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R O'Rahilly

Publications and source records attributed to R O'Rahilly.

At least 37 records · Page 2Linked to original sources
PubMed1990

The human rhombencephalon at the end of the embryonic period proper.

The human rhombencephalon at 8 postovulatory weeks (stage 23) is described and illustrated for the first time with the aid of silver-impregnated sections and graphic reconstructions. The motor and sensory trigeminal nuclei were among those studied, and the latter was found to be almost contiguous to the dentate nucleus. Fibers to the principal sensory nucleus join the mesencephalic trigeminal tract, which also seems to be connected with the motor fibers. Fine fibers from the sensory root join the tractus solitarius, which appears to receive connections from the facial, glossopharyngeal, and vagal nerves. Main and accessory abducent nuclei are evident. A part (the Kappenkern des Facialisknies) of the nucleus funiculi teretis is particularly prominent. The presence of the pyramidal decussation during the embryonic period is noted for the first time. The arrangement of nuclei and tracts at 8 weeks is shown to be closely similar to that present in the newborn, and it is likely that the rapid growth of the rhombencephalon during the embryonic period proper is associated with correspondingly early functional activity.

Cell Movement↗
PubMed1990

Ventricular system and choroid plexuses of the human brain during the embryonic period proper.

This morphological study, based on serial sections and graphic reconstructions at 4-8 postovulatory weeks (stages 11-23), is believed to be the first account of the ventricular system in staged human embryos. Closure of the caudal neuropore at stage 12 heralds the onset of the ventricular system and separates the ependymal from the amniotic fluid. After the appearance of the optic ventricle at stage 11, the cavity of the telencephalon medium is discernible at stage 13. At stage 14 the future cerebral hemispheres and lateral ventricles begin, and the rhomboid fossa becomes apparent. The medial and lateral ventricular eminences cause indentations in the lateral ventricle by stage 15. The hypothalamic sulcus is evident at stage 16. At stages 17-18 the interventricular foramina are becoming relatively smaller, and cellular accumulations indicate the future choroid villi of the fourth and lateral ventricles. The areae membranaceae rostralis and caudalis are visible in the roof of the fourth ventricle at stage 18, and the paraphysis is appearing. At stage 19 choroid villi are seen in the fourth ventricle, and a mesencephalic evagination (Blindsack) is detectable. Choroid villi are noticeable in the lateral ventricle at stage 20. An olfactory ventricle is present by stage 21. At about stages 21-23 the lateral ventricle has become C-shaped, so that anterior and inferior horns are visible. Several recesses, e.g., the optic, infundibular, and pineal, develop in the third ventricle during the embryonic period. Features of the ventricular system that do not become apparent until the fetal period include the posterior horn of the lateral ventricle, choroid plexus of the third ventricle, suprapineal recess, interthalamic adhesion, aqueduct, and apertures in the roof of the fourth ventricle.

Cerebral Ventricles↗
PubMed1990

The human brain at stages 18-20, including the choroid plexuses and the amygdaloid and septal nuclei.

The development of the human brain during the seventh embryonic week was studied in serial sections of 88 embryos, and graphic reconstructions were prepared. From stages 18 to 20 the cerebral hemispheres expand rapidly and become more and more distinct entities. The longitudinal fissure between them occupies approximately half of their rostrocaudal extent. In stage 20 they have progressed so far in organization that functional aspects (based on synapses in the primordial plexiform layer) are of importance. An advanced differentiation is also present in the amygdaloid body, which has at least four individual nuclei, and in the forebrain septum, which shows the nucleus of the diagonal band and the medial septal nucleus. This has a bearing on recent experimental studies that document the fundamental role of the septal nuclei with regard to behavioural and cognitive functions. Fibre connections between septal nuclei and hippocampus have appeared. A definitive internal capsule, however, is not yet present. The main connections with diencephalon and other parts of the brain are chiefly by fibres to or from the amygdaloid body by way of the lateral forebrain bundle. The olfactory areas are connected with the habenular nuclei by a well developed stria medullaris thalami. Globus pallidus externus, entopeduncular nucleus, and subthalamic nucleus are prominent features in the subthalamus. The main nucleus of the oculomotor nerve shows a dorsolateral and a ventromedial portion. The rhombic lip is mitotically active in all parts of the rhombencephalon, and seems to participate significantly in the formation of the intermediate layer of the cerebellum and of the cochlear nuclei. The sensory nucleus of the trigeminal nerve has appeared. In the cerebellum the cell layer thought to contain the future Purkinje cells develops. The cerebellar plate is organized into external and internal bulges, and is now connected with mid- and hindbrain through fibre bundles. The area thought to be the dentate nucleus and the supposed floccular region are especially rich in fibres. The accessory olivary nucleus appears in stage 19, and accessory nuclei of the abducent and hypoglossal nerves are evident in stage 20. The choroid plexuses of the fourth and lateral ventricles have appeared. In view of their advanced features, the study of embryos of stages 19-21 becomes increasingly relevant to questions of tissue transplantation.

Amygdala↗
PubMed1990

The human brain at stages 21-23, with particular reference to the cerebral cortical plate and to the development of the cerebellum.

The development of the human brain during the eighth embryonic week was studied in serial sections of 22 embryos, and graphic reconstructions were prepared. The cortical plate appears in stage 21 in the area of the future insula and is an excellent feature for staging. The internal capsule contains neocortical fibres. Its three main outlets begin to be present in stage 22 and lead to epithalamus, to dorsal thalamus, and to mesencephalon. At this time a well developed lateral olfactory tract can be seen. The anterior commissure appears in stage 23. A clear developmental relationship between claustrum and olfactory area is described for the first time in human embryos. The optic tract reaches the ventral area of the lateral geniculate body. Scattered fibres of the lateral lemniscus reach at least as far as the caudal mesencephalon, in which superior and inferior colliculi can be distinguished at stage 23; two caudal Blindsäcke containing ventricular recesses form in stage 23. The cerebellum is still present as a plate, but its internal bulge is considerably enlarged. It possesses radially- and tangentially-arranged cells; the latter form the external germinal layer. The dentate nucleus, as well as the inferior and superior cerebellar peduncles and some of the cerebellar commissures, are present. Compared with the highly developed and probably already functional remainder of the hindbrain, the cerebellar plate shows far less differentiation. Two caudal migratory streams (marginal and submarginal) are present and represent the corpus pontobulbare. The decussation of the pyramids appears in stage 23. This article concludes the study of the developing human brain during the embryonic period, from stage 8 to stage 23. The series was based on 340 serially-sectioned embryos and graphic reconstructions from 89 brains. No comparable investigation of the fetal brain is available.

Amygdala↗
PubMed1990

The human vertebral column at the end of the embryonic period proper. 4. The sacrococcygeal region.

The sacral and coccygeal vertebrae at 8 postovulatory weeks (the end of the embryonic period proper) have been studied by means of graphic reconstructions. The cartilaginous sacrum is now a definitive unit composed of five separable vertebrae, each of which consists of a future centrum and bilateral neural processes. The base of each neural process consists of an anterolateral or alar element, not present in the lumbar region, and a posterolateral part, which includes costal and transverse elements. The usual illustrations, in which the costal component is placed in the alar element, are incorrect. The future dorsal foramina (containing dorsal rami) face laterally in the embryo and are in line with the thoracicolumbar intervertebral foramina. Considerable differential growth is required to change the dorsal openings from a lateral to a dorsal positions. The intervertebral foramina transmit ventral rami, but pelvic foramina are not yet present. The lumbosacral plexus is completed by S.N.1-3; S.N.4, 5 and Co.N.1 form the pelvic plexus. The inferior hypogastric plexus and the hypogastric nerves are present. The sacrum takes part in the spina bifida occulta that characterises the entire length of the embryonic vertebral column. The coccygeal vertebrae, which are variable, were 4-6 in number in the present series. The first is the best developed. The ventriculus terminalis ends usually at the level of Co.V.1 and the spinal cord generally at Co.V.5. The coccygeal notochord ends commonly in bifurcation or trifurcation. 'Haemal arches' were not observed.

Anthropometry↗
PubMed1990

The human vertebral column at the end of the embryonic period proper. 3. The thoracicolumbar region.

The present study of the thoracicolumbar region continues an investigation of the vertebral column at 8 postovulartory weeks (the end of the embryonic period proper) by means of graphic reconstructions. The cartilaginous vertebrae have short neural processes associated with the normal spina bifida occulta present at this time. The separate cartilaginous centres that several authors believe to exist in the cervical and lumbar costal elements, but which have not been observed by the present authors, have been thought to be the forerunners of extrathoracic ribs. A distinction needs to be made, however, between such centres and ribs. Similarly, in the fetal period, ossific loci in the costal elements of CV 7 are very frequent, whereas cervical ribs in the adult are relatively rare. The neurocentral joints, and hence the boundaries between neural arches and centra, are unclear before ossification has begun and has progressed during the fetal period. The sternal bands are almost completely united and the scapula is high in position. Neural relationships aid in the determination of homologous parts within the vertebral column, but clarification of corresponding parts has not previously been possible within the embryonic period. Areas ventral to the dorsal rami are ribs in the thoracic region and costal elements in other regions. Areas underlying the dorsal rami are transverse processes in the thoracic region and minute 'true' transverse elements in the cervical and lumbar regions. Thus, the descriptive lumbar transverse processes correspond to the true transverse processes and the ribs in the thoracic region. The dorsal rami of the thoracic nerves pass between the transverse processes and the tubercles of the ribs and then divide. The ventral rami of lumbar Nerves 1 and 2 resemble the thoracic in their course, whereas those of Nerves 3-5 are similar to the sacral. The thoracic dorsal roots are sloping and, associated with the greater height of the lumbar centra, the lumbar roots even more so. The directions of the various dorsal roots reflect differences in growth gradients between vertebral column and spinal cord. The thoracic and lumbar portions of the column change little in proportion during the embryonic period proper.

Anthropometry↗
PubMed1989

Bidirectional closure of the rostral neuropore in the human embryo.

The length of each neuropore was measured in 23 human embryos of stages 10-12 (about 22-26 days), and the closure of the lips of the rostral neuropore was studied in 24 embryos of stage 11 (about 24 days), with particular reference to the terminal lip. Graphic reconstructions were prepared from two particularly suitable examples, and mitotic figures were plotted for one of these. The lengths of the rostral and caudal neuropores are basically similar, but the rostral opening closes 1 day earlier and more abruptly (within a few hours) than the caudal (which takes a day). Closure of the rostral neuropore in the human embryo is bidirectional, proceeding simultaneously from 1) midbrain and diencephalon 2 and 2) the telencephalic region adjacent to the chiasmatic plate. Species differences are emphasized. Closure at the terminal lip of the neuropore is by fusion of right and left neural folds, as occurs elsewhere during primary neurulation. The rostral end of the neural plate in the median plane is, in the human embryo, at the rostral limit of the chiasmatic plate. Histological differences, however, exist between closure at the terminal lip and that at the dorsal lip: the surface epithelium plays a more significant role at the terminal lip, and the seam is more visible and presumably stronger. In future anencephaly it has been found that fusion at the terminal lip may occur, although that at the dorsal lip is deficient.

Anencephaly↗
PubMed1989

Mediobasal prosencephalic defects, including holoprosencephaly and cyclopia, in relation to the development of the human forebrain.

Four very early synophthalmic embryos were studied in serial sections and reconstructed graphically by the point-plotting method. Three belonged to stage 16 (5 weeks) and one to stages 19/20 (7 weeks). Recently completed accounts and reconstructions of the normal brains of staged human embryos served as controls for comparison with the abnormal examples. The embryos shared in common: holoprosencephaly, arhinencephaly sensu stricto (absence of olfactory nerve fibers, bulbs, and tracts), presence of a proboscis, synophthalmia with two lens vesicles, a retarded telencephalic wall, absence of the mediobasal part of the telencephalon (the future septal area and the commissural plate: future anterior commissure and corpus callosum), irregularity of the diencephalon, mensural changes in the brain, absence of the rostral part of the notochord and consequent cranial defects, and small ganglia of the cranial nerves. Where it could be determined (at least in the three less advanced specimens), the adenohypophysial primordium was either small and isolated or was absent; a tentorial condensation appeared to be missing; and disturbances of the primordia of the orbital muscles and their innervation were noted. The corpus striatum is single and corresponds to only the diencephalic part (medial eminence) of normal embryos. Interference with induction by the prechordal plate at or before stage 8 (18 days) would be expected to affect the future mediobasal part of the neural plate (median prosencephalic dysgenesis) and the future optic primordium (cyclopia sensu stricto). Insufficient formation of material from the prechordal plate would account for disorders of the orbital musculature and, possibly, for inadequacy of the tentorium cerebelli. Disturbance a couple of days later (stage 9) would result in synophthalmia. Cyclopia and synophthalmia entail arhinencephaly and holoprosencephaly, both of which may arise independently. Defective distribution of the cephalic mesenchyme points to a derangement of the mesencephalic neural crest (stages 10 and 11), causing such features as an incomplete chondrocranium and reduction in size of the ganglia of the cranial nerves. Failure of bilateral division of the telencephalon would occur at or before 4 weeks (stages 13 and 14). It is concluded that all the above conditions arise during the first 4 postovulatory weeks.

Abnormalities, Multiple↗
PubMed1989

Interpretation of some median anomalies as illustrated by cyclopia and symmelia.

Anomalies that involve the median plane are heterogeneous, and their embryological basis varies widely. Cyclopia and symmelia present a number of similarities: 1) They would appear to arise by neither fusion nor merging but mainly through a failure in lateralization. 2) Mesenchymal deficiency is important in both: possibly disturbance of the prechordal plate in cyclopia and failure of the caudal eminence in symmelia. The caudal eminence is an important developmental feature that is only recently becoming clearer in the human embryo. 3) Disturbance of axial material seems to be essential in both. 4) The results of experimental teratogenesis and an analysis of normal human development confirm that these conditions arise early. The teratogenetic termination-periods in the human are probably 2 1/2 weeks for cyclopia sensu stricto (a median eye in a single orbit) and 3 weeks for cyclopia sensu lato, i.e., synophthalmia (paired ocular structures in a single orbit); 2 1/2 weeks for symmelia of the upper limbs (e.g., in cephalothoracopagus) and 3 1/2 weeks for symmelia of the lower limbs in a single individual. It is pointed out that in symmelia the limb buds, upper or lower, have failed to separate at their postaxial margins. This is in contrast to dimelia, in which the preaxial borders are missing and the postaxial margins are duplicated (postaxial dominance).

Humans↗
PubMed1989

The human brain at stage 17, including the appearance of the future olfactory bulb and the first amygdaloid nuclei.

The growth of the cerebral hemispheres rostrally and caudodorsally brings about a deepening of the interlongitudinal fissure, in which blood capillaries for the future choroid plexus develop. Accumulation of mesenchyme basally presages the formation of the nasal septum. The olfactory bulb and tubercle become outlined. The three main areas of the telencephalon, future archi-, paleo-, and neocortex, can be recognized. The amygdaloid body, which is related to the medial ventricular eminence, contains either one or two nuclei. Nerve fibres from the olfactory tubercle arrive and pass through the amygdaloid area. The first indication of a septal nucleus is recognizable. The lateral ventricular eminence is present but not pronounced. The hemispheric stalk joins the cerebral hemispheres to the ventral thalamus and to the diencephalic part of the medial ventricular eminence. The beginning of the future choroid plexus consists in the formation of blood vessels and necrotic changes in the roof of the telencephalon medium and in rostral growth of the anterior choroid artery. Necrotic processes in the future choroid epithelium are now localized at the periphery of the still multilaminar tissue. The sulcus medius and zona intrathalamica delimit the dorsal from the ventral thalamus. The dimesencephalic borderline passes through the commissural fibres in the roof: the rostral part of the commissure is the posterior commissure, the caudal part, the commissure of the superior colliculi. In the mesencephalon, the red nucleus has a laterorostral position with regard to the nucleus of the oculomotor nerve. The cells of the locus coeruleus are well distinguishable. The gustatory fibres begin to separate from the common afferent tract as the tractus solitarius. Inferior and superior salivatory nuclei are delineated.

Amygdala↗
PubMed1989

The human brain at stage 16, including the initial evagination of the neurohypophysis.

Thirty-nine sectioned embryos of stage 16 were studied. Up to this stage the amygdaloid body is derived entirely from the medial eminence, which was purely diencephalic in stage 14, but now extends also to the telencephalon. The area of the future olfactory bulb is indicated by the presence of olfactory fibres entering the brain wall; the future olfactory tubercle is characterized by cellular islands. The presence of the hippocampal thickening and various histological features make it possible to outline the main, future cortical areas already at this early stage: archi-, paleo-, and neopallium. Hippocampus and area dentata correspond to the areas identified by Hines (1922) and Bartelmez and Dekaban (1962) but not to those identified by Humphrey (1966). The interventricular foramen is wide. The cerebral hemispheres grow rostrally and dorsally, thereby forming the beginning of the longitudinal fissure. Apart from the commissure of the superior colliculi, which began to appear in advanced embryos of stage 14, fibres of the posterior commissure are now present in some specimens. The neurohypophysis is apparent in fewer than half of the embryos. The marginal ridge (zona limitans intrathalamica) separates the dorsal from the ventral thalamus. Cranial nerve 3 emerges from M2. M1 has become shorter. Important pathways are beginning: the olfactory route by the olfactory fibres and the medial forebrain bundle; the vestibular by vestibulocerebellar and vestibulospinal fibres; gustatory by chorda tympani, nervus intermedius, and tractus solitarius. Fibres of the cochlear nerve are noted. The first parasympathetic ganglia, submandibular and ciliary, are identifiable. Asymmetry of the cerebral hemispheres was noted in one specimen.

Brain↗
PubMed1989

Anatomical terminology, then and now.

Anatomical terminology, which had become chaotic by the nineteenth century, was codified in the BNA of 1895, when some 5,000 terms were carefully selected from among approximately 50,000 names. The BNA and its three major revisions (BR, INA, PNA) are here reviewed and placed in historical perspective. It is emphasized that many anatomical terms are very ancient and that the various nomenclatures are not 'new terminologies' but rather, for the most part, selections of already existing names. This can be seen clearly in the naming of the cranial nerves. Another example, the carpal and tarsal bones, is analysed in detail. Of the 8 carpal bones, for instance, the current names for 7 of them are those proposed by Henle in 1855. All the nomenclatures are, as they should be, in Latin, but it is understood that translations of many terms into other languages are necessary. Although views pro and con have been expressed, current usage favours the erect posture and the anatomical position as a basis, as well as the elimination of eponyms. In both teaching and research, the Nomina has been of great benefit in reducing drastically the number of unnecessary synonyms and in providing a coherent, internationally accepted system that is now the standard in anatomical textbooks. Hence, further use of the Nomina should be encouraged.

Anatomy↗
PubMed1989

[Atlas of stages of development of the human embryonic brain with graphic reconstructions of the median plane].

Reconstructed median sections of staged human embryos were prepared to provide fundamental insight into the developing human brain. 1. At its first appearance as neural plate at stage 8 and even better visible at stages 9 and 10, the rostral end of the CNS is at the chiasmatic plate, that is, at the rostral end of the diencephalon. The telencephalon is a later acquisition. 2. The main parts, already visible in the neural plate, become arranged as neuromeres as the neural tube develops. The neuromeres, defined by definite morphological features, are transversely arranged originally. As morphological entities they are visible longest in the rhombencephalon (up to stages 17-18). 3. The initially uniformly thick walls at stages 12 and 13 develop thickenings: medial and lateral eminences in the forebrain constitute parts of the corpus striatum. The hippocampus forms as a rostrocaudally growing C-shaped thickening in the medial wall of the cerebral hemisphere. The ventricular bulge develops in the alar areas of Rh 1 and the isthmic segment, and develops into the corpus cerebelli. 4. Internal sulci are important landmarks. The sulcus limitans separates alar and basal plates in the rhombencephalon and mesencephalon, but it does not continue into the forebrain. The hypothalamic sulcus separates the hypothalamus from the thalamus (epi-, dorsal, and ventral thalami); the sulcus circularis separates the olfactory bulb from the olfactory tubercle. 5. The embryonic brain develops from approximately day 18 to day 57, i.e., during a period of 5 1/2 weeks. It progresses from a neural plate to a highly complicated, almost spherical organ, parts of which, to judge from their morphology, should be capable of functional competence.

Anatomy, Artistic↗
PubMed1988

Computer ranking of the sequence of appearance of 40 features of the brain and related structures in staged human embryos during the seventh week of development.

The sequence of events in the development of the brain in human embryos, already published for stages 8-17, is here continued for stages 18 and 19. With the aid of a computerized bubble-sort algorithm, 58 individual embryos were ranked in ascending order of the features present. The increasing structural complexity provided 40 new features in these two stages. The chief characteristics of stage 18 (approximately 44 postovulatory days) are rapidly growing basal nuclei; appearance of the extraventricular bulge of the cerebellum (flocculus), of the superior cerebellar peduncle, and of follicles in the epiphysis cerebri; and the presence of vomeronasal organ and ganglion, of the bucconasal membrane, and of isolated semicircular ducts. The main features of stage 19 (approximately 48 days) are the cochlear nuclei, the ganglion of the nervus terminalis, nuclei of the prosencephalic septum, the appearance of the subcommissural organ, the presence of villi in the choroid plexuses of the fourth and lateral ventricles, and the stria medullaris thalami.

Brain↗
PubMed1988

The development of the human brain, including the longitudinal zoning in the diencephalon at stage 15.

Twenty-six embryos (6-11 mm) of stage 15 (approximately 33 days) were studied in detail and graphic reconstructions of three of them were prepared. Characteristic features of this stage include closed lens vesicles, presence of nasal pits, and retinal pigment. The neuromeric pattern is still visible. Each cerebral hemisphere is limited by the torus hemisphericus internally and by the di-telencephalic sulcus externally. The medial (diencephalic) eminence of the basal nuclei (previously misinterpreted by others as the lateral) had appeared in stage 14, and the lateral eminence, which is telencephalic, is now distinguishable. The amygdaloid body in stages 14 and 15 is derived from the medial eminence. The hippocampal thickening is identifiable in the dorsomedial part of the cerebral hemisphere. Medial and basal forebrain bundles are developing. The olfactory eminence is visible. Future olfactory bulb and tubercle possess an intermediate layer. The wall of the diencephalon presents five longitudinal zones: epithalamus, dorsal thalamus, ventral thalamus, subthalamus, and hypothalamus. The primordium of the epiphysis cerebri is beginning in the more advanced embryos. The sulcus limitans ends rostrally at the midbrain (M1) and is not continuous with the hypothalamic sulcus. Hence the alar/basal distinction does not arise in the forebrain. In the roof of the midbrain (M2) the mesencephalic evagination already noticed at stage 14 is characteristic. It is suggested that it may function as a temporary circumventricular organ. The precursors of some new tracts are identifiable: habenulo-interpeduncular, medial tectobulbar, and mamillotegmental fibres. Commissures include the supramamillary, that of the superior colliculi, and (in some embryos) the first fibres of the posterior commissure. Nuclei include the habenular, mamillary, and probably subthalamic. The cerebellum, the beginning of which was already noted at stages 13 and 14, consists of (1) a rostral part that arises from the alar plate of the isthmic segment and will form the superior medullary velum and part of the corpus cerebelli; and (2) a caudal part that develops from rhombomere 1. The involvement of the isthmic segment, first elucidated with stage 14, has not been observed in previous reports. All cranial nerves except the olfactory and optic are present in the more advanced embryos.

Brain↗
PubMed1988

The first appearance of the future cerebral hemispheres in the human embryo at stage 14.

Thirty-five embryos of stage 14 (32 days) were studied in detail and graphic reconstructions of four of them were prepared. Characteristic features of this stage include the beginning formation of the future cerebral hemispheres and the cerebellar plates. The ventral boundary between telencephalon medium and diencephalon is the preoptic recess. Although a velum transversum is not yet distinguishable as a dorsal boundary, its site is indicated by a change in the thickness of the roof of the forebrain. As the cerebral vesicles (future hemispheres) begin to evaginate, a di-telencephalic sulcus and a corresponding lateral ventricle and ventricular ridge (torus hemisphericus) develop. The telencephalic wall is mainly ventricular layer but three areas show advanced differentiation: olfactory area, future amygdaloid body (which lies at first mainly in the diencephalon), and primordium of the hippocampus. The telencephalon is growing in length, and the forebrain now occupies almost one quarter of the total length of the brain. The two neuromeres of the diencephalon are no longer as clearly delineated. The floor of D1 presents a thickened chiasmatic plate; that of D2 includes the infundibulum, which is closely related to the adenohypophysial pouch. The ventricular surface of D1 presents elevations for the dorsal and ventral thalami, separated by the sulcus medius. Other features of the diencephalon include the ventricular eminence (medial ventricular ridge) of the basal nuclei and the hypothalamic cell cord, from which the preopticohypothalamotegmental tract arises. The roof of D2 contains the evaginating part of the synencephalon. The mesencephalic angle continues to diminish. Two neuromeres, M1 and M2, are still distinguishable. The oculomotor nucleus emits nerve fibres, as does also the trochlear nucleus, which lies in the isthmic segment. Some extracerebral oculomotor fibres are present, but decussating and extracerebral trochlear fibres have not yet appeared. In the region of the tectum, two nuclei are discernible, and will form the medial tectobulbar tract and the mesencephalic root of the trigeminal nerve, respectively. The medial longitudinal fasciculus is present. A "median ventricular formation" is sometimes found in the mesencephalic roof. The cerebellum is the widest part of the brain. Two neuromeres (isthmic segment and Rh1) are involved in its formation. Most of the cerebellar plate has differentiated an intermediate layer, and the future rhombic lip is discernible. Indications of an efferent fibre system are present. In addition to the cerebellum, the rhombencephalon includes Rh1 to Rh7, and RhD.(ABSTRACT TRUNCATED AT 400 WORDS)

Amygdala↗