Paper - On the development of the hind-brain of the pig 1
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Bradley OC. On the Development of the Hind-Brain of the Pig: Part I. (1905) 40, 1-14.13. PMID 17232657
|pig neural development.
Bradley OC. On the Development of the Hind-Brain of the Pig: Part II. (1905) 40, 133-151.6 PMID 17232671
Bradley OC. Development and Homology of the Mammalian Cerebellar Fissures: Part II. (1903) J Anat Physiol 37: 221-240.13. PMID 17232554
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On the Development of the Hind-Brain of the Pig: Part I.
By O. Charnock Bradley, MB. DSc., FRS.E, Royal Veterinary College, Edinburgh.
Probably because the description given by the late Professor Wilhelm His of the development of the human hind-brain was so clear and detailed, only a comparatively small number of investigators, during the past twenty years, have turned their attention to this province of mammalian embryology. Nevertheless, in the somewhat sparse literature which does exist, several questions of importance have been raised, and conflicting opinions expressed. To mention only one point of surpassing interest and importance, all are not agreed as to the formation, degree of development, and histogenetic function of the rhombic lip (Rautenlippe) in embryos of mammals other than Man. There are those who arc not satisfied as to its formation even in the human embryo.
In the conviction that the examination of even an isolated species of maimmal may afford additions to the sum of facts from which valuable generalisations may be deduced, it was decided to investigate some of the steps of the development of the hind-brain of the pig.
In most cases two embryos, of equal size, were chosen from the same litter ; the one being sectioned in a sagittal, the other in a coronal direction. In the case of the younger material more than two embryos were examined.
With the exception of two embryos where such a proceeding was not deemed necessary, models were made, according to Born's wax-plate method, of the hind-brain of all the different stages herein described. In many instances it was considered advisable to make two models from the same embryo—one of the whole of the hind-brain, and another of part of that organ under greater magnification. By this means, it is hoped, greater accuracy of description has been attained. The following table shows the material employed in this research :—
|Age of Embryo.||Length of Embryo.||LD 1.|
|() x 50. (2) x 100. 2. 22 ,, 8 mm. (1) x 414.||(2) (part) x 834.
3 25 15 , (1) x 413. | | (2) (part) x 834. | 4. 28} . 23 (1) x 275% | | (2) (part) x 834. 5 30 25 (1) x 18. (2) (part) x 355. | 6. 35 32 (1) x 18. (2) (part) x 353. | 7. 37 n | 8. …. 43 l . 9. 40 52 x 355. 10. 48 80 . x 268. 11. 55 100 , x 358. 12. TO 150 , (part)-x 713,
Although the “age” of the different embryos is given above, it is not intended that this should signify more than the length of time which elapsed between the time of coition and the time when the mother was destroyed. An examination of Keibel's Normentufel (1) shows that coition and fertilisation of the ovum are not by any means contemporaneous, or even approximately so in the pig. In embryos taken from two litters, it not infrequently happens that those which should be farther advanced in development, judging from the period which has elapsed since sexual congress took place, are as backward as, or even more backward than, those of the “younger” litter.
In the embryo taken from the uterus 19 days after coition, the hind-brain is still in à rudimentary condition. The fissura rhombo-mesencephalica (Kupffer) is distinct, and the cervical flexure is well marked; but the pontine flexure has only begun to be formed (fig. 1). The hind-brain, as a whole, is in the form of a slightly bent tube, the calibre of which is greatest at about the junction of the anterior third with the posterior two-thirds. The roof of the tube is membranous from the cervical flexure to within a short distance from the constriction which marks the anterior limit of the hind-brain. The cerebellum is very rudimentary, and, indeed, is very difficult of accurate definition. Immediately behind the fissura rhombo-mesencephalica there is a short non-membranous dorsal union between the two halves of the neural tube; but, with this exception, the primitive Anlagen from which the future cerebellum arises are only connected by the membrana obturatoria quarti ventriculi (Külliker). (Fig. 9.)
Transverse sections taken anywhere, except at the most anterior part of the hind-brain, present a remarkable similarity, whether they are from the medulla or the region of the cerebellum. It is noteworthy that, though the alar and basal laminæ are clear and distinct in the spinal cord and in the mid-brain, it cannot be said that they are precisely defined in any part of the hind-brain. In fact, the hind-brain at this stage consists of two lateral plates (fig. 9), diverging from each other more and more as the widest part of the neural tube is approached: but joined ventrally at an acute angle, indicated on the surface of the model by a median ventral ridge, which, though more pronounced in the region of the future pontine flexure, can readily be traced all the way from the cervical to the cephalic flexure.
From the above it will be seen that there is practically a uniformity of construction of the whole of the hind-brain at this period of development.
In this embryo there are seven neuromeres lying between the cephalic and cervical flexures (fig. 2). Attention has been called to these structures elsewhere (2). It is, therefore, unnecessary to give more than a brief account of them here. The interior of the neural tube presents seven grooves, of unequal extent and depth, corresponding to elevations on the exterior. Following the lead of previous observers, these grooves, with the external elevations produced by them, are held to indicate the presence of neuromeres. Longitudinal sections show that the height of the ridges separating the neuromeral grooves is often disproportionate to the depth of the constrictions between the external elevations.
The first neuromere is large and from it the cerebellum arises, as has been shown by Orr (3), Hill (4), and Kupffer (5). The seventh neuromere is second in point of size, and has à deep and extensive internal depression. Whether it is a neuromere of the hind-brain or of the spinal cord may be open to question, since Hill, after careful examination of the neuromeres of fishes and the chick, has announced it as his conviction that there are never more than six neuromeres in the rhombencephalon.
In the hind-brain of an embryo 22 days old, the cervical tlexure is more abrupt and the pontine flexure better marked than in the 19-days’embryo (fig. 3). The two lateral parts of the cerebellum have increased in size, and their median connection is more extensive. As a consequence of these developments, the outline of the membranous roof of the ventricle (Rautenfeld) has changed. Those lateral borders of it which lie in front of the greatest transverse width of the rhombencephalon are now convex towards the middle line, instead of concave as previously. This, and the subsequent changes in the outline of the “ Rautenfeld,” are similar to those figured by Grünberg as occurring in Erinaceus (6).
The distinction between alar and basal laminæ can now be traced from the spinal cord into the medulla, but only for a short distance. The median ventral ridge, spoken of in connection with the younger embryo, is now confined to the district occupied by the pontine flexure and the isthmus rhombencephali, but in this situation it is even better marked than previously.
Seven neuromeres can still be detected (fig. 4). Their internal depressions are deep and very clear, but the corresponding external elevations and the intervening constrictions have become faint. The seventh depression is more shallow than before, and the suleus between the alar and basal laminæ (sulcus limitans) is continued into it.
An examination of the two youngest embryos herein described reveals in à very clear manner the nerve-connections of the neuromeres. Stated briefly, these arc as follows :—The N. trigeminus arises primarily from the second neuromere, but establishes à secondary connection with the third also. The acustico-facialis root-complex is associated with the fourth neuromere, and the glosso-pharyngeus and vagus with the sixth and seventh respectively. The nerve-relationship, as here stated, is almost in entire accord with the description given by Prenant (7) of the condition found by him in à 14-mm. pig embryo.
In the 22-days’ embryo the otic vesicle lies opposite the fifth and the greater part of the sixth neuromere. In the 19-days embryo it does not extend so far backwards, and it is therefore concluded that the real relationship of the vesicle is with the fifth neuromere, the association with the sixth being merely the result of expansion of the vesicle.
À more detailed description of the neuromeres is given in the paper already published.
Grônberg (6), in his description of the development of the brain of the hedgehog, attaches great importance to the appearance of two longitudinal grooves, lateral to the suleus centralis, in the anterior part of the floor of the ventricle in the earlier stages of development. The more lateral of the two he considers to be the true sulcus limitans; the more mesial he holds to be only à secondary structure, and names it the sulcus intermedius. This latter, however, he avers is equivalent to what His has recognised as the suleus limitans of the human embryo. Thus Grünberg desires to shift the sulcus between the alar and basal laminæ into a more lateral position.
A comparison of fig. 11 (transverse section through the hind-brain of a 22-days pig embryo) with Grôünberg’'s fig. 59 (Taf. 18) reveals a strong likeness. Grônberg’s figure is from a hedgehog embryo of about the same stage of development as the 22-days’ pig; and the section is from about the same level as that from which fig. 11 was taken. Both figures show two apparently longitudinal grooves ; but the model of the pig embryo discloses the neuromeral nature of the grooves. Is there not the possibility that the grooves in Grôünberg’s illustration, and called by him sulcus limitans and sulcus intermedius, may bear a like interpretation ? This is merely à query, not an assertion. Nothing short of making reconstructionmodels of the hind-brain of hedgehog embryos would justify one in saying that Grônberg’s reading is not the correct one. At the same time, it is strange that the two figures should be so much alike.
15 mm Embryo
In an embryo 15 mm. long (25 days old) the pontine flexure is well marked (fig. 5). This, in association with the increased size of the cerebellar lamina, has produced a great alteration in the shape of the outline of the Rautenfeld. It is now in the form of a triangle whose base, directed forwards, consists of à median notch flanked on each side by a convexity produced by the lateral portions of the posterior border of the cerebellum. Its posterior angle no longer quite reaches the cervical flexure : and, since the lateral recesses are beginning to form, its lateral angles are curved slightly forwards.
In addition to the increased antero-posterior dimensions of the lateral parts of the cerebellum, there has also been an augmentation of their thickness. The inner surface of each half is now convex; but this is not entirely due to an increase in thickness, for there has been the concomitant production of an external concavity. The median part of the cerebellum has begun to develop a marked convexity in the sagittal, and also in the transverse direction—a condition of some moment, as subsequent development will show.
Immediately behind the posterior edge of the cerebellar lamina a choroidal fold has begun to form. This begins laterally in the neighbourhood of the incipient lateral recess, and is continued for more than twothirds of the distance between this point and the middle line. Something less than the median third of the lamina shows no sign of folding.
The ventral median ridge, present in the earlier stages, has entirely disappeared, and has given place to a shallow groove continuous with the ventral groove of the spinal cord. The groove extends forwards to a level with the origin of N. trigeminus. Anterior to the origin of the N. acusticofacialis there is a low rounded ridge running along the floor of the groove.
Alar and basal laminæ can now be followed from the spinal cord for about half the length of the ventricle, ie. nearly to the most anterior root of the IX. and X. cranial nerves (fig. 12). Of the neuromeral grooves five are still evident (fig. 6). The first, however, is shallow ; the second is of good depth, and the ridge between it and the first is very prominent. The five grooves now present have the same nerve relations as had the most anterior five of the seven in the carlier stages. ‘The sixth and seventh neuromeres have, therefore, lost their internal grooves, and it appears that this has resulted from an anterior extension of the interzonal sulcus (suleus limitans); in support of which supposition may be cited the fact that the sulcus is much wider in the position of the former sixth and seventh neuromeres.
The fact that alar and basal laminæ cannot be distinguished in the early rhombencephalon, and that, as growth proceeds, they gradually extend, as recognisable entities, farther forwards, appears to justify emphasis.
23 mm Embryo
The pontine flexure is so pronounced in this embryo, that the floor of the ventricle slopes gently downwards and forwards from its posterior end to opposite the lateral recess (fig. 7). Anteriorly it is inclined steeply upwards and forwards.
The lateral recess is now deep and clearly bounded. Its posterior limit has become defined, partly as the result of the increase of the pontine flexure which has caused the ventricle to assume suddenly a much greater transverse diameter in front of the N. acustico-facialis; but partly, also, as the result of intrinsic development of the hind-braiïn in this region. The cerebellum, because of its increase in size and its greater backward projection produced by the accentuation of the pontine flexure, now completely roofs in the lateral recess.
The increase in the thickness of the floor of the ventricle has caused the distinction between alar and basal laminæ to become à matter of difficulty. But there is still à faint groove visible opposite the fasciculus solitarius.
It will be noticed that, in the pig, definite alar and basal laminæ sepurated from each other by a sulcus limitans have not been met with in the anterior part of the rhombencephalon. The farthest anterior point at which they can be distinguished is about the middle of the length of the ventricle, and this only in an embryo of 15 mm. In younger material they cease to be obvious at a more posterior level.
Although the indications of neuromeres are generally held to disappear at a comparatively early period, there are grooves in this and older embryos which it is difficult not to consider as the direct descendants of the neuromeral grooves of younger specimens. À comparison of figs. 6 and 7 will demonstrate the grounds for supposing that the grooves persist for a much longer time than is generally supposed. That grooves do exist in a 23-mm. embryo is shown in fig. 7; and these grooves—four in number—have the same topography as have similar depressions in fig. 6. If the grooves in the older embyro are not the descendants of those in the younger, it is strange that they should be so similar in position and relations. Until the contrary is shown to be the case, the depressions in the 23-mm. and larger embryos will be described as being identical with those of the earlier embryos.
The first and second depressions are partly combined, 4.e. they are now included in one large concavity ; but their individuality is not completely lost, since the grooves which extend laterally from them into the lateral recess of the ventricle are quite distinct from each other. The second depression is much deeper than the first.
At the present period of development the third groove has gained the supremacy s0 far as depth is concerned. There is yet a fourth depression, shailow and indistinct, and into it runs the possible representative of the sulcus limitans. Microscopic sagittal sections suggest the presence of even a fifth depression, but concerning its actuality there is some room for doubt.
25 mm Embryo
In an embryo only 2 im. longer than the one just described (25 mm., 30 days old) the pontine flexure has attained its maximum curvature. From now onwards it gradually becomes more obtuse. The extreme degree of the flexure at this period produces a great exaggeration in the depth of the medium fissure of the ventricle on a level with the lateral recesses. These latter are now very definitely bounded—above by the cerebellum, and below in a manner presently to be described (fig. 23).
The connection between the two halves of the cerebellum has increased in thickness in that part adjacent to the mid-brain. Posteriorly it gradually thins away. The lateral halves of the cerebellum are still concave on the outer surface—this being in striking contrast to a convexity occurring in the median region.
By this time the choroïid plexus is well formed, and is disposed in the form of a curve whose concavity looks forwards.
Of the depressions considered in the last embryo as vestiges of the neuromeral grooves, four can still be distinguished. Internal to the opening from the body of the ventricle into the lateral recess, the first and second grooves are more blended than in the 23-nun. embryo; but into the recess itself they can readily be traced as separate entities. Their relative positions, however, have been altered owing to the increase of the pontine flexure. The first groove now lies at a higher level than the second, instead of being directly anterior to it.
The third groove continues to be deep in the body of the ventricle, but it is with difficulty followed into the recess. The fourth groove is faint and indistinct, and can be satisfactorily demonstrated only in sagittal microscopic sections.
32 mm Embryo
The rhombencephalon of the next older embryo shows several features of interest. The pontine flexure is less abrupt. The transverse and vertical diameters of the medulla are much greater than those of the spinal cord; the change occurring rather suddenly at the cervical] flexure. The fourth ventricle fails to reach the cervical flexure; that is, the intercalated portion of the medulla (Sckaltstück of His) has begun to form. The dorsal median fissure of the spinal cord is continued into this part of the medulla, but gradually becomes shallower as it passes forwards, À short distance behind the posterior tip of the ventricle the bottom of the fissure develops à ridge, to which attention will be directed later (fig. 24).
Viewed from the surface, the two halves of the cerebellum do not meet at so sharp an angle as formerly. The outer surface of each half has still a slight concavity about its middle (fig. 31), but its posterior portion is decidedly convex (fig. 30). The external convexity of the median region of the cerebellum, which had made its appearance in the previous embryo, is now very prominent, and does not extend so far back as the posterior border of the cerebellum (figs. 30 and 31). Viewed from the inside, the two halves of the cerebellum are separated by à median saggital fissure which is very deep on a level with the external convexity just mentioned. This fissure has taken the place of a wide groove, previously present, corresponding to the thin median connection of the two moieties of the cerebellum. The change from à fairly wide groove into a contracted cleft has doubtless been due to two factors. In the first place, the outward bulging of this part of the cerebellum, without a commensurate increase in vertical thickness, must have contributed to the deepening of the groove. Secondly, the lateral walls of the groove have become steeper and taller because of the increase in thickness of the lateral halves of the cerebellum, and the resulting increase in the internal convexity of these parts.
In the posterior part of the cerebellar lamina, and in the wider part of the fissure just described, à prominent ridge has made its appearance (fig. 30). The ridge, however, disappears as the deeper, more anterior, portion of the fissure is approached. The occurrence of a ridge is no doubt associated with the initiation of the process of filling up of the fissure. It is of interest to note that Schaper (8) figures a similar, if larger, ridge at the bottom of the “ Medianfurche ” (ventral surface of the “ Deckplatte ”) in trout embryos (cf., for example, his fig. 28). It may be remarked that a similar ridge is present (and was also visible in the 25-mm. embryo) at the bottom of what may for convenience be called the ascending (anterior) portion of the median fissure (sulceus centralis) in the floor of the ventricle Even in the 23-mm. embryo the precursor of the ridge can be detected.
It should be here noted that at no time in its development does the cerebellum of the pig present the internal features described by Külliker (9) as occurring in the rabbit. Kôlliker remarks that frontal sections reveal, on the under (inner) surface of the developing cerebellum, a deep median and two lateral fissures. The surface, therefore, he says, has four longitudinal ridges running along it; of these the more lateral are the largest. This assertion is supported by the figure of a frontal section (fig. 338) through the cerebellum of a 16-days’ rabbit embryo. In its lack of the lateral fissures, the pig resembles the sheep (cf. Kuïithan’s figures).
The general character of the lateral recess has not altered materially, though its roof and floor are nearer together owing to addition to the thickness of the cerebellum by which the roof is formed. Because of a diminution in the transverse diameter of the ventricle just behind the opening into the recess, the isolation of the latter is rendered more conspicuous.
In the body of the ventricle there is a shallow depression, occupying the same relative position as the first and second grooves described in the preceding paragraphs as neuromeral in origin. From this depression a deep furrow runs outwards into the lateral recess, in the outer and anterior part of which it divides into two—thus, possibly, indicating its duplex constitution. The third groove is of great depth in the body of the ventricle, and can be readily followed into the lateral recess.
52 mm Embryo
In the next embryo (40 days) the intercalated part of the medulla has increased materially in extent. The cerebellum has developed rapidly, and, examined from the exterior, now shows a division into an elementary vermis and two lateral hemispheres. The vermis is most markedly developed anteriorly, where it forms a rounded projection overlying the rudimentary anterior medullary velum, and extending for some distance in front of the anterior limit of the hemispheres. Such an early indication of a vermis seems to militate against Bolk’s contention that it is not a fundamental division of the cerebellum (10).
There can be little doubt that the vermis has sprung directly from the median convexity present in the earlier embryos. Thus the median thin portion of the cerebellum only produces à very small part of the vermis, almost the whole of it being developed from the more mesial parts of the lateral halves of the cerebellar lamina.
The transverse diameter of the cerebellum is much greater than that of the underlying portion of the rhombencephalon (figs. 42 and 43). The sulcus, which has been described elsewhere (11) as demarcating the nodulus and flocculus from the rest of the cerebellum, is now present. The deep sagittal median fissure found on the inner surface of the cerebellum in the previous embryo has become a veritable cerebellar ventricle (igs. 43 and 44), such as has already been described in mammalian embryos by Blake (12). This narrow cavity extends forwards into the anterior projection of the vermis, so that its communication with the rest of the fourth ventricle is directed downwards and backwards. (Cf. figs. 48 and 44) In the depth of the cerebellar ventricle there is still a trace of the median ridge already mentioned (fig. 43).
The under surface of the anterior medullary velum is marked by a sagittal furrow, which may be looked upon as a shallow forward continuation of the fissure out of which the cerebellar ventricle has developed.
It is, perhaps, worthy of remark that a ridge has made its appearance where the cerebellum joins the floor of the fourth ventricle (fig. 42).
In the posterior part of the floor of the fourth ventricle the lateral halves of the medulla are inclined to each other at à very acute angle, a condition which has been evolved gradually since the earlier stages. A ridge can still be distinguished at the bottom of the median fissure in the anterior part of the ventricle (figs. 42 and 43), but this is the oldest embryo in which such a feature is found.
The depression occupying the position of the first and second neuromeral grooves is becoming very indistinct. The third groove, on the other hand, is deep and very evident in the body of the ventricle, and is continued into the lateral recess as a conspicuous furrow.
The opening from the ventricle into the lateral recess has become both relatively and absolutely narrower.
80 mm Embryo
The cerebellum in an 80-mm. embryo (48 days) has an unmistakable vermis and two lateral hemnispheres. Fissures are beginning to separate the lobes: the nodulus and floceulus being already completely demarcated from the rest of the cerebellum. The cerebellar ventricle has become filled up to a large extent, but is not as yet completely obliterated. Where the cerebellum meets the floor of the ventricle there are now three short ridge-like elevations in the place of the one in the younger specimen.
The groove occurring on the under surface of the anterior medullary velum is now very shallow.
The floor of the ventricle has altered scarcely at all in the interval between 40 and 48 days. The third neuromeral groove is still represented by a deep depression in the body of the ventricle; but it now lies at a level slightly in front of the opening into the. lateral recess, whereas, formerly, it was on a level with the opening. This change of position is to be associated with a backward growth of the anterior wall of the recess, rather than to a forward movement of the depression. The subsequent history of the third neuromeral groove shows that there is little reason to doubt that it becomes the anterior fovea of the floor of the fourth ventricle. It may be again repeated that, if the depression is not really a persisting neuromeral groove, there is nothing in the series of embryos examined which militates against the contention that it is so.
100 mm Embryo
The third neuromeral depression is again deep and occupies à position considerably anterior to the opening into the lateral recess, into which no continuation of it can now be demonstrated.
The three ridge-like eminences at the junction of the cerebellum with the floor of the fourth ventricle are more prominent than in the earlier stage. The anterior medullary velum is thin, its ventricular surface showing à median sagittal keel corresponding to a dorsal furrow. Even now the cerebellar ventricle has not entirely disappeared.
Continued - Part II
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Explanation of Figures
- The cost of reproduction of the figures has been defrayed by the Carnegie Trust for the Universities of Scotland.
The outlines of all the figures representing sections were made by means of a Leitz camera lucida.
The figures illustrating sections are arranged so that the first figure belonging to one particular embryo represents the most posterior section ; the last figure of the series representing the most anterior.
The following reference lettering is common to all the figures :—
a.c. ala cinerea. | fo. Flügelwulst. a.l. alar lamina. | h.b. hind-brain. a.m.v. anterior medullary velum. Lr. lateral recess. a.p. area postrema. m.b. mid-brain. b. backward projection of the n À. etc. nucleus of X etc. cranial roof of the fourth ventricle. : nerve. b.1. basal lamina. ob. obex. bl.vs. blood vessels. | o.v. otic vesicle. cb. cerebellum. pA. paraflocculus. c.e. central canal. p.m.v. posterior medullary velum. c.v. cerebellar ventricle. rl rhombic lip. ch.pl. choroid plexus. : s-a.s, subarachnoïid space. d.m. dura mater. | 1, 2, 3, etc. 1st, 2nd, 3rd, etc., neuroJloc. flocculus. meral gruoves. J.s. fasciculus solitarius. | V. etc. V. etc. cranial nerves.
f.sp. funiculus separans.
Fig. 1. 19 days’ embryo. Model of the hind-brain. Exterior. Fig. 2. 19 days’ embryo. Model of the hind-brain. Interior. Fig. 3. 22 days’ embryo. Model of the hind-brain. Exterior. Fig. 4. 22 days’ embryo. Model of the hind-brain. Interior. Fig. 5. 15 mm. embryo. Model of the hind-brain. Exterior. Fig. 6. 15 mm. embryo. Model of the hind-brain. Interior. More than half
of the brain has been modelled. The cut surface, therefore, is greater than it would be in the middle line.
Fig. 7. 23 mm. embryo. Model of the region of the lateral recess. Interior.
Fig. 8 150 mm. embryo. Model showing half of the most posterior part of the fourth ventricle and the central canal. From within and in front.
Fig. 9. 19 days’ embryo. Horizontal section through the hind- and mid-brain.
Figs. 10 and 11 22 days’ embryo. Transverse sections through the hind-brain.
Figs. 12-15. 15 mm. embryo. Transverse sections through the hind-brain.
Figs. 16-20. 23 mm. embryo. Transverse sections through the hind-brain. Fig. 19 shows the amount of development of the rhombic lip of the cerebellum in the region of the lateral recess.
Figs. 21-23. 25 mm. embryo. Transverse sections through the hind-brain.
Figs. 24-31. 32 mm. embryo. Transverse sections through the hind-brain.
Figs. 32-44, 52 mm. embryo. Transverse sections through the hind-brain. Figs. 34-40 illustrate the form and dimensions of the rhombic lip at different levels.
Cite this page: Hill, M.A. (2021, June 19) Embryology Paper - On the development of the hind-brain of the pig 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_On_the_development_of_the_hind-brain_of_the_pig_1
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