Paper - Embryological and morphological studies on the mid-brain and cerebellum of vertebrates 2
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Palmgren A. Embryological and morphological studies on the mid-brain and cerebellum of vertebrates. (1921) Acta Zoologica. 64(5): 2-94.
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Embryological and Morphological Studies on the Mid-Brain and Cerebellum of Vertebrates
II. THE Cerebellum
Cerebellum has been studied in Acanthias tmlgaris, zlcipenser ruthenus, Lepidosteus osseus, Salmo salvelimts and Osmerus eperlamts.
The structure of the Cerebellum is well-known, thanks to the investigations of BURCKHARDT (1897), EDINGER (I901), VOORHOE\7E (1917) and others.
Cerebellum is usually divided into a medial: Corpus cerebelli and two lateral parts: Aiuriculi cerebelli. As these latter are connected by a medial causdal part, which I will call: Pars medialis auriculi, it would be suitable to make a distinction «between an anterior pant: Corpus cerebelli and a posterior: Pars auricularis. The auricles consist of two laminae, an outer and an inner. I term them: Lobius externns and internus auriculi.
According to VOORHOEVE (1917) the Auriculi are separated from the Corpus by two lateral grooves, the Sulei paraauriculares, which are connected with a medial groove: Sulous postrem-us. In front of this groove lies another medial one: Sulcus posticus. This groove can, -especially in older specimens of /lcanthias, be traced frontad and la.-terad, and there form the dorsal boundary of a laterally projecting pant of the Corpus. Moreover, since Sulcus paraauricularis and Sulcus postremsus are already developed at stage 6,5 cm., but Sulcus postic-us first appears at stage 15,0 cm., the first mentioned grooves may be considered as the true boundary grooves between the Corpus and Pars auricularis.
The granular cells in Selachians are, as is known, principally concentrated in two longitudinal strings Or ridges (”Kleinnhirnlippe”) (S. g. c. fig. 106), one on each side of a pairediependymal thickening (E. th. fig. 106). These strings of ‘granular cells can be traced fnom the Deoussatio veli viz. the decussation of the Nervus IV, through Corpus and the part between Sulcus posticus and Sulcus postremus and then along the caudal surface of the P.ars medialis auriouli. After this the strings separate and form together with a Purkinje—cell and a molecular—layer the edges of the Lobi interni and externi.
These strings extend therefore without interruption through the whole of the Cerebellum and behind it along the edges of the Fossa rhomboidalis. It is however worthy Of mention, that in older embryos of Acanthias (I5,0 cm.) and in full-grown animals, there exist two very well-defined transverse grooves, which cut fairly deeply into’ the strings at a point which lies Opposite the Sulcus postremus. These grooves certainly appear comparatively late, but are, as far as I could find, invariable in occurrence and position. It seems therefore probable that these grooves are to be considered as boundary- grooves between the Corpus and Pars auricularis viz. Pars medialis auriculi.
Scattered granular cells are also to be found here and [there in the cerebellar substance (EDINGER ).
a) Acipenser mthenus
The Cerebellum has been investigated in Acipemer by GORONOWITSCH (1888) (A. ruthenus) and JOHNSTON (1898, 1901) (A. smrio).
Cerebellum in Acipenscr can likewise be subdivided into a medial part: Corpus, and two lateral pants, Auriculi cerebelli (Pedunculi cerebelli of GORONOWITSCH). Between the Corpus and Lobi inte-rni auriculi (firnbria of GORONOWITSCH, the lateral lobes of JOHNSTON) have developed longitudinal grooves (S. p. figs. I03, 104), which are clearly identical with the Sulci paraauriculares (Voorhoeve) in Selachians. NO Sulcus postremsus i. e. no boundary-groove between Corpus and Pars medialis auriculi has on the contrary been described.
GORONOWITSCH (1888), however, «describes ”in den distalen A-bschnitteu des dorsalen Teiles des Kiirpers ein stark entwickeltes Querfasersystem, welches das Kéirnergewebe cler fimbria beicler Seiten mi/t einander verbindet” (page 536). JOHNSTON (I9OI) say.s: ”The cerebellar crest — is not lost as a distinct bundle, but continues mesially and then caudo-mesially along the border of the lateral lobes at their junction with the plexus chorioideus, and finally forms the most caudal and dorsal portion of the molecular layer in the middle line’’ (page 90). This caudal and dorsal portion of the Cerebellum can be indicated as Pars rnedialis auriculi. I am not able to draw up a boundary between this portion and the Corpus, as I have only had a series of transverse sections at my disposal.
According to GORONOWITSCH and JOHNSTON, a large granular cell-mass (Seitenwulst of GoRoNow11‘scH)‘ lies on each side of the great medial molecular mass in the Corpus. These lateral granular cell-agglomerations extend into the Valvula cerebelli and al-so cover the frontal -parts of the Auriculi cerebelli. Granular cell-strings are further found along the Lobi interni and extemi as well as along the Cnista cerebelli. These granular cell-strings are connected with the granular cell-agglomerations in the Corpus at the frontal parts of the Auriculi.
fig. I00, section 2271. Represents a section through the most caudal part of the Cerebellum viz. Pars medialis aurieuli. Gvanular cells (s. g. c.) here cover the ventral and lateral walls of this part in a continuous layer.
fig. 101, section 212. The granular cells form two strings on each side: a lateral (1. s. g. c.) and a medial string (m. s. g. c.). These strings become fused caudally, i. e. merged into each other (fig. 100 s. g. c.).
fig. 102, section I86. The lateral as well as the medial strings remain (1. s. g. c. and .m. s. g. c.). The latter lie in a ’’prominent median ridge" (JOHNSTON), the ”Kiel” of GORONOWITSCH (K), but have not been described or figured by these investigatons. The bulk of gran.ular cells forms the two great cell-agglomerations (g. m.) one on each side of the Corpus between the above mentioned strings. The Crista cerebelli is further covered by a granular cell-string.
fig. 103, section H5. The lateral cell-agglomerations have been sub-divided into a dorsal (l. g. m.) and a ventral part (c. g. m.), which are clearly separated from each other. The dorsal agglomeration lies in the Lobus internus (L. i.), the ventral in the Corpus. The section has, to the right on fig. 103, touched the point of fusion between Lobus internus and externus. The lateral granular cell-string (l. s. g. c.) here passes continuously into the granular cell-string in Crista cerebelli, respectively Lobus externus. The medial strings (in. .s. g. c.) are to be found again in the median ridge, but are now somewhat smaller than further caudad.
fig. 104, section IOI. This section is on the right of the figure tangent to the Recessus lateralis (R. 1.). On the opposite side the section has intersected the auricle further caudad. Here the anterior part of the granular cell-strings of the auricles is still visible '(l. is. g. c.). In the left auricle (to the right of the figure) there appear only two larger granular cell-agglomerations. The dorso-lateral is the same as that represented on fig. 103 l. m. The ventro-lateral continues caudad and lies under the Crista cerebelli (fig. 103) 2. Both these agglomerations, which are seen to merge continuously into each other in fig. 105 l. g. m., cannot according to me, be said to be in continuous connection with the granular cell-strings of the auricles. These latter consist of very closely—packed cells. There are indeed, scattered granular cells between these strings and the just mentioned cell-agglomerations but no true fusion takes place. While the granular cell-strings thus form the edges of the Lobi and cover the inner wall of the_ Recessi laterales, the remaining granular cell-agglomerations lie on the frontal surface of the auricles, in the medial part of Lobi interni and ventral to Crista cerebelli.
- In the series of sections I have examined, the thickness of the sections was 25 14. The number of sections into which Cerebellum was cut was 235. fig. Ioo represents the 227th section.
- This ventral granular cellular tissue is clearly that described by GoRoNowITscH as: ”die' ventrale Kornerlage” (p. 536) and by J01-INSTON‘(I898) as Tuberculum acusticum (p. 586).
fig. 105, section 82. This section i-s tangent to the frontal part of the Auriculi, where the above mentioned granular oell-agglomerations merge continuously into each other. The granular cell—agglomerations in.the Corpus (c. g. m.) continue frontad and into the Valvula cerebelli, where they lie in !he same position as in the Corpus. The medial strings (m. s. g. c.), on the contrary, do not extend into the Valvula, but finish with a number of small groups of cells, irregularly scattered about in the ventro-lateral parts of the median ridge.
On section II 5 (fig. 103), the molecular layer (M. l.) is visible, partly in the Lobus externus, ventral to the granular cell-string, partly on the dorsal side of the Lobus internus. In both these parts the dendrites of the Purkinjescells .seem to lie in more regular order than in the medial molecular-mass and in the median ridge. Between the molecular layer in the Lobus intemus and the more -medial parts, it is therefore possible to draw a sharp boundary-line. By following the series of sections frontad, it is possible to see how the fusion of the molecular layers takes place (figs. I04, 105), and forms a mantle dorsal to the large cell-agglomeration (l. g. m.). If section 82, fig. 105 is now compared with a section which is tangent to the auricles in Acanthias (fig. 106), it becomes clear that the fused granular cell-strings (s. g. c. fig. 106) in this species cannot be directly compared with the granular cell-agglomerations in Acipenser (.l. g. m. fig. 105). The molecular layer (M. 1.) lies ventral to the granular cells in Acanthias, but dorsal to them in Acipemer. This section of Acantliias (fig. 106) ought clearl.y to be compared with ‘section IOI in Acipenser, where to the left of fig. I04 the fused granular cell-strings (l. s. g. c.) lie in the same position in relation to the molecular layer as in Acanthias.
fig. 98. Parasagital section through the posterior part of the mid-brain of Acanthias at stage 15,0 cm. (Microphotograph.) fig. 99. Transverse-section of posterior part of the mid-brain of Gallus at stage 8 days. (Microphotograph.) figs. Ioo-105. Transverse sections through the Cerebellum ‘of Acipenser. (Sketches.)
C, Cerebellum; c. g. m., the granular cell-mass of the Corpus cerebelli; E. th., ependymal thickening; F. rh. m., fissura rhombo-mesencephalica; g. m., granular cell-mass; K, the median molecular ridge; L. e., Lobus externus; l. g. m., the granular cell-mass of the Lobus intemus; L. i., Lobus intemus; l. s. g. c., the lateral string of granular cells; M. 1., molecular layer; m. s. g. c., the medial string of granular cells; Nc. i., Nuclei isthmi; P. m. a., Pars medialis auriculi; p. T. f., the posterior part of the Torus-formation; S. p., Sulcus paraauricularis; s. g. c., the granular cell—string of the Pars medialis; T. 0., Tectum opticum; V1. c., Valvula cerebelli; Z, the boundary-zone between Mes- and Metencephalon.
82 fig. I02. fig. 103.
It is further worthy of notice, that a number of larger cells are to be found in the lateral part of the Corpus, directly ventral to the line of connection with the Lobus in-ternus, i. e. between the dorsal and ventral granular cell-agglomerations (these cells have also been described by JOHNSTON [I901], page 92)-
Thus the granular cell—.string of the Lobus externus in Acipenser as in Acant/rias, bends -over on to the Lobus inter.nus, after which it passes away in a caudal direction. In Pars medialis the strings from the two sides unite, and can then be followed medially frontad into the Corpus 1. The above described medial strings .in Acipenser (.m. .s. g. c.), I assume to be homologous with the granular cell—ri:dges in Acamfhias. These strings in /lcipenser do not indeed appear to extend so far frontad as in Acam‘hias. Their exact position in regard to the decussation of the trochlear nerve I have not been able to determine, as this decussation is altogether untraceable on the haematoxilin— stained series which I have examined. Compared with the ridges in Aconthias, the strings in Acipenser give the impression of being rudimentary.
That the cells in these strings are true granular cells, appeans evident from the fact: I) that in the Pars rnedialis they pass continuously into the exactly similar strings in the Lobus internrus, 2) that their size and appearance, so far as I have been able to judge, agree completely with the granular cells in .the remaining parts of the Cerebellum. Medial to the strings are to be found two ependymal thickenings (E. th. figs. 1o3~—Io5). In Ammthias there are also to be found two distinct ependymal thickenings (E. th. fig. 106) medial to the ridges. The granular cell-strings in Acipemjer can therefore not be the medial ependymal thickenings, whvioh occur in A canthias and other fishes.
The bulk of granular cells in Acipenser should then correspond to the granular cells found scattered outside the ridges in Acanthias (EDINGER 1901).
With regard to ..the form of the Corpus cerebelli, I agree with JOHNSTON (1898), when he assumes that ”the body and valvula have apparently -been formed by a down-folding of the cerebellum in the middle-line, this folding has brought the molecular layer of the two folds into apposition and they have fused to form the single median molecular layer” (page 59I—592). If in other words, an imaginary medial groove, developed in that part of the Corpus which lies caudal to the decussation of Nervus IV, can be conceived as cutting down to the ependymal thickenings and then allowing it to develop and be magnified, it would be possibale to get a type of cerebellum, which in form would correspond with the Cerebellum in Selachii, namely in Acanthias. .
- In Acanthm: the strings of granular cells do not, indeed, pass into one another in the Pars medialis auriculi as in Acipenser. When, however, the Cerebellum is primarily a paired structure (SCHAPIER [I894]) and the corresponding cell-strings in Salmo at earlier stages are clearly also paired, it appears probably that the coalescence of the contra-lateral strings is secondary -in Acifvmser (and Lrffidosmis).
b) Lepidosteus ossens
I have examined a series -of transverse-sections of Lepidostens osseus (length 1,3 dm-.) of which 105 sections at 2o,u. fall to the share of the Cerebellum. The fixation was not by any means perfect, but the tissue was well enough preserved to make an investigati~on of the topographical conditions of the various sections possible.
fig. 107, section 88. A granular cell layer (s. g. c.) is here seen on the ventral side, which lies in the Pars medialis auriculi. Dorsally the section goes through the caudal part of the Corpus, the ventricle of which is seen in the figure. On both sides of the ventricle lies a mass of granular cells (g. m.), which dorsally and laterally is surrounded by a layer of Purkinje-cells and a molecular layer. A granular cellstring is lacking at the Crista cerebelli.
fig. 108, section 75 lies anterior to the Pars medialis. The Cerebellum ventricle is in open communication with the 4th ventricle. Laterally (s. g. c.) lie strings of granular cells, which form a direct continuation frontad of the lateral parts of the above rnentioned layer of granular cells (.5. g. c. fig. 107). The mass of granular cells in the Corpus begins dividing into two separate parts (c. g. m. and 1. g. m.), a subdivision which is completed further frontad.
fig. 109, section 48. On this are seen two dorsal grooves (S. p.), which divide a medial part: Corpus cerebelli, from two lateral parts: the Lobi interni. These grooves may therefore be termed Sulci paraauriculares. In the Lobus internus (L. i.) there lies ventrally a granular-cell-string (s.g.c.), the one have just mentioned. Laterally the Lobi is covered by a molecular layer (M. 1.). Medial to this lies a granular cell-agglomeration (1. g. m. ), which extends dorsally to the surface of the brain. A dorsally lying molecular layer reappears ‘then in the Corpus. Under this and medially in the Corpus, lies another granular cell-agglomeration (c. g. ‘m.), clearly distinguishable from the mass of granular cells, in the Lobus internus. Further caudad however, these granular cell—masses become fused (figs. I08, 107). Between the principal masses of granular cells, we find a few scattered ones here and there in the Cerebellum. There exists also a number of langer cells (* fig. 109) between the masses of granular cells and ventral to the granular ce1l—mass in the Lobus internus, which I cannot by their .appearance distinguish from the Purkinje-cells in the Purlcinje—c‘ell layer proper.
fig. 110,.secti'rm 39 goes thiiough the anterior part of the Corpus and the fusion—point of the Lobi interni and externi. On this poin-t we see the frontal part of the granular cell—string in the Lobus internus (s. g. c.). The molecular layers (M. l.) in Lobi merge into one another. Between the molecular layer and Sulcus paraauricularis is found again the granular cell- mass (1. g. m.) in the Lobus internus. The granular cell—mass in the Corpus cerebelli (c. g. m.) is somewhat smaller frontad and continues into the Valvula cerebelli.
fig. 111, section 31, is tangent to the anterior parts of the auricles. The granular cell-mass (1. g. m.) in this part is the frontal part of granular cell—rnass in the Lobus internus. I cannot discover any ventral granular cell—mass in the Lobu.s externus under the Crista cerebelli.
The granular cell—strings in Lepidosteus are thus considerably reduced, are entirely missing medially in the Corpus, so ‘far as I have been able to determine, as also in the Lobus externus. On the other hand, a cauda- medial part, connecting the granular cellastrings in the Lobi interni continues to exist. The remaining granular cells are disposed, as nearly as may be, in the same manner as in Acipenser. The granular cell—mass in the Lobus internus does not lie, it is true quite on the surface in Acipenser, the fibre substance which covers the same dorsally is however fairly thin and does not appear to be composed of typical molecular layer.
- In regard to the form, especially of the Corpus cerebelli, the difference is greater, The medial ependymal thickenings lie near the surface dorsally and approximately in the same position as in Acanthias. Moreover, the whole of the Corpus cerebelli rises above the lobes, while in Acipenser it lies on a level with or lower than the lobes.
(Salmo sal-velvinus, at stage E). Cerebellum in the Teleosts is usually subdivided into: I) Valvula cerebelli, 2) Corpus cerebelli, 3) Auriculi cerebelli with Velrum medullare posterius (Pars medialis auriculi). The embryonic development of the Valvula is treated below.
The boundary between the Corpus cerebelli and Pars auricularis is marked caudally by a well developed Sulcus postremus (fig. 52).
fig. I06. Transverse section through the Cerebellum of Acanthias at stage 15,0 cm. (Sketch) figs. I07——III. Transverse-sections through the Cerebellum of Lepidosteus. (Sketches.) figs. 112-I16. Transverse-sections through the Cerebellum of Salmo at stage E. (Sketches.)
Ca. c., Canalis cerebelli; c. g. m., the granular cell-mass of the Corpus cerebelli; E. g., Eminentia granularis; E. th., ependymal thickenings; g. m., granular cell—mass; L. e., Lobus externus; 1. g. m., the granular cell—mass of the Lobus internus; L. i., Lobus internus; M. 1., molecular layer; P. c., Purkinje-cells; P. m. a., Pars medialis auriculi; R. 1., Rccessus lateralis; s. g. c., string of granular cells; S. p., Sulcus paraauricularis; S. po., Sulcus postrcmus; T. 0., Tcctum opticum; V]. c., Valvula cerebelli.
fig. 112. This section goes through the caudal part of the Pars medialis which is oomposed of compactly disposed granular cells (s. g. c.). Dorsal to this part lies the dorsal portion of the Corpus, whose bulk consists of a granular cell—agglomeration (g. m.), surrounded by a Punkinje cell and molecular layer. Canalis cerebelli (SCHAPER [I894]) i.s transversely intersected -dorsally and ventrally (Ca. c.).
The granular cell-string in the Pars medialis continues frontad in the Lobus internus: fig. I13. This section goes through the anterio-r part of the Sulcus postremnis (S. po.). Ventral to this groove lies a molecular layer (M. 1.), which must thus be assigned to the Pars medialis. Dorsal to this groove lies the Corpus cerebelli. Granular c-ell—s~trings are lacking in the Crista cerebelli.
fig. 114. The molecular layer in Pars med-ialis here bends over to pass into the ventral molecular layer in the caudal part of the Corpus. At t-his point, or imnnedia-tely in front of the same, opens the ventro-caudal part of the Canalis cerebelli in ventricle IV. The granular cell—strings in the Lobus internus (s. g. c.) and the anterior part of the Recessi laterales (R. 1.) can be observed laterally.
fig. II 5. This section lies somewhat frontad of the preceeding one. The ventro—ca.udal part of the Canalis cerebelli (Ca. c.) is in open communication with the 4”‘ ventricle. The Lobi interni and externi become merged laterally. On this point lies the anterior part of the granular cellstring (s. g. c.). Close to the intersected portions of the"-Tectum (T. 0.), the granular cell- mass reaches the surface of the brain and «divides a dorsal part of the molecular layer from a lateral. If the series of sections is followed frontad, it can be seen how this ”nake.d” place Widens more and more.
fig. 116. On the right side of this section lies, lateral to the auricle, a granular cell—mass. (* fig. 116), which somewhat further on (see the left side of the figure) passes into the lateral part of the bulk of granular cells (Eminentia granularis [E. g.]).
A comparison between Salmo and Lepidosteus can now be easily made. The boundary between the Corpus and the auricles should in Salmo clearly be drawn where the dorsal moleeular layer in the Corpus finishes laterally. Between this layer and the corresponding layer in the auricles the granular cell-mass extends up to the surface as in .Lepidosteus. It is not, indeed, possible to draw up any boundary in Salmo between ‘the granular cell-mass in the Lobus -rinternuxs and in Corpus, but this can easily be explained by the assumption that the scattered granular cells which .are to be met with in Lepidosteus between the granular cell-agglomerati<on's, have increased in number, so that a continuity -has formed between them. Not till further frontad do the granular cell-niasses in Salmo divide into two parts, of which one continues into the 'Va1vula and the other covers the anterior part of the auricles. I am unable to determine in what meas.ure the lateral granular cells in the auricles, visible to the right on fig. 116 *, correspond with the ventral granular cell-mass in the Lobus externus of Acipenscr.
fig. 119. fig. 117. Transverse section through the posterior part of the Cerebellum of Osmerus. (Sketch.) fig. 118. Sagital section through the Cerebellum of Osmcrus. (Sketch.) fig. 119. Horizontal section of Salmo at stage D. (Microphotograph.) C, Cerebellum; D. IV, Decussation of Nervus trochlearis; g. m., granular cell-mass; P. rh. m., Plica rhombo-mesencephalica; R. p. m., Recessus posterior mesencephali; s. g. c., granular cell-string of the Pars medialis auriculi; T. ch., Tela chorioidea; T. 0., Tectum opticum; T. s., Torus semicircularis; U. g., ”Ubergangsganglion”; V1. c., Valvula cerebelli; Z, the boundary-zone between the Mes- and Metencephalon.
In Salmo a molecular layer pnojects into the Corpus in a medial direction from the molecular layer in the auricles * * fig. 116. This layer is connected caudally with the molecular layers in Pars medialis and with the ventro-caudal part of the Corpus. The co1'1‘e:spo~ndin.g Pu-nkinje—cell layer lies in the same position as the large cells, described above. in Lepidosteus, and also by JOHNSTON (1901) in Acipenser. It may be assumed that these cells, which at least in Lepidosteus, greatly resemble the true Purkinije-cells in Salmo, have been regularly arranged and form together with cell-neurites, a typical molecular layer. The granular cell—st:rings in the Pars auricularis are in complete conformity to the corresponding strings in Lepidosteus.
Thus the Cerebellum in Salmo differs from the Cerebellum in Lepidosteus, chieﬂy by the considerable increase in the number of granular cells. This must also be the cause of the Cerebellum—ventricle growing together to a canal rudiment: the Canalis cerebelli.
Before passing on to a description of the Valvula cerebelli, it seems suitable to say a few words about the Cerebellum in fully developed bony fishes. I have examined Osmerus eperlanus and Labrus rupestris.
Caudally in the Cerebellum in Osmems lies a -granular cell-agglomeration (5. g. c. figs II7, 118), which clearly is the granular cell—string in Pars medialis auriculi described in Salmo. In Osmerus, Sulcus postremus has disappeared, and Pars medialis become merged in the Corpus. It is further characteristic for this species, that the whole length of the Lobus internus is united with the Lobus externrus. The granular cell-string in the Pars medialis can be traced frontad as two lateral strands close to the ventricle. The granular cell—mass in the Lobus internus is powerfully developed and forms two eminenoes, one on each side of the brain (Eminentia granularis). In other respects Osmerus corresponds with Sal-mo.
A well-defined granular cell—string is lacking in the Pars medialis in the higher bony fishes, as e. g., in Labms. On the other hand, the strings in the Lobi interni are to be found lying in the same position as in Osmems. In the Pars medialis these strings turn into a small number of scattered granular cells, which form the remaining part of the granular cell-string in this part. The Eminentia granularis are not so powerfully developed as in Osmems. Corpus cerebelli is how-ever considerably larger in Labrus. The caudal part of the Corpus projects a good deal over the Pars medialis.
Valvula cerebelli. Already at stages 0,7 and 0,8 cm. in Salmo, it is possible, on haemalum—stained sagital—sections of the brain, to discern the decussation of the trochlear nerve as a lighter spot in the embryonic cell- mass (D. IV fig. 41). At these stages the decussation lies fairly close to the fissura rhomlbo—mesencephalica and thus in a position normal in the rest of the vertebrates. It is possible to observe during the process of development, how the decussation becomes more and more displaced in a caudal and ventral direction, so that it finally, in the fully developed brain‘, lies right under the valvula. As at earlier stages, fissura rhombo-mesencephalica lies where the thin" caudal wall of the Mesencephalon passes into the Valvula cerebelli, the boundary between the Mes- and Metencephalon should at later stages also be drawn at this point. The displacement of the trochlearis decussation can -therefore only be explained by the growth or expansion of the part between the boundary-plane and the decussation. Although this part takes the shape of a fold, under the Tectum, it is not thereby proved that it (the orirnent of the Valvula) increases frontad. There is also a possibility of the caudal part of the Tectum growing caudad. As a matter of fact, this latter seems to be the case, at any rate in the beginning. If the position of the anterior part of the Valvula fold is compared in relation to the Sulcus intraencephalicus posterior viz. the boundary-line, it becomes apparent that no change has taken place in this position although the Valvula has enlarged and developed more and more. It is not till the eldest stages (D. [fig. 52]) that a portion of the Valvula (that in front of ”Uber-gangsganglion” of MAYSER) lies in front of the boundary-plane, and has thus grown in a frontal direction in relation to the latter.
With the development of the Valvula cerebelli, the boundary-plane between the Mes- and Metencephalon becomes more and more curved. On horizontal sections (fig. 119) the boundary-line (Z—Z) forms curves with the convexity pointing frontad. These distinct lines form the anterior boundary to the Isthmus region.
Immediately behind the boundary-line lies a compact mass of cells, which forms the so-called ”Ubergangsganglion”, which "thus lies in the Metencephalon.
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