The anlage of the cerebellar nuclei in chick after early extirpation of the vestibular region

Transcription

1 /. Embryol. exp. Morplu, Vol. 12, Part 1, pp , March 1964 Printed in Great Britain The anlage of the cerebellar nuclei in chick after early extirpation of the vestibular region by SVEN-INGVAR RUDEBERG 1 From the Tornblad Institute of Comparative Embryology and the Institute of Zoology, University of Lund WITH TWO PLATES INTRODUCTION DIVERSE opinions have been advanced in the relevant literature concerning the embryological origin of the cells constituting the deep cerebellar nuclei in vertebrates. A number of different regions in the embryonic brain have in turn been regarded as the matrix region of the cerebellar nuclei; these are given below. 1. The vestibular nuclei. Dowd (1929, pig), Larsell (1935, opossum), Nistri, Fabiani & Cannizzaro (1959, man). 2. The cerebellar neural epithelium. Gentes (196, chick), Hajashi (1924, man), Tello (194, mouse), Baffoni (1956, cat), Baffoni & D'Ancona (1958, chick and pigeon), Baffoni (1959, toad). 3. The cerebellar neural epithelium, probably reinforced with cells from the superficial stratum granulosum, in species possessing such a cell-layer. Riideberg (1961, lamprey, dog-fish, pike, frog, chick, pigeon, cow and man). 4. The inner cell layer of the embryonic cerebellar cortex. Loewe (188, rabbit [nucleus lateralis cerebelli]). Nistri & Fabiani (1956, man) traced the anlagen of the cerebellar nuclei from the neural epithelium of the cerebellum. In more advanced developmental stages, however, they repeatedly pointed out that the cerebellar and vestibular nuclei are connected by cell strands, which would indicate a close morphogenetic relation between these two nuclear complexes. Among the different opinions the first, accepting the origin from the vestibular nuclei, is the most firmly established in the relevant literature. The phylogenetic origin of the cerebellar nuclei from the vestibular complex was early advanced by, for example, van Hoevell (1916, different vertebrates), and Ariens Kappers (1921, different vertebrates). The shifting of cells from vestibular centres into the cerebellum was explained in agreement with Kappers' so-called' law of neurobiotaxis'. The different opinions on the origin of the cerebellar nuclei are all based on 1 Author's address: The Embryological Institute, University of Lund, Sweden.

2 52 SVEN-INGVAR RUDEBERG purely morphological studies. No experimental investigation has been performed in this particular field as far as the present author knows. He has, therefore, tried to analyse experimentally the relation between the cerebellar nuclei and one of their supposed sites of origin: the vestibular nuclei. The problem of the present study is: Do the cerebellar nuclei in chick arise embryologically through a migration of cells from the vestibular nuclei? METHOD In very young chick embryos (about 1 days, 1-16 somites) the presumptive vestibular region ('vestibular neuromere'), as well as the surrounding parts of the neural tube, was extirpated. Incubation of the eggs then continued until the TEXT-FIG. 1 TEXT-FIG. 2 TEXT-FIGS. 1 and 2. Schematic drawings of the rostral part of a chick embryo at stage of operation. Figure 1 shows the part of the neural tube extirpated in the V-operation ('vestibular neuromere'). Figure 2 shows the part removed in the E-operation (from just caudal to the cerebellar anlage to level with the caudal part of the second pair of somites), c = cerebellar anlage, fn = 'facialis-abducens neuromere', m = mesencephalon anlage, s = somites, tn = 'trigeminal neuromere'. age that the anlage of the deep cerebellar nuclei becomes clearly visible in normal chick embryos. The operation within the brain was performed with a thin glass needle. Incisions were made in the brain, one at the rostral limit and one at the caudal limit of the lesion, and the brain tissue in between was carefully removed. After sealing the egg with a piece of tape it was returned to the incubator. For details of the operation the reader is referred to Hamburger (1942).

3 Anlage of the cerebellar nuclei in the chick 53 Two types of operations, V and E, have been performed by the present author. In the V-operation (Text-fig. 1), only the 'vestibular neuromere' was removed. In the E-operation (Text-fig. 2), the neural tube was extirpated from a point just caudal to the cerebellar anlage to a point level with the caudal part of the second pair of somites. Surviving embryos were fixed after a total incubation time of between 7 to 1 days, HH 3-HH 36 (HH followed by a number indicates the developmental stage according to the normal stages in the development of the chick, as described by Hamburger and Hamilton (1951)). During this period the cerebellar nuclear anlage in normal chick embryos develops and differentiates into its various parts. In all, 35 operations have been performed: 128 in the V-series, and 177 in the E-series. MATERIAL One hundred and nine embryos survived the operation until fixation: thirtynine in the V-series, and seventy in the E-series. However, many were to some extent malformed, showing, for example, a ventrally open thorax and abdomen, or failure of the face processes to fuse normally. Moreover, several embryos revealed a more or less high degree of so-called 'overgrowth' within the mesencephalon (concerning 'overgrowth', see, e.g., Bergquist 1959, chick). As expected, the lesions in the brain are very different in the two series of operations. In embryos belonging to the V-series, where the extirpation was restricted to the 'vestibular neuromere', the brain is certainly seriously damaged in the region of operation, but in almost all of them the continuity of the brain is maintained. Very few embryos in this series show a distinct gap in the brain for some few sections. Only one of these embryos will be described in the following text. In the E-series, where the neural tube was extirpated from just caudal to the cerebellar anlage to the level of the second pair of somites, many embryos show large gaps in the brains, amounting in some instances to almost 1 sections. However, the critical point is not so much the size of the gap, but rather that a distinct gap is present between the cerebellum and any anlagen of vestibular nuclei that may occur. But even in the embryos with a gap in the brain just caudal to the trigeminal entry, a tip of brain tissue is always present caudal to this nerve. In some embryos this caudal tip in front of the gap is fairly large and may contain some presumptive vestibular nuclear material, although no such nuclei have been identified in this part of the brain. To try to reduce this caudal tip of brain tissue rostral to the gap, the rostral incision in a series of twelve surviving E- embryos was made somewhat further rostrally at operation, i.e. at the very caudal limit of the cerebellar anlage. Abnormal cerebella were found in almost all these embryos, and most of them showed ' overgrowth' within the mesencephalon. The extension of the operation so far rostrally thus seems impossible without

4 54 SVEN-INGVAR RUDEBERG increasing highly the frequency of malformations. The whole experimental material of embryos in the E-series is given in Table 1. The material used for the following description was selected from embryos with no 'overgrowth' within the mesencephalon and with so short a tip of brain tissue caudal to the trigeminal entry that any vestibular nuclear anlagen can hardly be present in this part of the brain. Some microphotographs of sections from the caudalmost tip of the brain in front of the gap are shown. The developmental stage of the embryos to be described varies from HH 3 to HH 36. Thus, there is obtained a developmental series that covers the critical period during which the cerebellar nuclear anlage in normal chick embryos forms and differentiates into its different portions. All embryos were killed in Bouin's fixing fluid, embedded in paraffin, and sectioned serially transverse in relation to the long axis of the brain tube in the cerebellar region. The section thickness was 1 or 15 fx and the staining method haematoxylin (Mayer)-eosin. RESULTS The V-series: Extirpation of the 'vestibular neuromere' HH 32-33: Operation number V16 m Fig. B Fig. A TEXT-FIG. 3 TEXT-FIGS. 3-1 show graphic reconstructions of parts of the brains in the embryos described. Caudal to the trigeminal entry, more or less extensive gaps are present in all the brains. The rostral limits of the lesions are marked with dashed lines. The section lines indicate the levels of the microphotographs: c = cerebellar anlage, m = mesencephalon anlage, nv = entry of trigeminal nerve, nviii = entry of vestibular nerve. Magnification for all graphic reconstructions: x 2.

5 Stage at fixation (HH) Total number Normal Mesencephalon 'Overgrowth' Slight Strong TABLE 1 The E-series Cerebellum Operation region Brain tissue between nerve V and gap Normal Retarded Abnormal Gap caudal Gap caudal Continuous Uinut MS Short to nerve V to nerve VIII remnants Remnants Long remnants ll Comment $ C3- & ^ * 1 whole brain malformed 2 open ventrally 2 open ventrally 5 open ventrally 6 open ventrally 1 open ventrally s! '. ick

6 56 SVEN-INGVAR RUDEBERG The lesion in the brain extends rostrally to the level indicated by a dashed line in Text-fig. 3. At this level the median raphe disappears, as well as the median longitudinal sulcus. Further caudalwards the brain gradually decreases in size. Plate 1, A, shows a section through the caudalmost tip of the brain in front of the gap at the level indicated in Text-fig. 3. No vestibular nuclear anlagen can be identified in this part of the brain. For some few sections caudal to this level, no brain tissue is present as is seen in the graphic reconstruction. The entry of the vestibular nerve is indicated in Text-fig. 3, and somewhat further caudalwards distinct cochlear nuclei are present. The cerebellar anlage is well developed and appears to be normal (Plate 1, B). The superficial stratum granulosum, the inner cortical cell layer, and the medial migration layer (Migr. B), are readily identified. These structures have earlier been described by the present author in normal chick embryos of about the same age (Riideberg, 1961). The anlage of the cerebellar nuclei is well developed and appears to be normal. In the rostralmost sections, the anlage constitutes a clearly delimited, somewhat rounded cell mass. Caudally it gradually becomes elongated, with the long axis passing from dorso-medially to ventro-laterally. Further caudalwards, at the level depicted in Plate 1, B, the nuclear anlage shows a clear tendency towards differentiation into two portions: the lateral N 1} and the medial N 2, described earlier in normal chick embryos by the present author (1961). In the transverse sections, these portions of the nuclear anlage appear as two elongated cell bands, separated by nerve bundles but fused dorso-medially. PLATE 1 FIG. A. Section through the caudalmost tip of the brain in front of the gap in Embryo V 16, stage HH 32-33, at the level indicated in Text-fig. 3. No vestibular nuclear anlagen can be identified in this part of the brain. nv = entry of trigeminal nerve. FIG. B. Section through the cerebellar anlage in Embryo V 16, stage HH 32-33, at the level indicated in Text-fig. 3. The cerebellum appears to be normally developed as well as the nuclear anlage, which is differentiated into the portions N x and N 2, fused dorso-medially. Within the dorso-medial part of the cerebellum, the medial migration layer, Migr. B, forms a large cell mass, en = cerebellar nuclear anlage, ic = inner cortical cell layer, MB = medial migration layer of the cerebellum, ssg = superficial stratum granulosum. FIG. C. Section through the cerebellar anlage in Embryo E 17, stage HH 3, at the level indicated in Text-fig. 4. A distinct cerebellar nuclear anlage is present, but no differentiation into the portions N x and N 2 can be seen at this stage. Dorsal to the ventricular angle the rostral part of the 'ventro-caudal continuation' of the cerebellar nuclear anlage is seen, en = cerebellar nuclear anlage, MB = medial migration layer of the cerebellum, ssg = superficial stratum granulosum, vc ='ventro-caudal continuation' of the cerebellar nuclear anlage. FIG. D. Section through the caudalmost tip of the brain in front of the gap in Embryo E 122, stage HH 32, at the level indicated in Text-fig. 5. No vestibular nuclear anlagen can be identified. The trigeminal ganglia are connected with each other through an anastomosis of nerve fibres, ita = intertrigeminal anastomosis, tg = trigeminal ganglion.

7 SVEN-INGVAR RODEBERG ssg (Facing page 56)

8 /. Embryol. exp. Morph. Vol. 12, Part 1 SVEN-INGVAR RODEBERG PLATE 2 {Facing page 57)

9 Anlage of the cerebellar nuclei in the chick 57 Further caudalwards the distinction between N x and N 2 disappears, and a rounded nuclear mass is formed, which can be followed far caudalwards through the cerebellar anlage. The E-series: Extirpation of the neural tube from a point just caudal to the cerebellar anlage to a point level with the caudal part of the second pair of somites HH3: Operation number E17 The lesion in the brain begins rostrally at the level indicated by a dashed line in Text-fig. 4. At this level the median raphe disappears, as does, somewhat further nv Fig. C TEXT-FIG. 4 caudally, the median longitudinal sulcus. From the level of the trigeminal entry the brain rapidly decreases in size. No vestibular nuclear anlagen can be identified in the tip of the brain rostral to the gap. Further caudalwards the brain disappears PLATE 2 FIG. E. Section through the cerebellar anlage in Embryo E 122, stage HH 32, at the level indicated in Text-fig. 5. The cerebellar nuclear anlage, as well as its 'ventro-caudal continuation', is clearly seen, en = cerebellar nuclear anlage, ic = inner cortical cell layer, MB = medial migration layer of the cerebellum, ssg = superficial stratum granulosum, vc = ' ventro-caudal continuation' of the cerebellar nuclear anlage. FIG. F. Section through the caudalmost tip of the brain in front of the gap in Embryo E 114, stage HH 34, at the level indicated in Text-fig. 7. oc = otic capsula, tg = trigeminal ganglion. FIG. G. Section through the cerebellar anlage in Embryo E 114, stage HH 34, at the level indicated in Text-fig. 7. The cerebellar nuclear anlage is differentiated into the portions Nj and N 2, in the transverse section appearing as two elongated cell bands, fused dorsomedially. ic = inner cortical cell layer, ssg = superficial stratum granulosum. FIG. H. Section through the cerebellar anlage in Embryo E 145, stage HH 36, at the level indicated in Text-fig. 1. The caudal part of the cerebellar nuclear anlage is clearly differentiated into the portions N 3 and N 4.

10 58 SVEN-INGVAR RUDEBERG completely for about fifty sections. The otic vesicles are dislocated and very rudimentary, as also are the vestibular nerve and ganglion. The cerebellar anlage appears to be normally developed (Plate 1, C). The superficial stratum granulosum, as well as the medial migration layer, Migr. B, is easily identified. Further caudalwards the faint rudiment of the inner cortical cell layer is also present. As is seen from Plate 1, C, the cerebellar nuclear anlage is quite distinct. No differentiation into the portions Nj and N 2, however, can be seen at this developmental stage which agrees with the findings in normal chick embryos of the same age. Further caudalwards in the cerebellum, no such a distinct nuclear anlage is organized as yet. This, too, agrees with the conditions seen in normal chick embryos of the corresponding age. The so-called' ventro-caudal continuation' of the cerebellar nuclear anlage, described earlier in normal chick embryos by the present author (1961), is clearly seen just dorsal to the ventricular angle and can be followed far caudalwards through the cerebellar anlage. HH 32: Operation numbers E122 and E162 The rostral limits of the lesions are indicated in Text-figs. 5 and 6. Especially in Embryo E 122, the median part of the medulla protrudes strongly into the ventricle, as is seen from the graphic reconstruction of this embryo. A section through Fig. D TEXT-FIG. 5 the caudalmost tip of the brain rostral to the gap in Embryo E 122, is shown in Plate 1, D. No vestibular nuclei can be identified here. This Figure also shows that the trigeminal ganglia in this embryo are connected with each other through an anastomosis of nerve fibres. Further caudalwards the brain completely disappears, and a large gap is present. In Embryo E 122, the otic vesicles are rudimentary and no vestibular nerves or ganglia are found. Nor is there any vestibular nerve in Embryo E 162. The cerebellum, as well as its nuclear anlage, appears to be normally developed in both embryos (Plate 2, E). The nuclear anlage is further developed and has a

11 Anlage of the cerebellar nuclei in the chick 59 TEXT-FIG. 6 greater rostro-caudal extension than in the preceding stage. In its rostralmost part, the nuclear anlage is rounded, as seen in transverse sections, but caudally it becomes more elongated, the long axis passing from dorso-medially to ventrolaterally. The * ventro-caudal continuation' of the cerebellar nuclear anlage is seen in both embryos. HH 34: Operation numbers E114, E116 and E119 Graphic reconstructions of the actual brain parts in these embryos are shown in Text-figs. 7-9, where the rostral limits of the lesions are indicated by dashed lines. Fig. G TEXT-FIG. 7 Fig. F A section through the caudalmost part of the brain in front of the gap in Embryo E114 is seen in Plate 2, F. In this embryo the dislocated otic vesicles, as well as the

12 6 SVEN-INGVAR RUDEBERG vestibular nerve, are rudimentary. No vestibular nuclei can be identified. As far as can be judged from the haematoxylin-stained sections, the vestibular nerves in TEXT-FIG. 8 Embryo E119 appear to anastomose with each other in the mid-line of the embryo within the region where the brain would normally be situated. The cerebellar anlage seems to be normally developed (Plate 2, G). The cells of the medial migration layer, Migr. B, in earlier stages forming a large, dorsomedial cell cluster, have spread out within the cerebellum. This has also occurred TEXT-FIG. 9 in normal chick embryos of the same developmental stage (HH 34). The superficial stratum granulosum and the inner cortical cell layer, on the other hand, are readily identified., The rostral part of the cerebellar nuclear anlage is differentiated into the portions Nj and N 2. At the level depicted in Plate 2, G, these portions constitute, in the transverse sections, two elongated cell bands, fused dorso-medially. The

13 Anlage of the cerebellar nuclei in the chick 61 so-called ' ventro-caudal continuation' of the cerebellar nuclear anlage, seen in earlier stages, appears to form the ventral part of N 2. This condition has earlier been demonstrated in normal chick embryos by the present author (1961). The caudal part of the cerebellar nuclear anlage is differentiated into two rounded cell masses: the dorsal N 3, and the ventral N 4 (cf. Plate 2, H). As in normal chick embryos, these two portions are not clearly separated from each other in every section, but in most instances the limit between N 3 and N 4 is at least marked by an isthmus. HH 36: Operation number E145 A graphic reconstruction of this embryo is seen in Text-fig. 1. The gap in the brain is not very large, although quite distinct. The cerebellum is now relatively Fig. H TEXT-FIG. 1 advanced in its development and possesses several furrows. The different portions of the cerebellar nuclear anlage: N l5 N 2, N 3 and N 4 are readily identified. The differentiation of the cell masses N 3 and N 4 is especially clear in this embryo and a section through these nuclear anlagen is shown in Plate 2, H. DISCUSSION The preceding description of a number of chick embryos, in which the vestibular region had been extirpated in very early developmental stages, seems to show that the morphological development of the cerebellum, as well as its central nuclei, proceeds quite normally during the actual period (HH 3-HH 36). The develop-

14 62. SVEN-INGVAR RUDEBERG ment of the superficial stratum granulosum, the inner cortical cell layer, and the medial migration layer, Migr. B, follows the same pattern as seen in normal chick embryos. This also applies to the anlage of the cerebellar nuclei: the morphological appearance of the nuclear anlage, its rostro-caudal changes in form, and its differentiation into the different portions N l5 N 2, N 3 and N 4 all agree with the conditions seen in normal chick embryos of a corresponding age. This result must be considered as a serious objection to the opinion that the cerebellar nuclei in chick develop embryologically through a migration of cells from the vestibular nuclei. On the other hand, it is possible that single cells from the vestibular nuclei in normal embryos migrate into the anlage of the cerebellum without this being revealed by studying the pure morphology of the cerebellar nuclear anlage, as done in the present paper. In order to tackle such a problem, it would be necessary to make careful quantitative studies of the cell populations in the cerebellar nuclear anlagen and to compare the results in normal and operated embryos. But even though such an addition of cells occurred, it must reasonably be considered of secondary importance, as the anlage of the cerebellar nuclei in chick develops morphologically normally also in the absence of the vestibular nuclei. The wellknown, strong tendency of the central nervous system towards self-differentiation during early development also apparently applies to the cerebellum and its nuclear anlage. The experimental result seems thus to support Riideberg's (1961) opinion, based on morphological studies, that the cerebellar nuclei cannot, at least in their principal part, be considered as taking their embryological origin from the vestibular nuclei. SUMMARY 1. An experimental investigation was made into the question of whether the deep cerebellar nuclei arise embryologically through a migration of cells from the vestibular nuclei, as has been suggested in several morphological papers dealing with different vertebrate species. 2. In very young chick embryos (about \\ days), the neural tube was extirpated from a point just caudal to the cerebellar anlage to a point level with the second pair of somites. Incubation of the eggs then continued until the age at which the anlage of the deep cerebellar nuclei becomes clearly visible in normal chick embryos. 3. From an extensive material, comprising 19 serially sectioned embryos, a few stages are described. These embryos were selected according to the purity of the operation and the developmental stage of the embryo. A developmental series is thus described from HH 3-HH 36, in which stages the cerebellar nuclear anlage forms and differentiates into its different portions in normal chick embryos. 4. Despite large gaps in the brain and lack of vestibular nuclei, the morphology of the cerebellum, as well as its nuclear anlagen, appears to be normal during the actual period.

15 Anlage of the cerebellar nuclei in the chick The conclusion reached is that the deep cerebellar nuclei in chick cannot, at least in their principal part, be considered to develop embryologically through a migration of cells from the vestibular nuclei. RESUME L'ebauche des noyaux cerebelleux chez le poulet apres extirpation precoce de la region vestibulaire 1. On a etudie experimentalement l'hypothese de Porigine embryologique des noyaux profonds du cervelet par migration de cellules des noyaux vestibulaires, comme l'ont, suggere plusieurs articles morphologiques se rapportant a differentes especes de Vertebres. 2. On a extirpe le tube nerveux de tres jeunes embryons de poulet (environ 1 jour ), entre un point situe juste en arriere de l'ebauche cerebelleuse et un point situe au niveau de la deuxieme paire de somites. L'incubation des oeufs a ete pursuivie jusqu'a l'age auquel l'ebauche des noyaux cerebelleux profonds devient clairement visible chez les embryons normaux de poulet. 3. On a decrit quelques stades a partir d'un materiel important, comprenant 19 embryons debites en coupes seriees. Ces embryons ont ete choisis en fonction de la reussite de l'operation et du stade de developpement de l'embryon. Une serie embryologique est ainsi decrite des stades 3 a 36 (Hamilton et Hamburger) au cours desquels l'ebauche cerebelleuse se forme et se differencie en ses differentes parties chez l'embryon de poulet normal. 4. En depit de larges breches dans le cerveau et de l'absence de noyaux vestibulaires, la morphologie du cervelet, ainsi que ses ebauches nucleaires, s'averent normales pendant la periode etudiee. 5. On en conclut que les noyaux cerebelleux profonds du poulet, au moins dans leur majeure partie, ne peuvent etre consideres comme se developpant embryologiquement par migration de cellules a partir des noyaux vestibulaires. ACKNOWLEDGEMENT The investigation was supported by a grant from The Swedish Natural Research Council. REFERENCES ARIENS KAPPERS, C. U. (1921). Die vergleichende Anatomie des Nervensystems der Wirbeltiere und des Menschen. 2:2. Harlem. BAFFONI, G. M. (1956). Contributo alia conoscenza delta morfogenesi e dell'istogenesi cerebellare. Arch. zool. (jtai), Napoli, 41, BAFFONI, G. M. (1959). Osservazioni sulla morfogenesi ed istogenesi cerebellare in un anfibio anuro (Bufo bufo L.). Riv. neurobiol. 5, BAFFONI, G. M. & D'ANCONA, G. (1958). Osservazioni sulla morfogenesi ed istogenesi cerebellare in uccelli a prole inetta ed a prole precoce. Ace. Naz. Lincei, 8, 24, BERGQUIST, H. (1959). 'Overgrowth' im Kiickengehirn, experimentell dargestellt. Anat. Anz. 16,

16 64 SYEN-INGVAR RUDEBERG DOWD, L. W. (1929). The development of the dentate nucleus in the pig. /. comp. Neurol. 48, GENTES, L. (196). Recherches sur le developpement des noyaux centraux du cervelet chez le poulet. C.R. Ass. Anat. 8, HAJASHI, M. (1924). Einige wichtige Tatsachen aus der ontogenetischen Entwicklung des menschlichen Kleinhirns. Dtsch. Z. Nervenheilk. 81, HAMBURGER, V. (1942). A Manual of Experimental Embryology. Chicago University Press. HAMBURGER, V. & HAMILTON, H. (1951). A series of normal stages in the development of the chick embryo. /. Morph. 88, HOEVELL, J. J. L. D. van (1916). The phylogenetic development of the cerebellar nuclei. Proc. Acad. Sci. Amst. 18, LARSELL, O. (1935). The development and morphology of the cerebellum in the opossum. Part I. Early development. /. comp. Neurol. 63, LOEWE, L. (188). Beitrdge zur Anatomie und zur Entwickelungsgeschichte des Nervensystems der Sdugetiere und des Menschen. 1. Berlin. NISTRI, M. & FABIANI, F. (1956). Contributo alia conoscenza dell'ontogenesi dei nuclei cerebellari nell'uomo. Riv. neurobiol. 2, NISTRI, M., FABIANI, F. & CANNIZZARO, C. (1959). Considerazioni sui rapporti ontogenetici tra nuclei vestibolari e nuclei cerebellari nell'uomo. Riv. neurobiol. 5, RUDEBERG, S.-I. (1961). Morphogenetic studies on the cerebellar nuclei and their homologization in different vertebrates including man. Diss. Lund. TELLO, J. F. (194). Histogenese du cervelet et ses voies chez la souris blanche. Trab. Inst. Cajal Invest, biol. 32, {Manuscript received 25th April 1963; revised 18th July 1963)