although theparts of the lentiform nucleus could not be distinguished yet, nevertheless, it wasnow possible to discern how the globus pallidus could be partially or completelyderived from the lower part of the medial lamina and the putamen could be formedfrom the lower part of the lateral lamina (Text-fig. 1).

In the 35 mm. embryo (P1. 1, figs. 4-8) the internal capsule consisted of a singlethick mass of fibres directed from the diencephalon, through the cerebral peduncle,obliquely forwards and laterally in the hemisphere and its course could only beunderstood by following it through serial sections with the aid of the reconstruction.Caudally the internal capsule (P1. 1, figs. 4, 5, I.C.) was visible as a thick mass offibres extending through the cerebral peduncle between the side of the diencephalonand the mass of the basal ganglia. The sections transacting this part of the internalcapsule were passing through the mantle zone deep to the posterior end of the de-veloping head of the caudate nucleus where its components were almost unitedinto the tail. It was, however, still possible to discern vaguely the two striatalelevations forming the head of the nucleus. Deep to the caudate nucleus the twocurved laminae of densely packed cells were even more clearly visible and nowobviously continuous with the cells of the striatal elevations deep to which they wereeach placed as before (P1. 1, figs. 4, 5, M.L., L.L.). The internal capsule extendedbetween the medially directed concavity of the medial lamina to the diencephalon.Below the internal capsule the cells of the medial lamina formed a tongue-likeprocess separated from the apex of the triangular mass, of the lower part of thelateral lamina, by a thin layer of paler-staining loosely packed cells. This tongue-

194 W. Hewittlike process was probably the portion of the medial lamina involved in the formationof the globus pallidus. In addition, a strand of cells was also given off from thelower part of the lateral lamina and joined the tongue of cells from the mediallamina. This may also have been concerned with the formation of the globuspallidus. Proceeding rostrally (PI. 1, fig. 6) the internal capsule transacted bothlaminae and the lentiform nucleus was apparent and resembled that seen in the25 mm. embryo. The only other feature was the tongue-like processes from bothlaminae.Even further rostrally (PI. 1, figs. 7, 8) the cells of the mantle zone deep to the

developing caudate nucleus were undifferentiated and had no characteristic arrange-ment. Because of its oblique course, in those sections traversing the anterior endof the forwardly directed internal capsule, only the outer part of the internal capsulewas visible just deep to the junctional zone between caudate nucleus and pallium(P1. 1, figs. 7, 8, I.C.). Here the internal capsule had fanned out and was arranged inbands or bundles with intervening strands of cells resembling the adult anteriorlimb of the internal capsule, where this is similarly split by strands of grey matterextending between the putamen and caudate nucleus.

In the 70 and 135 mm. foetuses the internal capsule, caudate nucleus and lenti-form nucleus were similar in form and resembled the adult appearance, and thesetwo foetuses were considered together. In both specimens the third element of thecaudate nucleus, the intermediate striatal elevation, had intervened between theother two (Hewitt, 1958). At these stages of development the internal capsulecompletely separated the cells forming the caudate and lentiform nuclei from oneanother, except rostrally where they were in continuity over the region of theolfactory tubercle. In earlier stages of development cells of each lamina forming thetwo nuclei were in continuity and these were able to keep pace with one anotherin their growth in width before the internal capsule split them. Nevertheless, in spiteof the intervention of the internal capsule between the two nuclei in these 70 and185 mm. foetuses the lentiform nucleus continued to increase in width and had keptpace with the increasing width of the caudate nucleus resulting from the additionof the intermediate striatal elevation. In the 70 mm. foetus the caudate nucleusconsisted of a thick deeply staining ependymal layer on its surface and deep to thisless densely staining, more loosely packed mantle layer cells as in previous embryosand foetuses. The lentiform nucleus consisted of loosely packed cells similar to thosein the deep part of the caudate nucleus. For the most part the globus pallidus wasstill not distinguishable in this foetus. In almost all sections of this specimen thestructure and staining reactions were uniform throughout the lentiform nucleus. Inone or two sections through the posterior end of the nucleus, however, its two partswere faintly distinguishable because the staining reaction of the globus pallidus wasslightly less intense than the putamen, and between the two a thin line was visiblebecause of the reduced density of the cells. On the other hand the globus palliduswas clearly defined in the 185 mm. foetus (Text-fig. 2). In this foetus the materialcomposing the caudate nucleus deep to the now thick ependymal layer and theputamen of the lentiform nucleus had not the homogeneous appearance of theprevious specimen, but consisted of scattered irregular masses of cells intermingledwith fibres. The cells comprising the globus pallidus on the other hand had retained

The human internal capsule and lentiform nucleustheir homogeneous appearance so that this was now clearly distinguishable. Inaddition, the material comprising the globus pallidus had become more eosinophilicin its staining reaction than the material comprising the putamen and deep part ofthe caudate nucleus and this provided another distinguishing feature. The palliumin both foetuses was also by now much thicker and more complex in structure, andthe internal capsule could easily be traced into the intermediate layer or deeperparts of the mantle zone.

DISCUSSION

Early in the development of the cerebral hemisphere a cup-shaped lamina of cellsforms in the mantle zone deep to the surface of the developing head of the caudatenucleus. The mouth of this lamina is directed towards the diencephalon and betweenthese a small bundle of fibres can be traced; this constitutes the beginnings of theinternal capsule. At a slightly later stage of development two concentric curvedlaminae of cells are present in the mantle zone. These lie deep to the medial andlateral striatal elevations, which at this stage of development are the only compo-nents present out of the three which ultimately contribute to the formation of thehead of the caudate nucleus. The cells of these laminae are in continuity with thecells deep to the striatal elevations; the medial lamina lying deep to the medialstriatal elevation and the lateral lamina lying deep to the lateral striatal elevation.The concavities of these laminae are also directed towards the diencephalon. Theselaminae can be clearly seen in the illustrations of Hochstetter (1929). When thesetwo laminae are first seen the internal capsule consists of three parts. The upperpart extends between the medial lamina and the diencephalon. This lamina andthis part of the internal capsule are probably equivalent to the single lamina andbundle of fibres first seen. The lower part of the internal capsule extends betweenthe diencephalon and the apical region of the triangular mass of cells into which thelower edge of the lateral lamina thickens. Between these two parts of the developinginternal capsule a third set of fine fibres, interspersed with cells, extends between thepallium anddiencephalon transacting bothlaminae. Thus a simple internal capsule hasfor med and below and lateral to this it is now possible to discern the triangular shapeof the lentiform nucleus formed from both laminae. The cells in the striatal elevationson the ventricular side of the internal capsule contribute to the formation of thecaudate nucleus (Text-fig. 1). Later by the further addition of fibres the internalcapsule becomes a single bundle of fibres transacting the two cellular laminae.The precise contribution made by the two laminae to the two parts of the lenti-

form nucleus is not completely clear. Undoubtedly much of the putamen is derivedfrom the lower part of the outer lamina, but whether it contains cells derived fromthe medial lamina adjacent to it is difficult to determine by the examination ofserial sections alone. It is very reasonable to conclude that the globus pallidus isderived from that part of the inner lamina which lies below the site of its transactionby the internal capsule, and this lamina being the first of the two to develop wouldconform with the belief that the globus pallidus, often called the paleostriatum, isphylogenetically the oldest part of the lentiform nucleus. The globus pallidus may,however, also receive a contribution either from the tongue of cells from the laterallamina or from its lower lip where this curves below the medial lamina (Text-fig. 1

195

196 W. Hewittand PI. 1, figs. 4, 5). The determination of the structures contributing to the globuspallidus might be aided by locating the iron which is said to be present in structuresderived from the paleostriatum.The third and major component of the caudate nucleus or intermediate striatal

elevation develops when the internal capsule has completely transacted the twolaminae except over the region of the olfactory tubercle. Thus it is impossible for athird lamina to develop as a continuous sheet alongside, or between the other two,having continuity with the intermediate striatal elevation and developing lentiform

Lateral Lateralventricle and intermediate

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,-Pallium %% Medial

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Medial striatalI/ elevation ,nai

Putamen - - capsuloe

Globus pallidus Puaent~,~ 'Internal \ / AdIGlobus pallidus rn

capsule \ , c

Diencephalon Anterior commissure

Fig. 1 Fig. 2

Text-fig. 1. Diagram of a coronal section through the basal part of an embryonic left cerebralhemisphere. The curved inner and outer laminae of cells in the mantle layer have been madecontinuous with the medial and lateral striatal elevations contributing to the formation of thehead of the caudate nucleus. The internal capsule can be seen extending between the dien-cephalon and pallium transacting the inner and outer laminae. The uninterrupted lines of theinternal capsule represent early stages in its development from the concavity of the mediallamina. The interrupted lines depict extension of the internal capsule laterally to the palliumnthrough both laminae subdividing them. The illustration demonstrates the globus pallidusforming from the lower part of the inner lamina and the derivation of the putamen from thelower part of the outer lamina. The part of the lateral lamina (putamen) marked in whitebelow the inner lamina is to show how this could contribute to the globus pallidus.

Text-fig. 2. Line drawing of coronal section through the left cerebral hemisphere of a 135 mm.C.R. length foetus. The caudate nucleus consists of three striatal elevations. The lentiformnucleus is divided into putamen and globus pallidus. Note the section has transacted theanterior commissure where it lies immediately below the lentiform nucleus. The dotted lineshave been drawn upwards from the inner and outer borders of the putamen following itscurvature. The lateral and intermediate striatal elevations of the caudate nucleus lie betweenthese lines suggesting that the putamen is equivalent to these parts of the caudate nucleus.

The human internal capsule and lentiform nucleus 197nucleus such as existed with the first two laminae before their transaction by theinternal capsule. Nevertheless, the caudate and lentiform nucleus keep pace withone another in their subsequent increase in transverse width. This can be shown ina coronal section of a fully differentiated or adult lentiform nucleus by drawing twolines upwards from the inner and outer borders of the putamen following theircurvatures, as in Text-fig. 2, and it will be found that these intersect the caudatenucleus at the junction of its medial and intermediate striatal elevations and itsouter edge, respectively. Thus it may be concluded that the putamen increases inwidth by the addition of further cells at the same time as the caudate nucleusbroadens after the intermediate striatal elevation appears. This occurs even thougha third continuous lamina cannot be formed between the putamen and intermediatestriatal elevation because by this time the internal capsule is intervening.The orientation of the medial lamina and the direction of its mouth determines

the path of the internal capsule. The internal capsule may be likened to a bulletfired from a rifle. The path it follows is governed by the line of aim or direction inwhich the rifle is pointing. When the internal capsule cuts through the lamina itfollows the line of the fibres emerging from the mouth of the lamina just as thebullet, if it could fire backwards through the breech, would follow the same line ofaim as before. The internal capsule joins the pallium at its point of junction with thelateral edge of the caudate nucleus, but it could pass through the basal ganglia inany direction to join the pallium below this point. It could not, however, join thepallium at a higher point, otherwise it would have to pass through the cavity of theventricle. No reason can be advanced, however, to explain why the internal capsuledoes not traverse the basal ganglia in a different line or even pass below them tojoin the pallium at a lower point.When the fibres forming the internal capsule first begin to intersect the cells of the

laminae the fibres and cells are intermingled; the cells predominating. As furtherfibres are added to the internal capsule the cells diminish in number until theydisappear except in the anterior limb of the internal capsule where they remain asstrands connecting the putamen and caudate nucleus and at the extreme anteriorend over the olfactory tubercle where the two nuclei are completely fused. Thisstate of affairs is first seen in the 35 mm. C.R. length embryo (P1. 1, fig. 7). Fromthese facts it would be reasonable to deduce that the fibres of the internal capsuleare being built up from behind forwards so that in the anterior limb of the internalcapsule the number of fibres present is insufficient to completely obliterate thestrands of grey matter connecting the caudate nucleus and putamen. The part of thecaudate nucleus fused with the putamen over the olfactory tubercle could reasonablybe expected to occur with that part of the caudate nucleus derived from the lateraland intermediate striatal elevations, if the deductions discussed and illustrated inTex-fig. 2, concerning the corresponding parts of the putamen and caudate nucleus,are correct. A number of efforts to demonstrate this have been made but they havealways implicated the medial striatal elevation in the fusion. One possible explana-tion for this is that the globus pallidus is derived from the central portion of themedial lamina of cells and the lower end of this contributes to part of the putamen.

W. Hewitt

SUMMARY

1. The development of the human internal capsule and lentiform nucleus hasbeen studied using serial coronal sections of the heads or cerebral hemispheres ofembryos and foetuses of C.R. lengths ranging from 7-5 to 135 mm.

2. In the mantle zone deep to the developing caudate nucleus in a 15 mm.C.R. length embryo a cup-shaped lamina of densely grouped cells appears with itsconcavity facing towards the diencephalon. Later in the 25 mm. C.R. length embryoanother dense collection of cells in the mantle zone forms a second curved laminalateral to the first and both laminae become continuous with the cells deep to theirrespective striatal elevations.

3. The internal capsule first appears as a small bundle of fibres in the con-cavity of the first or medial lamina of cells in the 15 mm. embryo. These can befollowed between the lamina and diencephalon. In the 25 mm. embryo the mediallamina becomes larger and the fibres from it increase in length and number. A secondbundle of fibres also develop from the lower edge of the lateral lamina. These twobundles are parallel, directed towards the diencephalon and between them the cellsof both laminae are less densely arranged. Among these cells fine fibres can be seenextending to the pallium. In the 35 mm. embryo all three parts of the internalcapsule seen in the last embryo have united into a single thick bundle directedobliquely between the diencephalon and the pallium splitting both laminate of cells.

4. When the internal capsule has split the laminate the parts of these deep to thestriatal elevations contribute to the formation of the caudate nucleus. The part ofthe medial lamina below the internal capsule forms much if not all of the globuspallidus, although it may contribute to the putamen where it lies adjacent to thelateral lamina. The part of the lateral lamina deep to the internal capsule forms theputamen but it is possible that it contributes to the globus pallidus also.

5. The 70 and 135 mm. foetuses in many ways resemble the adult appearance.The globus pallidus is faintly visible in the 70 mm. foetus and clearly distinguish-able in the 135 mm. foetus. In both these foetuses the third or intermediate striatalelevation has commenced developing in the caudate nucleus, and although the in-ternal capsule is separating the caudate nucleus from the lentiform the width of thelatter keeps pace with the increasing width of the caudate nucleus. It appears thatthe putamen is the part of the lentiform nucleus which corresponds to that part ofthe caudate nucleus derived from the lateral and intermediate striatal elevations.

6. The direction of the internal capsule is discussed and is probably originallydetermined by the orientation of the medial lamina when this is first formed. Theinternal capsule forms along an axis at right angles to the centre of the mouthof this lamina.

7. The parts of the developing laminate involved in the fusion between the frontend of the putamen and the head of the caudate nucleus are discussed but noconclusions are reached.

My thanks are due to Mr A. V. Freeborn for his technical assistance and toMr J. S. Fenton for carrying out the photography.

198

Journal of Anatomy, Vol. 95, Part 2 Plate 1

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The human internal capsule and lentiform nucleus 199

REFERENCESAREY, L. B. (1954). Developmental Anatomy, 6th ed. Philadelphia and London: Saunders.FRAZER, J. E. (1931). A Manual of Embryology, 1st ed. London: Baillibre, Tindall and Cox.HAMILTON, W. J., BOYD, J. D. & MOSSMAN, H. W. (1952). Human Embryology, 2nd ed. Cambridge:

W. Heffer.HARRISON, R. G. (1959). A Textbook of Human Embryology. Oxford: Blackwell.HEwiTT, W. (1958). The development of the human caudate and amygdaloid nuclei. J. Anat.,

Lond., 92, 377.HOCHSTETTER, F. (1929). Beitrage zur Entwicklungeschichte des menschlichen Gehirns, 1st ed.

Wein und Leipzig: Deuticke.JOHNSTON, T. B., DAVIES, D. V. & DAVIES, F. (1958). Gray's Anatomy, 32nd ed. London, New

York and Toronto: Longmans Green.PATTEN, B. M. (1946). Human Embryology, 1st ed. London: Churchill.STREETER, G. L. (1912). Manual of Human Embryology, ed. by F. Kiebel and F. P. Mall, vol. 2,

1st ed. Philadelphia and London: Lippincott.

EXPLANATION OF PLATECoronal sections through the cerebral hemispheres of human embryos. Except Fig. 1, all are

sections of left cerebral hemispheres. Figs. 2-8 only include the region of the basal ganglia. Allsections have been stained with haematoxylin and eosin and are 10/is thick.Fig. 1. Cerebral vesicles of a 7*5 mm. C.R. length embryo showing striatal elevations in each side

wall composed of an undifferentiated mantle zone deep to the darker staining ependymalzone. x 25.

Fig. 2. 15 mm. C.R. length embryo. In the mantle zone is a small curved lamina of cells deep tothe groove between the medial and lateral striatal elevations. In the concavity of the lamina,which is directed medially, lies a small collection of fibres, the early form of the internalcapsule. x 30.

Fig. 3. 25 mm. C.R. length embryo. The internal capsule is composed of three parts. The upperpart is emerging from the centre of a dense mass of mantle zone cells, the medial lamina, deepto the medial striatal elevation. The lower part of the internal capsule is somewhat smallerthan the upper part and is continuous with the lateral lamina of cells lateral to the mediallamina. The lower part of this lateral lamina is most conspicuous by its triangular shape andhas the form of the lentiform nucleus. When followed up this lamina becomes continuous withcells deep to the lateral striatal elevation. Between the two parts of the internal capsule, andin line with these but extending further laterally, the cells are much less densely arrangedbecause intermingled with them are the fine fibres of the third part of the internal capsuleextending to the pallium. These fibres cannot be seen in the photograph. Note also the corticallayer in the pallium extending over the surface of the upper part of the basal ganglia. x 25.

Fig. 4. 35 mm. C.R. length embryo. This section is through the posterior end of the internal capsule,a solid mass of fibres in the cerebral peduncle proceeding obliquely forwards as far as themedial lamina of cells. Note the tongue of cells from the lower edge of the medial laminaand another strand of cells from the lateral lamina joining the lower part of the mediallamina. x 10.

Fig. 5. Same embryo as Fig. 4, but further rostrally. The internal capsule is just beginning toextend through the medial lamina of cells. x 10.

Fig. 6. Same embryo as Figs. 4 and 5 but further rostrally. The internal capsule is beginning tointersect both laminae. x 10.

Fig. 7. Same embryo as Figs. 4, 5 and 6 but further rostrally. The internal capsule has reached thepallium and consists of bundles of fibres separated by bands of cells similar to the appearanceof the adult anterior limb of the internal capsule. x 25.

Fig. 8. Same embryo as Figs. 4 to 7. This shows the most rostral end of the internal capsule whichis small but in a similar position to that in Fig. 7. x 25.

ABBREVIATIONSD. Diencephalon L.L. Lateral lamina of cells M.L. Medial lamina of cellsI.C. Internal capsule L.S.E. Lateral striatal elevation M.S.E. Medial striatal elevation

13 Anat. 95