Paper - Embryological and morphological studies on the mid-brain and cerebellum of vertebrates 1

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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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This historic 1921 paper by Palmgren describes the embryological development of teh cerebellum in vertebrates.



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Embryological and Morphological Studies on the Mid-Brain and Cerebellum of Vertebrates

by

Axel Palmgren

I. The Mesencephalon

In order to have fixed points from which to determine e. g. the morphological position of a nucleus, I have tried as accurately as possible, to draw the boundary—lines between the Mesencephalon and adjacent parts of the brain and then subdivided the Mesencephalon into fixed smaller portions. The anterior and posterior boundary-planes of the mid-brain have been determined in conform-ity with the boundary-planes of the Encephalomeres or segments 1, in the same manner to be found in all vertebrates. These segments are indeed, distinct as such, i. e. as dilatations separated by external constrictions, and internal ridges, only in comparatively early embryonic stages. I think, nevertheless, that I have suceeded in following their original boundanies to later stages, and in determining these boundaries in relation to dif1'erentia.ted structures. A transversal division of the Mesencephalon has thereby been obtained.

A longitudinal division has been obtained partly guided by ventricular sulci, partly also by the cell-formation itself.

I «begin by describing the development of the mesencephalic segments.

‘ By segments is meant in the following, the so-called secondary segments. The question of the importance of these segments, from the point of view of the segmentation of the head, lies outside the scope of this investigation.


1. The Mesencephalic Segments

MCCLURE (1889) and KUPFFER (1885, 1893) were the first to state that the segmentation of the Met— and Myelencephalon, earlier -described, extends forward to the anterior part of the brain. The number of «segments in front of Eissura rhombo—mesencepha1ica have, by ZIMMERMANN (I891), FRORIEP (1892), WATERS (I892), KUPFFER (I893), Locv (I894, 95), NEAL (1898), HILL (1899) and others, been fixed at five. ZIMMERMANN, NEAL, FRORIEP and ICUPFFER (I893) assigned three segments to the Mese.nce— phalon, namely, segments 3, 4 and 5; VVATERS, MCCLURE, HILL and KUP1«‘1=1«:R (1905) two segments (4 and 5). The Mesencephalon was previously considered by, e. g. ORR (1887), and others, to be formed of only one segment.

I agree with this last—nam.ed investigator in so far as that the Mese-ncephalon is formed by the fourth segment, while, in all the animal types I have examined, the fifth segment is completely reduced. At early embryonic stages however, two segments, the fourth and fifth, must be assigned to the mid—brain.

The third segment has by KUPFFER been termed Synencephalon and by him been assigned to the ’tween—brain (Dienoephalon) together with the second segment or Parencephalon.

The earlier assignment of the third segment -to Mesencephalon was on the following grounds: as KUPFFER, NEAL and others stated, it is possible at certain embryonic stages of all veitebrates, to distinguish two folds on the ventral side of the brain, the posterior one is the Plica encephali ventralis; the anterior one is a fold which bounds the hypothalamic region caudad. This latter fold contimies as a groove, fissura proisoameseiicephalica, round the bmin, and divides an anterior part, Prosencephalon, from a posterior, Mesencephalon of iHUXI.EY. This posterior part is later subdivided into three segments.

NE'AL (1898) has however, stated that the Commiissuura posterior developes between segments 3 and 4. At the dorsal anterior edge of the third .segment, the Epiphysis is formed. Nervus III ‘starts from .segment 4.

Thus the boundary-line Commissura posterior-Tuberczulum posterius (HIS) (or behind Tuberculum posterius [KUPFFER]) does not coincide with the segmental groove between Pros- and Mesencephalovn in earlier stages. I believe it to be most suitable, «on grounds which I give later, to draw the boundary between the ’tween-brain and the mind-brain, between segments 3 and 4, and thus in accordance with KUPFFER (I905), assign segment 3 (Synencephalon) to the ’tween-brain (Diencephalon). To the Mesencephalon I thus assign segments 4 and 5. A. S e 1 ach ii {Acanthias 7/ulgaris).

In parasagital sections of the brain of an embryo of Aamthias at stage 1,1 cm., it is easy to distinguish the segments (fig. 1). (Compare KUPFFER [I905], fig. 89). Of the five segmental ~dilatations, which lie before the fissura rhombo-mesencephalica, the first forms, as is known, the Telencezphalon the second and third respectively Parencephalon and Synencephalon, and the last two, Mesencephalon. As long as the segments are distinct, they are sepanated from each other, partly by grooves or constrictions, partly by inner ridges, corresponding with these grooves. Between the grooves the segments protrude somewhat outwards (ORR [I887]). The boundary- grooves as well as the ridges at stage 1,1 cm. are only developed on the lateral walls of the brain, and thus do not appear on the medial sagital sections. This does not however apply to fissura rhornbo—rnesencephalica, which is a circular fissure or nearly so. So far as I know, it has not hitherto been possible to observe the two mesencephalic segments of Acanthias in later stages of development; the Mesencephalon is .said to become unified very soon.


At stage 1,5 cm. it is stillrpossibvle to observe the segrnenrts in the same manner as in stage 1,1 cm. An important change h.as, however, taken place. VVhile at stage 1,1 cm. the two mesencephalic segmenlts are approximately of the same size, at stage 1,5 cm. the anterior segment is considerably larger than the posterior (fig. 2).

On fig. 3, which represents a more medial section of the same series as that shewn in fig. 2 the boundary-ridge between the Synencephalon and the finst mesencephalic segment (boundary—ridge S——M,) is touched, the boundary-ridge M,—M2 is intersected.

A section in the direction given by the line 4——4 F.ig. 3, is given in fig. 4. The considerably larger Mfsegment is here sepanated fromthe M2- segment by a small depression ‘pin the lateral walls of the brain, boundary- groove M,—M2. fissura rhomibo-mesencephalica. also appears distinctly.

A comparative study of stages 1,5, 1,8 and 2,0 om. gives the answer to the question as to what becomes of the segment M2. A parasagital

fig. I. Parasagital section of Acanthias at stage 1,1 cm. figs. 2, 3. Parasagital sections of Acanfhias at stage 1,5 cm. fig. 4. Horizontal section of Acanthias at stage 1,5 cm. fig. 5. Parasagital section of Aczmthias at stage 2,0 cm. fig. 6. Horizontal section of Aczmthiax at stage 2,0 cm. (Microphotographs)

C, Cerebellum; F. mi-ma, the boundary-groove between the mesencephalic segments; F. rh. m., fissura rhombo-mesencephalica; F. sm., fissura. syn-mesencephalica; M1, the anterior mesencephalic segment; M2, the posterior mesencephalic segment; N. III, Nervus oculomotorius; P, Parencephalon; r. ml-ma, the boundary-ridge between the mesencephalic segments; 1'. s-m1, the boundary-ridge between the Syn- and Mesence— phalon. S, Synencephalon; T, Telencephalon.

fig. 4. section of stage 2,0 cm. shews the now imrrnense difference in size between the M1— and the M2—segments (fig. 5). The boundary-1‘idge S—M, and likewise the ridge M1—M2 are touched. It is now easy by means of a horizontal section in the direction given by the line 6—6, to «prove that the M2-segment has been reduced, or as it appear.s, oompressed by the growth of the M,- segment, so that a groove has been formed. This groove is Sulcus intra- encephalicus posterior of KIJPFFER (fig. 6).

A distinct impression of the compression of the M2-segment during the process of development is given by the features received from sections of a somewhat later stage (2,7 cm.) (fig. 7).

On the parasagital section, fig. 7, structural features corresponding to those in fig. 5 of the 2,0 cm. stage are shewn. Sulcus intraencephalicus posterior has become considerably deeper and more sharply defined, which is best seen on horizontal sections in the direction given by the line 8—8 fig. 7 (figs. 8 and 9). At stages 1,8 to 3,2 cm. in Acanthias, Sulcus intra- encephalicus can be traced. from the region of the Velum antieum downwards along the inner side -of the lateral walls. There is no dorsal or ventral connection between the two laterally «developed sulci. In other wonds, Sulcus intraenoephalicns posterior is not Visible on the medial sagital section.

In further suppont of my View on the connection between the M2—seg— ment and Sulcus intraencephalious posterior, it may be stated that the posterior oculomotor fibres, which, like the whole Nervus III, emerge from the brain between the ridges S—M1 and M,—M2 (fig. 3, compare also NEAL [I898]) at the same time as the vigorous development of the M1—se-gment and the compression of the M2—segment, will lie nearer to fissura rhombo— mesencephalica, without however, passing the boundary between the M,— and M2-segments.

In order to prove that Sulcus intraencephalicus posterior is the inner concavity of the reduced Mfsegrmaent, it may be asserted: I) At stages I,5—2,7 cm. a reduction in the size of the M2-segment can be traced step by step; 2) The bounndary-ridges M1——M2 are displaced more and more caudad, and limit the Sulcus intraencephalicus posterior fnontad; 3) Nervus oculomotonius, which, at stage 1,5 cm., li-es comparatively far in front of

fig. 7. Parasagital section of Acanthiar at stage 2,7 cm. fig. 8. Horizontal section of Acanthia: at stage 2,7 cm. fig._ 9. Detail of the section fig. 8 showing the Sulcus intraencephalicus. fig. I0. Sagital section of Acanthias at stage 2,0 cm. figs. II and I2. Sagital and parasagital sections through the ventro-medial part of the mid- and hind- brain of Acanthias at stage 2,7 cm. (Microphotographs.)

C, Cerebellum; E. i., Eminentia interpeduncularis; F. i., Fovea isthmi; F. mg-ma, the boundary-groove between the mesencephalic segments; F. rh. m., fissura rhombomesencephalica; M., Mesencephalon; M1, the anterior mesencephalic segment; P., Paren- cephalon; S_., Synencephalon; S. i. p., Sulcus intraencephalicus posterior; T., Te1encephalon. _c ‘4:.1 9 ‘I -. I0


fissura rhombo—mesencephalica within the M,-segment, is displaced caudad (while netaini-ng its morphological posit-ion) in the same degree ‘as the M2- segment becomes reduced. '

By Sulcus intraencephalzicns posterior, then I mean the groove, described by KUPFFER and others, which ~,in certain embryonic stages recurs in the whole of the vertebrates in the inner side of the lateral walls of the brain between the Mes- and Metencephalon or”-more correctly in the caudal part of .the M;esencephalon. These grooves are not, as will -directly be shewn, connected with the Fovea i-sthrni of HERRICK (”hintere Mit.telhirngrenze” of BURCKI-IARDT, ”Isthrrmsgrt1be” of H15, Sulcus interencephalicus of I-IALLER), which are sometimes, in wonks bearing on the subject (by KUPFFER, amongst others), named Sulcus intraencephalicus and described as the ventral part of the lateral Slulci intraencephalici.

In order to prove this more clearly, it is necessary first to "determine the position of the fissura rhombo-mesencephalica in‘ relation to Sulcus intraencephalicus and ‘Fovea isthmi. As already stated, this is the .most distinct of the boundary-grooves. At the earlier stages (1,I cm.) the groove is about equally well-diefined all round .the brain. Later (stage .I,5 cm. etc.) it becomes laterally more obsolete, while ventrally and donsally it appears continually more distinct and clearly -defined. At stage 2,0 om. (fig. 10) fis»s,ura rhombo-mlesencephalica 1 lies immediately in front of the -d«e~press.ion in the floor of the brain, called by HERRICK Fovea isthmi.

In front of fiss-wura rhomxbo-mesencephalica appears at stage 2,0 cm. (fig. 10) another fairly distinct groove ‘with about the «same lateral extension as this fissure. This just-mentioned groove lies opposite the boundary- ridge M,—'M,, and can therefore be described as the boundary—groove between the M,— and M2-segments (F. m,———m, fig. I0, comp. also NEAL [I898]).

The boundary-groove M,—M2 becomes later continually clearer and more distinct (stage 2,7 cm. fig. 11) and then forms the apex of the cranial flexure. Somewhat behind this, fissura rhombo-mesencephalica is to be found. (F. rh. m. fig. 11) 2. Thus Fovea isthmi clearly lies behind fissura rhombo-mesencephalica, that is ,to say, in the cerebellar neuromere, while Sulcus intraencephalicus lies in front of fissura rhornbo—mesencephalica (comp. KUPFFER [I9o5]). ,

A longitudinal section immediately lateral to the medial plane of stage 2,7 cm. (fig. 12) is especially illuminating. Between the mborundary-groove

’ In spite of the groove having already become considerably indistinct laterally, it can yet be maintained with certainty, that the groove marked F. rh. m. in fig. 10 is fissura rhom-bio-mesencephalica. In the series of sagital sections of the stage 1,5 cm., it can namely be traced as a distinct groove, at least far enough in a lateral direction to prove that it corresponds to the boundary-ridge between the Mes- and Metencephalon.

’ It is to be noticed that both at stage 2,0 and 2,7 cm. fissura rhombo-mesencephalica lies immediately in front of Fovea isthmi, and is thus the same in both cases.


M1—M2 (F. m,~.m2 fig. 12) and fissura rhomJbo—mesencephal-ica (F. rh. III. fig. 12) lies Sulous intraencephalicus posterior (S. i. p. fig. 12) and a little behind this a groove (F. i. fig. 12) issui-ng from Fovea isthmi (comp. below). By following the series of sections it is easy to prove that this interpretation of the grooves is the correct one. It is not possible to point out the slightest connection between them.

Sulcus intraencephalicus posterior, which in stages 2,7 to 3,2 cm. is a deep groove, does not remain as such in later stages. This is, «on the contrary the case with Fovea isthmi, which is to be found, as we know, in the adult brain. Sulous intraencephalicus posterior however, does fortunately not disappear without leaving traces. While still at its clearest (stage 2,7 cm.) it is already surrounded by a relatively cell—free substance 1, which can be found again in later stages (fig. 9).

At stages 5,0 t.o 8,0 cn1., when the ependym1al—layer has been differentiated, a distinct ependym~a1—t»hickenin-g ha.s been formed, which surrounds the above—mentioned cell—free substance (r. S. .i. p. F.ig. 13). Of this ependymal-thiclcening is later formed (stage 15,0 cm.) a layer of clo.sely—packed, typical epen-dyamal cells, which lies under an inner layer with only a few of such cells (r. S. i. p. fig. 14). This latter layer, deficient o-f cells, has been formed from the above—mentioned relatively cell-free substance. In consequence of this peculiar arrangement of cells, it is.possible even in the fully «developed brain, to ascertain the exact position ‘of the rudiment of Sulcus intraencephalicus posterior. The ependymal—strand, which lies under the rest of the ependymal-layer, can easily be ‘traced from the region of the Velum anticum down to the ”I-Iaubenwiulst” of HIS, thus in the same position and extension as Sulcus intraencephalicus posterior.

From the anterior part of Fovea isthmi issues a groove (comp. stage 2,7 cm. fig. 12) which, at stage 15,0 cm., becomes exutraordinarisly distinct (F. i. fig. I 5). This groove does not extend further than to the inner side ofithe ”Ha1rbenvwulst”, and is thus a transverse groove in Sulcus longitudinalis ventralis. Fovea isthmi itself, becomes (stages 6,5 .to 15,0 cm.) more and more oompressed -so as to form the ventral part of the transverse groove just—mentioned. ,

Simultaniously with the disappearance of Sulcus in-traenceph.alicus, the distance between fissura .rhom-bo—mesencephalica and the b-ou=ndary—groove M1-—M2 is shortened; these grooves, as already mentioned, appear on the ventral side of the brain at stages 2,0 to 2,7 cm. (figs. 10, II). Already at stage 3,2 cm. (fig. 16) these grooves lie nearer each other than at stage 2,7 cm. and at last at stages 3,9 to 5,5 cm. (fig. 17) they lie so close together, that one can hardly speak of them as separate grooves. The unified groove or depression thus formed HIS (1892, a) names the Fossa interpeduncularis. As it however clearly lies within the bo«unclary—plane, I continue to call it fissura rhombo-rnesencephalica.


  • Comp. ORR’s (I887) definition of neuromeres: ”The nuclei are generally nearer the outer surface, and approach the inner surface only toward the apex of the ridge” (1). 335).



Tanking also into consideration, that only very few cells are formed from the neuroblastic layer of the M2-segment it should be rightly asserted that little more than a lamel of the ll/I2—s.egment remains between the Mes- and Metencephalon at the later stages and in the adult brain. The rudiment of the M2—segment, which really lies immediately anterior to the bo~undary between the Mes- and Metemephalon, can practically be said to form a boundary—plane between these parts of the brain in the later stages of development.

B. T-eleostii (Salmo salvelinus).

According to WATERS (1892), the mid-brain of Teleosts (Gadus) contains two segments.

According to HILL (1899), who has likewise found two segments in the Mesencephalon of Salmo, these segments are bounded by outer and inner grooves, instead of inner ridges, as for the most part, at least in later stages, in the other types of animals.

HILLS, figs. 12—15, Taf. 28, and Textfigure D. (page 410), of Salmo at the stages of 24-32 days’ incubation, seem to agree with my fig. 18, which represents a parasagital section of the brain of an embryo of Salmo salz/elinus at stage 0,6 cm. The inner groove (S. i. p. fig. 18), which, according to HILL, bounds the posterior mid—brain segment caudad, is in this case, Sulcus intraencephalicus posterior.

This groove, as well as that described by HILL, as the boundary—groove between the mid—brain segments, lies clearly, at least at «stage 0,6 cm., intrasegmentally and not intensegmentally. I have not had the opportunaity of investigatinug what the case is in earlier stages. As a distinct Sulcus intraence—


figs. I3 and I4. Details of transverse-sections of Acanthias at stages 5,0 and 15,0 cm. fig. 15. Horizontal section through the Sulcus longitudinalis ventralis in Acanthias at stage 15,0 cm. figs. 16 and I7. Sagital sections through the ventro-medial part of the mid- and hind—brain of Acanthias at stages 3,2 and 5,5 cm. fig. I8. Parasagital section of Salmo at stage 0,6 cm. fig. I9. Sagital section of Salmo at stage 0,6 cm. (Microphotographs)

C, Cerebellum; E. i., Eminentia interpeduncularis; F. i., Fovea isthmi; F. rm-ma, the boundary-groove between the mesencephalic segments; F. rh. m., fissura rhombomesencephalica; F. sm., fissura syn—mesencephalica; H, the hypothalamic region; M, Mesencephalon; Nc. IV, Nucleus trochlearis; P, Parencephalon; r. S. i. p., the rudiment of the Sulcus intraencephalicus posterior; S, Synencephalon; S. i. p., Sulcus intraence— phalicus posterior; S. l. v., Sulcus longitudinalis ventralis; T, Telencephalon.

I2 fig. 19. 4 I AXEL PALMGREN

phalicus exists at certain stages of Salmo, it may readily be assumed that this groove has arisen in like manner in the Tueleorsts, as e. g. in A canthias. What however is of greater importance for this investigation, is that Salmo at 0,6 cm. an.d older stages is in conformity with the other animal—types at corresponding stages.

At the earliest stages (0,6 and 0,8 cm.) available to me for. investigation, Sulcus intraencephalicus is thus already fully developed. In the lateral -sections of a series of sagital sections of stage 0,6 cm., the msesencephalic ventricle has the form of a triangle, whose ventral apex is constituted by Sulcus intraencephalicuas. Th-is groove can be followed section by section down towards Sulcus longitudlinalis ventralis. In the medial plane in front of fissura rhomabo-mesencephalica there is indeed to be found a -depression in the floor of the mid—brain (-1fig. 19) (comp. KUPFFER [1905] fig. 148), but this is limited towards the front by a groove, which later will be shewn to be the boundary—g'roove between the Syn— and Meseneephalon, and thus -to «be fissura syn—mesencepha.lica. This groove must therefore not be compared with the boundary—groove M,—M.,, which in Acanthias bounds Sulcus inxtraencephalicus frontad. The medial depression is thus not the ventral part of Sulcus intraencephalicus, as KUPFFER asserts. The boundary—groove M,——M2 does not occur in Salmo at the stages which I have had available for investigation. It may possibly be found at earlier .-stages.

Immediately behind Sulcus intraencephalicus (S. i. p. fig. 20), which distinctly lies in front of the fissura nhomubo-msesencephalica, lies another similar groove (F. i. fig. 20) in the same position an-d with the same extension as that which -issues from Fovea isthmi in Accmthias (F. i. fig. 12). An insignificant ridge divides these grooves: the boundary-ridge M2—Mt. (r. m2—— .t. fig. 20). Fovea isthmi, like the lateral groove emerging from it, lies immediately behind fissura rhomlbo—rniesencephalica. Neither can I, in Salmo, discover any connection between these grooves.

In all these respects, Salmo i.s in accord with Acanthias.

By reason of the ventro-lateral parts of the m-id-brain in Salmo not being vertically raised as in Acanthias embryos, but lying more or less horizontally (comp. -‘figs. 57 and 71), Sulcus intnaencephalicu-s posterior describes a curve from Svulcuslongitudinaliis ventnalis upwards. (In consequence of this, the dorsal parts of Sulcus intraencephalicus are not visible on horizontal sections as in Acanthias).

As late as stage 0,9 cm._.t.here is no trace of a ventral communication between the lateral Svulci intraencephalici. Such a communication is, however, found later. The cause ofthis seems to be, that the boundary-ridge M,—Mt. becomes ventrally consi-derably higher than at earlier stages, while at the same time the ventro-medial part of the mid—brain thickens. At stage I,O cm. two distinct ventral grooves have thus been formed (fig. 21) (comp. KU1>I=FER’s [1905] fig. I 50). The anterior groove is Sulcus intraencepha— licus (S. i. p. fig. 21), the posterior is Fovea isthmi (F. i. fig. 21) (not Sulcus intraencephalicus as KUPFFER affirms), which, in Salmo also has been compressed to a well-defined groove. .

Step by step, in the stages up to 1,2 cm. a fusion of these grooves can be observed, that is to say the boundary-ridge M2—Mt. (r. m2—mt. fig. 2r) disappear more and more. In this way i.s formed the large and well—defined medial groove or depression, which, in the fully developed brains of e. g. Salmo, Osmems, Labrus and others, lies on the floor of the brain under the Va.lv.ula cerebelli (F. i. fig. 52). It is thus established that in Salmo there is a fusion between Fovea isthmi, which lies behind the borundary-plane between the Mes- and Metencephalon, and Sulcus -intraencephalicus, which lies in front of the same. This entitles us all the move, at later stages, to use the rudiments of the po~st~er\ior Mesencephallon segment as boundary- marks. The lateral parts of Srulcus intraesnoephalicus disappear completely a few days after hatching. (Regarding the remaining traces of it and the boundary-zone, see page 16).

C. Amphibia (Rana temporaria).

The segmentation of the amphibian brain has been exarnsinedby WATERS (I892) (Amblystoma), MCCLURE (I889) (Amblystoma), KUPFFER (I885, I905) (Salamandra) and others. The number of segments in front -of fissnura rhombo-mesencephalica is five, two of which belong to the midi-brain.

The stage 0,4 cm. of Rana can be said to correspond with stage 1,5 cm. of Actmthias, in so far as that, in the former, the anterior Me~sencepha.lon segment is distinctly larger than the posterior. The segm-ents are mostly clearly defined on horizontal sections (fig. 22). The protrusion of the posterior segment, forms, in Rana, Su-lcus i-ntraenoephalicus posterior, which, even at only slightly later stages (o,5 cm.) becomes charactenistically developed as a well-«defined groove, after which it increases in distinctness and depth. T.owands the dorsal side it can be traced in the same way as in Gallus (see page 20). At the stages of about 3 cm. body-length, Sulcus intraencep-halicus commences growing together, and by, e. g., stage 3,6 cm., becomes almost defaoed.

Already at stage 0,7 cm., the floor of the groove is formed of cells, which, by reason of their bi-polar shape and greater receptibility to stains, are distinguishable from the neighbouring cells. At later stages these cells remain in the form of an ependymal-thickening on the point where Sulcus intraencephalicus was situated. From these cells issue thick ependymal-fibres towards the surface of the brain (e. f. fig. 23). Owing to the fact that Sulcus iriutraencephalicus in comparatively late stages, lies near or in the b0undary—plane M2—Mrt. itself, this ependymal li-ne must be considered as a boundary-zone (2. fig. 24). A similar boundary-zon.e is also to be found in Salmo at stages 0,9 cm. — D. (2. fig. 119).


In «none of the stages, which I have examined, Sulcus intralencephalicus has been ventro—rnedia1l_v developed; it is, however, possible, e. g. at stage 3,0 cm., to trace Sulcus intraencephalicus to sections which lie immediately beside the medial plane, where a very well-defined fissura rhombo—mesencephalica is ventrally developed. This groove clearly lies behind Sulcus intraencephalious. On the medial section a rather indistinct ventral eminence (the boundary-ridge M2—M-t. [r. m2—mt. fig. 25]) is perceptible which lies opposite tr: fissura nhomsbo-mesencephalica (F. rh. m. fig. 25). At stage 3,6 cm. this eminence is to be found .in the same position as in stage 3,0 cm. (r. m2—~m.t. fig. 26). Caudally to this, the floor part of the Metencephalon is somewhat concave. This concavity, Fovea isthmi (F. i. figs. 25, 26) d-evelopes more and more in later stages, and remains in the fully udeveloped brain as a well—defined depression in the Isthmus region.

D. Reptilia.

In regard to reptiles, I have had access only to a series of sagital sections of Tropidonotus matrix (at the stage with 4 boIdy—s.piral.s) and some older stages of Igmma sp. This lack of material is so much the more to be deplored, as it is exactly in Tropidonotus, (the 2% body-s.pi.ra.l stage) that KUPFFER (1905) «described and figured a larger posterior and a lesser anterior Mesencephalon segment (see fig. 245 of KUPFFER). The posterior segment would thus in Tropidonotus form the greater part of the mi.d—brain, the M,- segment being reduced.

It is clear that sueh an interpretation of the figuned structural features

fig. 20. Parasagital section of Salmo at stage 0,6 cm. fig. 21. Sagital section through the ventral part of the mid-brain in Salmo at stage 1,0 cm. fig. 22. Horizontal section of Rana at stage 0,4 cm. fig. 23. Detail of an horizontal section of Rana at stage 2,0 cm. fig. 24. Parasagital section of Rana at stage 3,6 cm. fig. 25. The ventra- medial part of the mid-brain in Rana at stage 3,0 cm. (Microphotographs)

C, Cerebellum; D. IV, the decussation of Nervus IV; e. f., ependymal fibres; F. i., Fovea isthmi; F. rh. m., fissura rhombo-mesencephalica; F. sm., fissura synmesencephalica; H, the hypothalamic region; I, the infundibular region; M, Mesence— phalon; M; and M2, the two mesencephalic segments; M. 0., Medulla oblongata; r. ma-mt, the boundary-ridge between the Mes— and Metencephalon, R. p. m., Recessus posterior mesencephali, S. i. p., Sulcus intraencephalicus posterior; T. p., Tuberculum posterius; T. s., Torus semicircularis; Z—Z, the boundary-zone between the Mes— and Metencephalon; v, the vacuolization in the Tuberculum posterius; V]. c., Valvula cerebelli. of Tropidonotus postulate a different course of development of the mesencephalic segments, than that above described. At the stage examined by me, which is considerably older than that described by KUPFFER,‘ I have been unable to find any trace of a (lesser anterior Mesencephalon—segment. KUFFFER (1893) states that these mesencephalic segments occur in, like manner in Binds_(ducks). If this is the case, I think I can give an explanation of the so-called ”anterior” Mesencephalon-segment in Tropidonoms (see below page 22).


E. A v e s (Gallus domesticus).

figs. 27, 28, of G-allus domesticus at stages of 50 and 60 hours of incubation, represent parasagital sections through the point where Nervxus III emerges from the brain. At the stage of 50 hours, the oculomotor nerve (N. III fig. 27) lies oompa.ratively far in front of F issura rhombo-mesencephalica (F. rh. m. fig. 27) and immediately in front of a groove which, by com~paring with Acam‘hia.s at the 1,5 an-d 2 cm. stages, may be defined as’ the bounsdary-gtnoove M,_—M, (F. m,—-‘me fig. 27). A gradual disp.lacement of Nervus III caudad towands fissura rhombo-«mesencephalica can be clearly -established (comp. TANDLER AND KANTOR [19o7]), i. e. the region between the groove M1—-M, and the fis-sura rhombo-mesencephalica (the M2—segment) is clearly reduced 1.

At the stage of 60 hours a well-«defined posterior segment can be observed in the horizontal section (M2 fig.‘29), corresponding to that of Acanthias at stage 1,5 cm. figured in fig. 4. It is obvious that in a parasagital section of this stage," a stnuctural feature approximately corresponding to fig. 2 (stage 1,5 cm.) of Acanthias would be received if the line a—a fig. 29 were parallel with the longitudinal axis, i. e. the direction of the section.

’ It should be observed, that the Nervus III the whole time behelds the same position to the cell-structure. The displacement cannot, thus be explained by new fibres having formed behind those already existing.

fig. 26. The ventro-medial part of the mid-brain of Rana at stage 3,6 cm. fig 27‘. Parasagital section of Callus at stage 50 hours. fig. 28. Parasagital section of Gallu: at stage 60 hours. fig. 29. Horizontal section of Gallu: at stage 60 hours. fig. :50. Sagital section of Gallus at stage 50 hours. fig. 31. Sagital section of Acanthias at stage I,8 cm. (Microphotographs)

C, Cerebellum; F. i., Fovea isthmi; F. mi-ma, the boundary—groove between the mesencephalic segments; .F. rh. m., fissura rhombo-mesencephalica; F. sm., Ftssura syn-mesencephalica; I, the infundibular region; M, Mesencephalon; M: and M2, the mesencephalic segments; N. III, Nervus oculomotorius; P, Parencephalon; r. mrmt, the boundary-ridge between the Mes- and Metencephalon; r. s-rm, the boundary-ridge between the Syn— and Mesencephalon; S, Synencephalon; S. i. p., Sulcus in1~‘raence— phalicus posterior; T., Telencephalon; T. p., Tuberculum posterius.


The anterior mesencephalic segment is, in other Words, more laterally expanded in Callus than in the corresponding Acanthias stages. That the posterior lesser concavity forms Sulcus intraencephalicus is clearly seen at later stages. (The anterior mesencephalic segment of KAMON [1906] [figs 1 and 2] appears to me to be identical with the segment D fig. 3 and 4, comp. also HILL [1899]).

Already at the stage of 50 hours, a distinct little depression of the Tectum has formed immediately do.rsal to and in front of Velum medullare anticum (S. i. p. fig. 30). This depression is also to be found at the 60 hours stage. Partly in sagiittal sections, and partly in total preparations, Sulcus intraenoephalicus can be proved to be connected with -this recess, which thus constitutes a dorsal connection between the lateral Sulci intraencephalici. A similar dorsal part of Sulcus intraenoep-hal_icus is to be found in Rana at, e. g. stage 3,0 cm. Here however Su‘lcu_>s‘ inntraencephalicus is dorsally not nearly so well-defined as in Gallus. In other types I have not been able to establish any medial dorsal pant of the posterior segment (Sulcus intraence— phalicrus). In Aamthias, however, at stages I,5—2,7 c-m., in a medial longitudinal section, the hindmost part of the Tectum is nevertheless, seen to be distinctly thickened (* fig. 31). Apart from the fact that this thickening is devoid of a cavity, this part greatly resembles the dorsal part belonging to the M2—se-gment in Gallus (comp. fig. 30 with fig. 31). I do not therefore endorse HERRIcK’s (I917) opinion, when he says that Sulcus isthrni (Sulcus intraencephalicus) in Amblystoma can be followed in a dorsal direction ”to end in the Recessus posterior :m-esencephali”).

At the stages of 4 and 43/2 days of Gallus, the roof of the Diencephalon is seen subdivided into three parts instead of two, as is usually the case (fig. 32). The foremost part belongs to the Parencephalon. Behind this, on the boundary to the central part, lies the oriment of the Epiphysis (E. fig. 32). Between the Epiphysis and Tectum lie two parts separated by a

fig. 32. Sagital section of Gallus at the 4.day stage. fig. 33. Parasagital section of Mus at stage 0,5 cm. fig. 34. Parasagital section of Mus at stage 0,8 cm. fig. 35. Parasagital section through the posterior part of the mid-brain of Acanthia: at stage 5,5 cm. fig. 36. Transverse section through the posterior part of the mid-brain of Mus at stage 1,0 cm. fig. 37. Sagital section through the ventro-medial part of the mid-brain of Mus at stage 0,5 cm. (Microphotographs)

b. M., the bundle of Meynert; C, Cerebellum; C. p., Commissura posterior; D. c., the dorsal part and column; E, Epiphysis; E. i., Eminentia interpeduncularis; F. i., Fovea isthmi; F. mi-ma, the boundary-groove between the mesencephalic segments; F. rh. rn., fissura rhombo-mesencephalica; F. sm., fissura syn-mesencephalica; I, the infundibular region; M, Mesencephalon; M1 and M2, the mesencephalic segments; P, Parencephalon; P. i., Pars intercalaris; R. p. m., Recessus posterior mesencephali; S, Synencephalon; S. i. p_., Sulcus intraencephalicus posterior; S. 1. t., Sulcus lateralis tecti; T, Telencephalon; T. 0., Tectum opticum; Z, the boundary-zone between the Mes- and Metencephalon.


well-defined gnoove. It can now, at somewlhat older stages, 5—6 days, be seen that the whole of the central part becomes continually pushed upwards into a perpendicular position, and is included‘ in the formation of the Epiphysis. The hindmost part clearly constitutes the Synencephalon-segment. It is in this part alone that commissural—fibres develope. (comp. KAMON [1906] fig. 17).

Cormesponading conditions clearly ex_ist also among‘Reptiles (TANDLER AND KANTOR [1907] fig. 3, Taf. 40, 4I). It appears to me very probable, that it is the hindmost of these three diencephalic parts, therefore in reality the roof of the Synencephalon, that I{UPFr1«:R (1893,, I905) denoted as the anterior lesser M~esencephalon-segm-ent in Tropidonotus and ducks. Commissura posterior would hardly be developed at so early a stage of Tropidonotus as that figured and -described by KUPFFER. A mistake in regard to the position of the commissure could therefore easily have occurred.

F. ‘Mam malia (Mus musculus var. albino).

The segmentation of the mammalian brain has been described by ZIMMERMANN (1891) and FRORIEP (1892). According to FRORIEP two segments belong to the ’tw:een-brain and three to the mid-brain. If we assign two segments to the mid—brain, the number of diencephalic segments should be three. Even in Zlfus nmscuilus var. albino these three diencephalic segments are discernable (fig. 33). To the first diencephalic ”segment” I have found no corresponding part in lower animals.

At stage 0,5 cm. of Mus the posterior mesencephal~ic segment is so dlistinotly developed that it appears on a parasagital section in the same manner as in Acanthia; at stage I,5—I,8 cm. (M2 fig. 33). The anterior segment M, is at this stage considerably lar.ger than the posterior, and rather extended in length; between the segments lies a well-defined boundary-ridge. Of the caudal miesencephalic seg-menrt’s inner concavity is formed a typical Sulcus intraencephalicus posterior, which at stage 0,8 cm. is deepest and best-defined (S. i. p. fig. 34). Later, at stages 1,0 to 1,6 cm., this groove grows together more and more, to vanish altogether at last. At these later stages the part where the fusion takes place is marked by a distinct ependyma1—cell thickening, from which a relatively cell-free zone (ependymal-fibre zone?) stretches to the surface of the brain, or fissura rhombo-mesencephalica (z. fig. 36). A similar zone also exists in Acanthias (z. fig. 35) and Gallus (z. fig. 99) at certain stages. As long as Sulcus intraencephalicus remains, it extends dorsally about as far as in Acanthias.

On medial longitudinal sections of stage 0,5 cm. (fig. 37), two outer grooves, about equally well-developed, are discernable, of which the posterior (F. rh. m. fig. 37) lies immediately in front of Fovea isthmi (F. i. fig. 37) and thus is fissura rhombo-mesencephalica (comp. GR6NBERG’s [I901] stage A of Erinaceus fig. 24 b, c). The anterior of these grooves (F. m,~m2 fig. 37) must be the b0undary—groove M,—M2. That this is the case appears, firstly, from the posterior mesencephalic segmentis-concavity, in «other words, Siulcus intraencephalicus, (which however, does not reach as far down as to the medial plane) lying between the latter and fissura rhombo—mesencephialica; secondly from a comparison with the corresponding stage (2,o cm.) i11 Accmthias (fig. 10). (In determining the position of S.ulcus intraenoephalicus, as also in the comparison with Acanthias, good help is afforded by the eminence [Eminentia interpeduncularis of HIS] [E. i. figs. 11, I2, 16. 37——39] corresponding with the Fovea isthmi [F. i. in the figs], with its charaeteristic arrangement of cells: as in Acanthiayat stage 2,7 cm. [fig. 11], the peripheral layer «of fibres is fairly broad in segment M1, comparatively thin in segment M2, and completely lacking under the Fovea isthmi [comp. figs. II with 38]). At a slighrtly .later .stage (0,6 cm.) (fig. 38) the boundary—groove M,—M.,, (F. m, m2 fig. 38) has, as in Acanthias at stage 2,7 cm. (fig. 11) become the best-defined of the two grooves in question. At this stage and at stage 0,55 cm. a faint ooncavity, i. e. a ventral connecting part of Sulous intraencephalicus has formed ventrally on the floor of the posterior mesencephalic segment. This concavity soon disappears again, however. At stage 0,7 cm. it has definitely vanished. At stage 0,6 cm. (fig. 38), which may be compared with stage 2,7 cm. in Acanthias (fig. 11), the fissura rhombo-mesencephalica can .still be clearly »distinguished from the boundary- groove M,—M2. Stage 0,7 cm. (fig. 39) can be compared with stage 5,5 cm. in Accmthias (fig. I7) fissura rhombo-mesencephalica. and the boundary- groove M,—M2 lie so close together, that they almost form a single groove (Fossa interpedunculanis of HIS).

It is necessary in Mus also to distinguish a Sulcus intraencephalicus from a Fovea isthmi, both of which are found simultaneously and distinct from one another at stage 0,6 cm. The latter disappears later, at the same time as the Sulc-us longitudinalis ventaralis in its entirety grows together, and is thus not to be found at later stages, as is the case in lower vertebrates.

2. The Morphological Delimitation of the Mid-Brain

In drawing up the boundary-lines, I have first and foremost sought to determine the natural boundaries between the «different parts. A boundary- definition ought further to be such that it can apply to all the vertebrates. As the same number of segments are simila.rly developed in all the vertebrates, it should be possible to consider the boundaries between rthese segments as natural and homologous for all vertebrates. It is clear that the boundary between two segments cannot be fully expressed by a straight line (in e. g. a medial sagital section), as the boundary is a surface. It is, a priori, to be expected, that,during the process o-f development, a boundary plane, which from the beginning is level, becomes move or less ourved, as some parts develope more than others. It is thus clearly imposssible in such a case that boun(lary~grooves and corresponding interneuromeric ridges should alone suffice. Straight lines drawn up between these (on sections) are boundary-lines only -in case of the boundary—surface being level. If this surface later becomes curved, these lines become entirely artificial, and only approximate as boundary-lines.


In young embryos the cells lie more or less distinctly arranged in rows, which radiate from the ventricle. Ependymal—fibres also radiate in the same direction so far as I could discover in Golgi— and Haemalum-preparations, in which latter ependymal—fibres can often be traced 1. Parallel with these there lies a number of lesser blood—ve.ssels. A natural boundary should therefore run parallel to the rows of cells, ependymal—fibres and in certain cases blood—vessels.

A. The Boundary between the Mes- and Metencephalon

The boundary between the Mes- and Metencephalon is marked, partly by fissura rhombo—mesencephalica and the boundary—ri:dge ciornezsponding with it; partly by Sulous intraencephalicus posterior or the epen»dy:mal—-thickening which remains after this Sulcus has disappeared 2.

‘ Comp. BURCKHARDT (1891): ’’In diesem beinahe geometrischen Geriist von Stiitz— substanz (in the mid—brain of Triton) gleiten die Neuroblasten peripheriwarts; die neugebildeten lehnen sich jedes Mal an die vorherigen an; so entstehen Reihen von Ganglienzellen, welche in gewissen Stadien durchs ganze Centralnervensystem zu erkennen sind, bedingt durch die Stiitzsubstanz.” See page 378.

‘ As has already been mentioned, Sulcus intraencephalicus posterior can practically be said to lie in the boundary-plane between the Mes- and Metencephalon.


fig. 38. Sagital section through the ventro-medial part of the mid-brain of Mu: at stage 0,6 cm. fig. 39. Sagital section through the ventro-medial part of the mid- brain of Mus at stage 0,7 cm. fig. 40. Parasagital section of Acanthias at stage 15,0 cm. fig. 41. Sagital section through Cerebellum of Salmo at stage 0,8 cm. (Microphotographs.)

b. M., the bundle of Meynert; C, Cerebellum; C. p., Commissura posterior; D. IV, the decussation of Nervus IV; F. i., Fovea isthmi; F. m,-mg, the boundary- groove between the mesencephalic segments; F. rh. m., fissura rhombo-mesencephalica; F. sm., fissura syn-mesencephalica; G. i., Ganglion intcrpedunculare; I, the infundibular region; M, Mesencephalon ; Nc. III, Nucleus oculomotorius; Nc. IV, Nucleus trochlearis; P. rh. m., Plica rhombo-mesencephalica; r. s-mt, the boundary-ridge between the Syn- and Mesencephalon; S. i. p., Sulcus intraencephalicus posterior; T. o., Tectum opticum.

24 fig. 40.

In Salmo and Rana there emanate from Sulcus intraenoepuhalicus or the ependymal-thickening well—defined ependymal—fibres, Which'togethe1' form a characteristic boundary—line (figs. 23, 24, 119). These ependymal—fibres, remain up to such late stages, (in Salmo, at least up to stage D; in Rana, (during the whole larva—stage) that a precise definition of th-e position of the boundary-plane in relation to the cell-structures is made possible. In Awnthias, Gallus and Mus (an-d also as far as I could make out from the Iguana sections available to me, in Reptiles) there is no such boundary—line of thick ependymal-fibres. On the other hand in these species the posterior mesenoephalic segment is distinguished by its deficiency in cells, so that from Sulous intraencephalicus or the ependymal—cell-thickening a comparatively cell—free bou-n;dary—z0ne (z. figs. 35, 36, 99) can be traced out towards fissura .rhombo—me~sencephalica. It is true that in older stages this zone beoomes more indistinct or even disappears, but by that time it has generally been possible to determine the position of the b~oundary—plane in relation to the differentiated structures.

The dorsal part of fissura rhombo—mesencephalica is in Acanthias, at stages 2,0 cm. to 3,9 cm. sharply V~shaped on sagital sections. Already at the 3,9 cm. stage the decussation of the trochlear nerve is visible as a colourless spot imm-ediantely behind this groove. In consequence, it would appear, of the Decussatio veli’s increase in size, Plica nhombo—mesencepha1ica (P. rh. m_. fig. 40) becomes later |_|—shaped on sections. The anterior of the closely-lying grooves (fig. 40) is clearly the fissura rhombo—mesencephalica proper. The .decus.sat.ion of the trochlear nerve (D. IV fig. 40) li-es immediately behind this. (The ventricular groove * in front of D. IV fig. 40 has nothing to do with Sulcus intraencephalicus, but developes secondarily at stages 8,0 to 15,0 cm.).

Lasterally fissura rho1nbo—mesencephalica is at stages 2,7 to 3,8 cm. faixrly shallow and vanishes entirely later on.

On a medial longitudinal section of Salmio, stage 0,6 cm. (fig. 19) fissura rh»ombo—mesencephalica appears as a well—defined groove between the Tectum and Cerebellum. Behind -this gnoove, at stage 0,8 cm., a new one (* fig. 41) has developed. The anterior and upper of these gnooves (F. rh. m. fig. 41) should be the fissura rhombo—rnesencephalica proper. Right inwards to (thus in no case behind) the lower groove, the decrussation of the trochlear nerves emerges distinctly at this stage (D. IV fig. 41). Thence-fourth fissura nhombo-mesencephalica retains its position between the Teotum opticum and Valvrula cerebelli, even at stage 1,0 cm., but disappears later completely, dorso«medially. The powerful transfo»rmation:s, which take place in this region in connection with the development of the Valvula cerebelli, cause, so to say, a straightening of that part in which fissura rhombo— meseneephalica was situated. The angle (fissura rhombohresencephalica) which forms between the caudal surface of the V alvula and Tectum, at stage 0,8 cm., becomes in other words more and more obtuse, till it entirely disappears. At ‘later stages, in spite of this, it is quite easy, to define the boundary between the Valvula and Tectum in consequence of the differing structure of these parts.

fissura rhombo-mesencephalica remains ventrally sharp and well—defined up to stage 1,0 cn1. (F. rh. m. fig. 21). At this stage another distinct groove (** fig. 21) lies behind fissura rhomrbo-mesencephalica. At stage 1,1 cm. this caudal groove remai-ns, while the fissura rhombo—mesencephalica has totally vanished 1. A homologon to the groove ** fig. 21, is developed 1) in Acanthias at stages 6,5——I5,o cm. (* fig. 42); 2) in Rana at stage 3,6 cm. (* fig. 26); 3) in Gallus at the stage of I 5 days,'(possibly earlier), and in M us at stage I,0 to 2,3 cm. In all these species, the groove lies immediately in front of Ganglion interpedunculare (the greater cellrich part of this Ganglion), developes comparatively late, and is therefore, highly probably homologous. In Salmo, during stages B and D, the distance between the groove and the bunch of cells becomes somewhat shortened, so that an enroneous interpretation of this groove as fissura rhombo-mesencephalica can easily be made.

In Rana, as in Gall-us and Mus, the ventro-lateral part of the fissura rhombo-meseincephalica is very slight and vanishes soon completely. Dorsally, the Plica rhombo-mesencephalica in Rana is we'll-defined up to stage 1,5 cm. and becomes later [_|—shaped (stage 3,0 cm.). The part between the two angles or grooves (Velum medullare anticum) becomes later more and more fronto—caudally extended (figs. 24, 96). It is easy by means of the position of the boundary-zone to determine that the foremost of these angle-s is the fissura rhombo-mesencephalica. The position of the decuissation of the trochlear nerve .is difficult to define in Rana-larvae.

In Mus on the contrary, where the fold between the Cerebellum and Tectum (Plica nhombo-mesencephalica) soon appears 1_]—shaped on sections, it is possible, e. g. at stage 0,6 cm. to determine with certainty that the deoussation of the trochlear nerve lies behind the fissura rhombo-mesencephalica. This groove, which thus bounds the Velum medullare anticum frontad, as well as the posterior groove or angle, which bounds the Velum caudad towards the Corpus cerebelli, remains very well-«defined in all investigated embryonic stages. This is not the case with Gallus, where fissura or Plica rhomnbowm-esencephalica becomes U—shaped. At the stage of 4 days, however, it can with certainty be proved ‘that the troohlear—decussati0n lies behind the fissura rhombo—mesencephalica. With the depression of the medial part of the Tecturn, fissura rhombo—mesencephalica disappears dorsally entirely.


  • It is to be noticed, that from the Sulcus intraencephalicus and Fovea isthmi issues, in a ventral direction, a pointed plug of cells, which at stage 1,0 cm. lies right inwards to the fissura rhombo-mesencephalica, but at stage 1,1 cm., a good distance in front of the now sole remaining groove. This characteristic plug of cells (which KUPFFER [1905] figures in fig. 149), should be considered homologous with the cell- mass, which in Acanthias, at certain corresponding stages (2,7—3,2 cm.) lies under the Fovea isthmi and extends at certain stages to the ventral surface of the brain (compare page 23). A similar cell-arrangement has been described by BURCKHARDT (1892) in Triton and by KINGSBURY (1895) and HERRICK (1917) in Necturus.



There is no indication of the fissura rhoniibo-mesencephalica ventromedially in Rana at stages 0,5 to 3,0 cm. or in Gallus at the 50 hour—stage. At stage 3,0 cm. in Rana there lies however a groove in such a position in relation to the Sulcus intraencephalieus and Erninentia intenpeduncularis, that it may be defined as Fvissura rhombo-mesencephalica (F. rh. m. fig. 25). The stage 3,6 cm. corresponds with the 1,0 cm. stage in Salmo, in so far as that a groove (* fig. 26) has developed behind the fissura rhombo—mesence— phalica. This caudal groove is found again in post-larval «brains immediately before the Ganglion interrtpedunculare. The fissura rho«rnbo-mesencephalica has at these stages again become obliterated., In Gallus there soon -developes (stages 60 hours etc., fig. 32) a very clear and well—defined ventral part of the fiss-ura rhombo—mesencephalica which remains at least to the I 5 day stage. I do not therefore agree with KAMON (1906) when he says, that in the 94 hour stage (Embryo VII) ”vorn oder hinten keine aussere Grenzfurche firndet” (page 607).

It is evident from the above, that the Trochlearis—d-ecussation lies immediately behind the boundary between the Mes— and Metencephalon. This is also the case with the lateral Trochlearis—tracts, which can be followed from the decussartion to the Nucleus IV. This is most easily demonstrated, e. g. in Sal-mo : stage 0,6 cm., and in Callus: the 6 day stage, where the Trochlearis— tracts are visible on haemalum preparations as white strea.l<s in the embryonic cell—mass. As the Trocxhlearis—nucleus lies in the Isthmus region in all the investigated animals, so the Nervus IV, with its nucleus lies altogether outside and behind the Mesencephalo-n, a fact which has already frequently been pointed out by HIS and other embryologists. In like manner Ganglion interpedunculare undoubtly lies in the Metencephalon (HIS).

B. The Boundary between the Syn— and Mesencephalon.

It is comparatively easy to define the caudal boundary of the mid- brain by means of the rudiment of the Suleus intraencephalicus, fissura rhombo—mesencepha1ica, Fovea isthmi and, in Rana and Salmo also by the ”boundary—zone”.

It is considerably more difficult to draw up definite boundary-lines between the Syn— and Mese-ncephalon. In the earlier stages we are chiefly obliged to rely on the bo.un;dary—groove and the corresponding ridge. In the following I term the boundary—groove: fissura syn—mesericephalica. (This boundary—groove is not, as I have pointed out above, identical with the fissura proso—mese<ncepha1ica nor with the fissura Clla-I1’l€S6I1Cepf13.liCa).

The boundary between the ’tween-brain and the mid-brain is, as is known, generally drawn from the Commissmra posterior down to (HIs)‘ or immediately behind (KUPFFER) Tuberculvum posterius. In the following I shall try to shew that this boundary—line is artificial and approximative.

The fisssura syn—mesencephalica is, in the earlier stages of Acanthias (e. g. 1,5 cm.) only a faint convex curve between segments S and M1. fig. 43 represents a parasagital section (5 .sections 51 I0 ,4» to the side of the medial plane) of the 1,5 cm. stage. Guided by the boundary—groove and especially by the boundary-ridge, it is possible to draw up the bovundary—line. If the series of sections is followed from this section in a medial direction, the position of the fissuna syn—mesencephalica can be approximately determined on the medial section.

It is further clear, that the boundary between the Har— an.d Synence— phalon should be drawn from the posterior edge of the oriment of the Epiphysis (E. fig. 43) down to the caudal edge of the hypothalamic depression, thus where the fislsura proso—mesencephalica surrounds the brain (comp. figs. 2, 3, 43).

The b0undary—riclge S——M1 is sharply—defined and clear at stage 1,5 cm. At later stages considerable transformations take place in the Synencephalon. On comparing the parasagital sections of stages 1,5, 2,7 and 3,2 cm. (figs. 43, 50, 44) it appears that the dovrso-lateral part of the Synencephalon thickens so that the dorsal part (Bars intercalaris [P. i. figs. 44, 50]) in parasagital sections, from having been curved outward at stage 1,5 cm. finishes by becoming curved inward at stage 3,2 cm. A more lateral parasagital section (fig. 45) of this later stage touches, in consequence of these transformation-s, a secondary ”boundary—ri-dge” (Prominentia »dia—tn'e:sencephalica of the authors) which will be found in the middle of the Synencephalon—segment (P. d-m. fig. 45). On the section fig. 45, the primary borundary—ridge (r. s—m,) is, however, also visible, ventrally and ndomsally intersected. In consequence of the position of the primary true boundary—ridge in relation to the structural features, I have drawn the conclusion, that the boundary between the Syn— and Me-sencephalon runs immediately behind the Commissura posterior, along the sharplyedefineld line fonmed frontad by the Tectum— ce1l—mas:s, and down to a point which for the present can be said to lie directly in front of the oculomotor—fibres (NEAL [1898]).

It is from several points of view unsuitable to «draw the boundary-line dow-n -to the Tuberculrum postezrius. \/Vithorut going into the question of the position of the Tuberrculum posterius in relation to the diencephalic segments,

29 30 AXEL PA‘LMGREN

I will only point out -that in any case it lies far in front of the primary bounrlary—riidge between the Syn- and Mesencephalon (see figs. 2, 3, 43). A line drawn from the Commissura posterior to the Tuberculum posterius would go obliquely through the Synencephalon. KUPFFER (1905) and others have therefore assigned Tubereulum posterius to the Diencephal.on, without however detemnining the position of the boundary plane more closely.

At stage I 5,0 cm., in the ventral part of the now bipartite Tuberculum posterius, a number of lymphatic cavities (v. fig. 42) have been formed under the ependymal layer, so.that the point of this part becomes vacuolized. The same curious structure is to be found i.n the Tuberculum posterius in Petromyzon and Ammocoetes. (Comp. also below p. 34). '

figs. 47—50 represent parasagital sections from a series of sagital sections of stage 2,7 cm. Guided by the position of the primary boundary—ridge, the boundary—line can be drawn as the line in fig. 50. In the basal part the cell—layers form an angle, whose apex lies in the boundary—ridge. This amrangemerit of cells is found again in more laterally lying sections (figs. 49, 48), where the boundaryplane is marked by a straight line in the cell-mass. Still more laterally (fig. 47) it is tangent to a dorso—ventr.al system of fibres, whose anterior part consists of the bundle of MEYNERT (b. M. figs. 47, 51) and the posterior part of the Commissura posterior—fibres (C. p. figs. 47, 51). The hindmost of these fibres lie close to the line which the Tec-tum-cell-mass forms in front and can thus be said to form semicircles in the boundary- plane. The bundle of MEYNERT runs obliquely downwands in a ventro- caudal direction and converges with the Comrnissura posterior—fibre~s. (The part between the Synencephalon-cell—mas.~s [S. figs. 47, 48] and the Mesencephalon corresponds to the secondary boundar '—ri-dge. As the embryonic cellmass is less compact in the periphery, and as a parasagital section cuts deepest into the cxell—mass S and M [figs 47, 48], thus giving the part that lies between, the appearance of a separate segment).

It is thus shewn that the boundary-plane is determined by the structural features themselves, and must therefore be looked upon as a natural boundary-plane. Medially to the section fig. 50 there is also to be found a structural, though not sharply-defined boundary. In the floor of the mi-d—brain there is a stratum of fibres considerably thicker than in the floor of the Synencephalon where the cell-masts extends almost down to the surface of the brain. Developing later (stage 3,9 cm. etc.) in this mesencephalic fibre—stratum, a oell-mass somewhat more compact than the corresponding one in Synencephalon, is to be found (fig. 46).



fig. 42. Sagital section through the ventral part of the brain of Acanthia: at stage 15,0 cm. fig. 43. Parasagital section of Acantlzias at stage 1,5 cm. figs. 44, 45. Parasagital sections through the Synencephalon of Acanthia: at stage 3,2 cm. fig. 46. Sagital section of Aczmthias at stage 3,9 cm. (Mic:-ophotographs.)

C, Cerebellum; C. p., Commissura posterior; E. Epiphysis; F. i., Fovea isthmi; F. mi-ma, the boundary-groove between the mesencephalic segments; F. rh. m., fissura rhombo-mesencephalica; F. sm., fissura syn-mesencephalica; G. i., Ganglion inter- pedunculare; I, the infundibular region; M, Mesencephalon; P, Parencephalon; P. d. m., Prominentia dia-mesencephalica; P, i., Pars intercalaris; r. s-m1, the boundary- ridge between the Syn- and Mesencephalon; S, Synencephalon; T., Telencephalon; T. p., Tuberculurn posterius; T. 0., Tectum opticurn; v, vacuols in the Tuberculum posterius.



To sum up, it may be said, that the boundary—plane between the Syn- and Mesencephalon ‘is determined by the fissura syn-mesencephalica and by the position of the primary boundary—ridge. This boundary-plane coincides: I) with the disappearance of the Tect;urn—format.ion in front; 2) with the dorsal and lateral parts of the Comrndssura posterior-fibres; 3) with t.he ventro-medial part of the bundle of MEYNERT, i. e. with the point where this bundle bends over in a horizontal direction, and 4) with a point immediately in front of the oculomotor nerve. A more precise definition of the position of the bo.undary—plane in relation to the structuinal features is given below (page 46). The boundary—plane thus determined, clearly lies considerably behind the line generally accepted in the literature as the boundary- line between the Di— and Mesencephalon.

In Salmo fissura syn~mes.encephalica is not dorsally and laterally developed as a sharply—defined groove at the earliest stages I have had access to (0,6 cm.), and can thus not be traced round the brain down to the ventral side. Four sharply-defined grooves have at this stage (o,6 cm.) developed ventrally. Of these the most caudal is undoubtedly fisxsrura rhombo— mesenoephalica (F. rh. -m. fig. 19). As the posterior mesencephalic segment in Salmo at stage 0,6 cm. is already reduced to a groove (Sulcus intraence— phalicu-s posterior) it must be assumed that the boundary—groove M,—M., here, as in Acanthias at stages 3,9 to 5,5 cm., has already approached the fissiira rhombo—mesencephali<:a and caused the distinct |_|-shaped appearance, which is to be found also in Salmo (fig. 19). A comparison with an earlier Acanthias stage (I,5 cm.) (fig. 43) and an examination of the series of sections would shew that the two foremost (*, ** fig. 19) of the above- mentioned four grooves, are identical with those which in Acanthias lie

directly -dorsally to the infurrdibular depression. One or both of these probably answer to the original fissura proso—mesencephalica. There remains then only one groove, which must be the fissura syn—mesencephalica (F. sm. fig. 19).


figs. 47-50. Four parasagital sections of the same series of Acanthia: at stage 2,7 cm. fig. 51. Parasagital section of Acanthia: at stage 3,2 cm. (Silver impregnation according to Cajal.) fig. 52. Sagital section of Salmo at stage D. (Microphotographs.)

b. M., the bundle of Meynert; b. v., the ”boundary-vessel”; C, Cerebellum; C. p., Commissura posterior; F. i., Fovea isthmi; F. sm., fissura syn-mesencephalica; G. i., Ganglion interpedunculare; H, the hypothalamic region; M, Mesencephalon; N. III, Nervus oculomotorius; P, Parencephalon; P. i., Pars intercalaris; P. m. a., Pars medialis auriculi; S, Synencephalon; T. 0., Tectum opticum; T. p., Tuberculum posterius; V1. c., Valvula cerebelli.

32 fig. 51.


The points of agreement with corresponding Acanthias stages are also very marked at later stages. It should here suffice to compare e. g. stage 1,0 cm. of Salmq with corresponding stages of Actmthias.

In Salmo, as in Acanthias (up to stage 8,0 cm.), the Tuberculum posterius is undivided, but in Salmo it is by this time already vacuolized (v. fig. 21), as is the case at later stages of Acanthias (v. fig. 42). Right outside the Tuberculum posterius and thus in the place where at stage 0,6 cm. in Salmo (fig. 19) and 1,5 cm. in Acanthias (fig. 43), two grooves were developed dorsally to the infundi-bular depression, in both cases there is only one groove (comp. figs. 21, 42) and behind it in Salmo, a very sharp1y—defined groove, the same which has already been assumed to be the fissura syn—mesencephalica (F. sm. fig. 21). Towands this groove the bundle of MEYNERT and the Comumisswra, posterio~r—fibres converge in the same manner as in Acanthias. The groove lies also in the prolongation of the line formed frontad by the Tectuim-cell-mass. Between this groove and the fissura rhombo-mesencephalica issues the oculomotor nerve. In relation to the ”secondary” boundary-marks, the groove in question thus occupies the same position as the fissura syn—mesencephalica in Acanthias. The only difference being, that the fissura syn—mesencephalica is very well- defmed in Sa-lmo, but in Acanthias, on the contrary, is particularly faint (F. sm. figs. 42, 46).

The boundary—plane between the Syn- and Mesencephalon can consequently be determined in the same way in Salmo, as in Acanthias. As 2. further help in the work of definition, in the case of Salmo, can be used a characteristic blood-vessel (b. V. figs. 52, 76), which, at an already early stage (0,8 cm.) is clearly developed. It enters the brain ventrally and medially at fissura syn—mesencephalica, rises later obliquely towards the ”Haubenwiilste”, after which it ram-ifies into two lateral vessels, which extend dorso-laterally towards the Commissura posterior (com-p. FRITSCH [I878]). A study of series of sagital sections of earlier stages shew clearly that this blood-vessel lies .in the actual boundary-plane, and thus i-ntersegmentally.

A comparison between the earlier and later stages of Salmo shews, that the boundary-plane, during the process of development becomes more and more cup-shaped, with the concave side facing forwards. The anterior part of Tectum grows frontad and over the Cornmissura post-erior, which, seen topographically, thus lies behind and under the same. The floor-part of the mid-bra-in becomes raised, so that the ”boun.dary-blood-vessel” takes, a nearly upright position (fig. 52). This curving of the boundary-plane occurs also in Acanthias, though in a clonsiiderably less degree.

34 5 STUDIES ON THE MID-BRAIN AND CEREBELLUM 3

It is very difficult to compare the ventro—medial part of the Syn- and Mesencephalori in Rana with the corresponding parts in Aczmthias and Salmo. To obtain points of contact, it is necessary to take very young embryonic stages (o,5—0,7 cm.) of Rana, before the transforzmations which characterize this species have set in.

In Rana at e. g. stage 0,6 om. the fissura syn-rnesencephalica is circular, and its position on the medial sec-tion can consequently be determined (F. sm. fig. 53). The groove (_* fig. 53) which lies in front of the fissura syn- mesencephalica can be traced in a lateral and frontal direction, where it forms the boundary between the hypothalamic region and the remaining part of the Diencephalon. The part which, in the medial section, lies immediately dorsal to the infundibular depression and rightopposite this groove, is doubtless Tuberculum posterius. Not till stages 3,0 and 3,6 cm., does this part become vacuolized in the same manner as in Acanthias.

The distance between the ventral parts of the grooves in question becomes lessened at later stages (0,7——0,8 cm.) till they can no longer be distinguished from each other, in other words, the grooves converge down to the same ventral point (fig. 54). The whole of the powerful floor—part of the Synencephalon, which in Acanth-ias lies between the fissura syn-mesencephalica and Lobus posterior is thus considerably reduced in Rana. At later stages, while the groove * fig. 53 remains, the fissura syn—mesencephalica disappears entirely laterally and ventrally.

In Rana the fibres of the bundle of MEYNERT lie so scattered, that they cannot be identified in a haemalum—preparation. The Commissura posterior complex and the Tectum—cell—mass remain however, as good landmarks. The boundary-plane in Rana lies very nearly vertical, and almost level in contrast to what is the case in Acanthias and Salmo.

N-either Salmo nor Rana are, independently of Acanthias, suitable subjects for a study of the boundary—plane between the Syn- and Mesencephalon. Gallus, on the contrary, seem.s in several respects, to be an even more suitable subject than Acanthias in this connection. While in Acanthias the fissura syn-mesence.phalica is very slightly developed, and it therefore becomes necessary to rely almost entirely on the position -of the boundary- ridge when determining the delimi-tations, in Gallus there is besides a distinct fissura syn-mesencephalica, also a particularly well-defined and clear primary boundary-ridge.

At the stage, during which the Commissura posterior developes (4———5 days) the fissura syn-mesencephalica is dorsally very well-defined, and lies immediately behind -the cornmissures, which forms the roof of the Synencephalon (Pars intercalaris). The caudal part of this commissural plate is composed of the Commissura posterior (C. p. figs. 55, 56). The fissura synmesenoephalica ca.n, e. g. at the stage of 60 hours — 4 days, be traced on a series -of sagital sections, in a lateral direction to a section, which ‘is tangent to the very sharply-«define-d boundary—ridge (r. s—«m, fig. 28). On the ven1tro—late:ral side of the brain, the fiss/ura syn—mesencephalica can still be perceived although faintly, immediatetly in front of the Nervus oculomotorius (fig. 28). The’ stage of 50 hours corresponds with these stages. At older stages (5 and 6 days) transformations take place in the Synencephalon, which cause the formation of a secondary boundary-ridge. The edge of this ridge (Prominentia wdia—mesencephalica), lies in front of the primary ridge, or about the middle of the Synence-phalon-segment (P. dm. fig. 56).

The Commissura posterior—fibres, the bundle of MEYNERT and _the line which the Tectum cell—mass forms frontad, lie in the same morphological position in relation to the boundary—p1.ane as in Acianthias. This is also the case in M us.

In Mus there is no boun»dary—groove between the Syn- and Meseu1ce— phalon at the earlier stages (0,5—0,7 cm.). A groove certainly does form dorsally in front «of the Tectum through the later growth of the latter (at stages 0,8 cm. and so on), but this groove cannot be considered of the same importance as a boundary-groove as. the fi-ssura symmesencephalica, which e. g. in Gallus is developed at earlier stages. ‘In Callus, as well as in Acan— thtlas and in the other investigated species, a secondary groove, lying opposite to Commissura posterior, developes at later stages, but only in connection with the formation of the secondary ridge (Prominentiia dia-mesence— phalica). It must have been these completely secondary ”Sulcus” and ”Pro— minentia dia-rnesencephalica” which have caused the borundary—plane to be generally located right through the Commissura postenior.

fig. 53. Sagital section ‘through the ventro-medial part of the mid-brain of Rana at stage 0,6 cm. fig. 54. Sagital section through the ventro-medial part of the mid-brain of Rana at stage 0,8 cm. fig. 55. Parasagital section through the Synencephalon in Gallus at stage 4 days. fig. 56. Parasagital section through the Synencephalon in Gallus at stage 5 days. fig. 57. Transverse-section through the mid-brain of Acanthias at stage 3,9 cm. fig. 58. Transverse-section of the mid-brain of Acanthias at stage 5,0 cm. (Microphotographs.)

C. p., Commissura posterior; D. c., the dorsal part and column; F. 1. d., Fasciculus longitudinalis dorsalis; F. sm., fissura syn-mesencephalica; F. rh. m., fissura rhombo- mesencephalica; I, the infundibular region; L. c., the lateral part and column; M, Mesencephalon; M. c., the medial part and column; m., 1., the medial, lateral portion of the part in question; M. o., Medulla oblongata; N. III, Nervus oculomotorius; Nc. III, Nucleus oculomotorius; P, Parencephalon; P. dm., Prominentia dia-mesence-phalica; P. i., Pars intercalaris; p. nc., the peripheral nucleus in the lateral part; S, Synencephalon; S. c. e., Stratum cellulare externum; S. c. i., Stratum cellulare internum; S. e., Stratum ependymale; S. 1., Sulcus lateralis; S. 1. v., Sulcus longitudinalis ventralis; T. f., the Torus-formation; T. 0., Tectum opticum; T. p., Tuberculum posterius; V. c., the ventral part and column; vl, the ventro-lateral portion of the ventral column.

36 fig. 56.

ig. 54.

TANDLER AND‘ KANTOR (I907) denominate the two ventral angles of the cranial flexure of Gecko embryos respectively ”vordere” and ”hintere Scheitelkriimzn-ung”. In stage I., a ridge _”Prominentia dia-mesencephalica” proceeds from ”vor-dere Scheitelkriim.mung”. The same remains in stages II. and III., where, however, Sulcus and Prominentia dia-mesencephalica are only developed dorsally and dorso-laterally. The distance between ”vordere” and ”hin«tere Scheitell<riimmung” becomes lengthened in these stages.

In stage IV., on the other hand, Sulous and Prominentia dia-mesence— phalica are directed towards ”hintere Soheitelkriimmung”! This peculiar change of position may, perhaps, be accounted for in the following manner. In stage I. (figs. I and 2 of TANDLER), Pars synenoephalica has not yet been formed. The groove represented in this stage is evidently identical with fissura proso—mesencephalica. The groove that subsequently divides the Syn— from the Mesencephalon, at all events in Gallus, goes «down to a point tliat is somewhat behind the ”vordere Scheitelkriirnmrung”. Judging from TANDL1~:R’s figs. 3 to 6, the same seems to be the case with Gecko. Owing to the distance between the fissura rhombo—mesencephalica and the Fnisisura syn-mesencephalica subsequently beoorning shonter, not only in Gallus -but also in all those types of animals subjected by me to examination, the boundary-plane S—M, like the boundary-plane M—Mt proceeds from almost the same ventral point.

The base of the Mesencephalon thus becomes very short, more especially in birds and mammals.

In a later work on the development of the brain in Tarsius, TANDLER AND fi_EIssIG (I915) appears to have assigned the Synencephalon to the mid-brain.

3. The Embryological Development of the Nuclei

Outside the thick cell—mass, which in young ernbryons (e. g. Acanthias) forms the walls of the brain," a cortical layer of fibre-matter soon developes. The cell-layer (the neuroblastic layer) differentiates later into two layers, an inner-: Stratum ependymale (S. c. fig. 57), and an outer: Stratum cellulare internum (S. c. i. fig. 57). Cells wander out from the cell-layers. into the cortical layer and there form a Stratum cellulare externum (S. c. e. fig. 5,7);

The histological structure of the Tectum and of the basal parts not being identical, it is necessary to distinguish between a dorsal part: Tuectum mes'encephali and a ventral part in Mesencephalon. In the following I name this latter part: Regio subtectalis, or -simply Subtectum 1.

‘ According tO'EDINGER (1908), by the term Tegmentum is meant ”bei den Saugern die ganze graue Substanz, welche dorsal von den aus dem Grosshirn stammenden Fussbahnen liegt”, ”fiir die niederen Vertebraten kommt eigentlich‘ der ganze centrale

38 9 STUDIES ON THE MID-BRAIN AND CEREBELLUM 3

The boundary between these parts should clearly be drawn where the Tectum—formation ventrally ceases. The boundary-line would then run, I) immediately ventral -to the point where the mi.d—brain ventricle has its greatest lateral extension (1 term the more or less sharply defined groove which sometimes developes here, Sulcus lateralis tecti); 2) between the Tec-tum—formation and Torus semicircularis or Nucleus lateralis mesence— phali; 3) along the rows of cells and ependymal fibres outwards to the surface of the brain, where the boundary would then be dorsally close beside the optic tract.

A. Selachii (Acanthias vulgaris).

I have examined in detail the development of the nuclei in Acanthias at stages: 3,9, 5,0, 8,0 and 15,0 cm.

Stage 3,9 cm. (fig. 57): Stratum ependymale (S. e.) is at this stage in the Tectum (T. 0.), considerably thicker than in the Subtectum, where it is only ventrally and medially (as under the Sulcus longitudinalis ventrali-s [S. l. v.]) somewhat thickened. Stratum cellulare insternum (S. c. i.) is likewise broader in Tectum than in Subtectum. This Subtectum layer is subdivided into thicker and thinner parts: it thickens medially and ventrally; and becomes much thinner close to the Fasciculus longi-tudinalis dorsalis (F. l. d.), only to increase in thickness again dorsally. This latter thickened part extends up to a ventricular longitudinal groove that I name Sulcus lateralis (S. 1.) 1. Dorsally to this groove Stratum cellulare internum decreases again continuously passing over into the corresponding Tectum layer.

Cells have migrated from these different subrtectal parts of the Stratum cellulare internum into the fibre—layer, where groups or columns of cells corresponding to these parts, have formed. These columns, and also the parts of the Stratum cellulare internum extend through the whole of the Mesencephalon, from the fnontal to the caudal boundary.

. Guided by these structures, I have been able to carry out a subdivision of the Subtectum as follows: the part of the Subtectum which lies between the 'ventro-medial line and an imaginary line drawn directly laterally to the Fascicu-lus longitwdinalis dorsalis and parallel to the spindle-shaped cells which radiate towands thesurface of the brain, I have called the ventral part (V. c.). This part is bounded laterally in certain animals (Gallus) and at certain stages by a ventricular longitudinal groove. Between this groove and Sulcus longituclinalis ventralis the ventral part is at times somewhat raised and forms a longitudinal, hounded-off ridge, called ”Haubenwulst” (HIS). The Stratum cellulare internum of the ventral part forms -the Nucleus III. A number of cells lie in the fibr-e—layer forming the Ventral cell-column.


  • Abschnitt des Hinter-, Mittel und Zwischenhirn als Tegmentum in Betracht” (p. 139). In order to obtain a term for the basal part of the Meseneephalon applicable to the lower as well as the higher vertebrates, I have introduced this new appellation ”Subtectum”.
  • This groove is not identical with the Sulcus limitans of HIS. According to H15 (I893) the Sulcus limitans is to be found in the mid-brain of adult mammalian brains. In Mus the Sulcus lateralis tecti is the only remaining groove. When HIS, however, states the Sulcus limitans of adult specimens not to be the same as in embryos, there appears to me suitable to introduce the new terms Sulcus lateralis and Sulcus lateralis tecti.



I have named the medial part (M. c.) that which lies between ”Haubenwuls ” and Sulcus laterali-s, with corresponding parts of the Subtectum. I assign to this part the thickened portion of the Stratum cellulare internum and the part of the Stratum cellulare externum which corresponds with it: the medial column. This is, at certain stages, frequently divided into two parts, a medial and a lateral (M. c., m. and l.).

To the lateral part (L, c.) belong the thinner part of the Stratum cellulare internum which lies between the Torus-formation and the medial part, and also a lateral column in the fibre-layer.

Lastly, I assign to the dorsal part (D. c.) the Tonus-formation (see below) and a corresponding part of the Stratum cellulare externum: the dorsal column.

The ventral part at stage 3,9 cm. (V. c. fig. 57) is formed of the following parts: medially of a thick Stratum cellulare internum and outside this a fibre—layer in which there as yet are no cells; laterally to the Nervus III (N. III) and outside -the Fasciculus longitudinalis dorsalis (F. l. d.) a number of cells issue from the Stratum cellulare internum .surroun— ding the dorsal longitudinal bundle.

No sharp b-oundary—line can be drawn between these cells and those which radiate from the medial part of the Stratum cellulare intemum. The Stratum cellulare externum i-s formed of con.siderably fewer cells in the lateral (1. fig. 57) than in the medial portion (m. fig. 57) of this part (M. c. fig. 57).

The lateral column (L. c. fig. 57) consists of closely-packed cells, in this respect differing from the dorsal column (D. c. fig. 57), which is less rich in cells. There is however no sharp boundary—line between the lateral and dorsal columns at this stage.

The development -of the nuclei is already practically completed at stage 5,0 cm. The only real change that takes place at later stages, is that the columns and nuclei become more diffuse and the boundaries in consequence more difficult to -disti-nguish.

A comparison with stage 3,9 cm. is easily made. The Stratum cellulare internum has formed a distinct magno-cellular Nucleus oculomotorius in the ventral part. A small-cellulafed cell-agglomeration, which corresponds to the vezntro-medial thickened part -of the Stratum cellulare internum lies

40 41 STUDIES ON THE MID-BRAIN AND CEREBELLUM

between the two Nuclei and N ervi III. The part, relatively deficient in cells, which at stage 3,9 cm., lies ventrally to the Fasciculus longitudainalis dorsalis (F. l. d. fig. 58) has at stage 5,0 cm. become filled by a cell—mass (1. fig. 58), whose origin is difficult to determine. There issue to be sure, from the Stratum cellulare internum round the Fasciculus longitudinalis dorsalis, bi-polar cells, which seem to be connected with the formation of this cell-mass. On the other hand however, the cells in the more medial parts are also bi-polar and arranged with their longitudinal axes in a lateral direction. These ce.lls are in complete continuity with the cell-mass just mentioned (1. fig. 58). It is impossible for me to decide whether these structural features should be interpretated as shewing‘, the formation of this cell—agglomeration to be in part from the medial part of the ventral column.

There are also bi-polar cells, horizontally arranged in the ventro—lateral part of the ventral column (vl. fig. 58).

The two parts of the medial column are singularly difficult to distinguish from each other, they are almost completely fused. Peripherally in this part lie a few cells, ‘which probably -originate from the column in ‘question.

The following transformations have taken place in the parts lying dorsally to the Sulcus lateralis. The first oriment of the Tor-us semioirculanis or, as I will call it the Torus—formation, becomes visible dorsally in_ the Stratum cellulare internum towards the boundary to the Tectum. The differentiation in thevstratum cellulare intern-um, which I call the Torus-formation, lies in the lateral (dorsal) part of the Torus semicircularis of higher animals, as be.st seen in birds (Gallus).

The cell-mass lying in the centre of the Torus—formation I name Nucleus lat-eralivs mesencephali. This is like the To.nus—fo.rmation a differentiated part of the Stratum cellulare internum. On transverse sections the Stratum cellulare internum here forms a little semicircle (T. f. fig. 58), in which a lesser cell—aggl-omeratiion is found, the Nucleus lateralis mesencephali 1. Outside this fonmation a cell-condensation in the Stratum cell-ulare externum is clearly visible, namely, the dorsal column.

Following the series of transverse sections frontad, the dorsal column is seen passing continuously into a greater oell-mass: the oriment of the Corpus geniculatum laterale of the authors, Ganglion genie-ulatum mediale of WALLENBERG (1907). According to VVALLENBERG (1907), the same seems to be the case in Carcharias. He writes (page 389): ”An Stelle cles medialen Anteils des Mittelhirnhaubenkernes liegt bei C archarias eine machtige Kernséiule mitten im lateralen Léingsbiindel, die sich flrontalwarts

‘ It would be suitable to introduce a more precise definition of the Nucleus lateralis mesencephali than that hitherto used in the literature. If we assign the whole cell-mass occupying the Torus semicircularis to this Nucleus, this will not be homologous in the different animal types, as will be shewn below.


in ventraler Richtung keulenformig verbreitert und schliesslich mit dem Ganglion geniculatum mediale zusammenfliesst.” This ”m5.chtige Kernsiiule” appears to be identical with the dorsal column of Acanthias. Judging by WALLEaNBERG’s description and illustrations, his ”media1e.r Teil des Mittel- hirnhanbernkernes” in Scyllium must be synonymous with what I have-called the Torus-formation, and his ”lateraler Teil des Mittelhirnhaubenkernes” in the same species with the dorsal column in Acanthias.

The lateral column remains in the same manner as at stage 3,9 cm. At this part however, the oriment of a peripheral nucleus (p. nc. fig. 58) can already be discerned.

In fig. 427 STERZI (1912) gives a transverse section of a 3,6 cm. Aconthias-embryo. STERZI has also evidently noticed the cell-condensations, which I have named the dorsal, lateral and medial columns. He calls the lateral column Form-azione reticolata; the medial one Nucleo laterale del msesencefalo; but he does not mention the dorsal column. The part between the Sulcus lateralis and Sulcus longitudinalis ventralis he calls Toro semicircolare.

The nucleus, p. nc. fig. 58, is completely developed at stage 8,0 cm. As no important changes can otherwi.se be noticed, I pass on to a description of stage 15,0 cm.

Stage 15,0 cm. : The cells in the Stratum cellulare internum of the ventral part (V. c. fig. 59) are more widely diffused than at the pveceeding stage and form together with the ventral column (the cells in the Stratum cellulare externum) :1 difiuse cell-agglomeration between Nervi III. I have not found any specially -differentiated nucleus i-n the vicinity of, and lateral to the oculomotor fibres in Acanthias. Such a nucleus however has been described in Scyllium by WALLENBERG (1907) under the name of Nucleus ruber. According to STERZI (1909), a ”Nucleo rosso” exists close to the Fasciculus longitudinalis dorsalis in Acanthias.

The thickening in the Stratum cellulare internum in the medial part (S. c. i. m. fig. 59) remains distinct. The whole of the medial column (M. c. fig. 59) is faimly diffused.

The Stratum cellulare internum continues thinly cellulated in the lateral part (S. c. i. 1. fig. 59). Outside the—lateral column (L. c. fig. 59), which in compactness of cells corresponds with the medial, is found a peripheral, well-defined nucleus (p. nc. fig. 59), identical with that mentioned at stages 5,0 and 8,0 cm.

Dorsal to the latter nucleus, we find the optic tract in the dorsal part. The dorsal, like the other columns, has become more diffuse, but can still

be clearly distinguished (D. c. fig. 59). The Torus-formation (T. f. fig. 59) '

has, in the Stratum cellulare internum developed into a very well-defined semicircular structure, in which lies a little nucleus or cell-c-olumn (Nucleus lateralis mesencephali [N. l. m. fig. 59]).


The boundary to the. Tectum is clearly marked by the Stratum cellulare internum being more vigorous and compact in the Tectum than in the Subtectum. In the Stratum cellulare externum a fairly sharply defined boundary is visible between the Tectum—forrnation (T. 0. fig. 59) and the dorsal column (D. c. fig. 59).

Before continuing the description of the development of the nuclei, panticularly in the caudal parts of the mid-brain, it would be suitable to define the boundary between the Mes- and M etencephalon in relation to the already described cell—colum.ns in the Subteetum. In consequence of -the boundary-plane lying at about 45 degrees’ angle to the direction of the sections, the metencephalic parts first appear ventrally on a series of transverse sections. In more caudal sections the boundary-lines are to be sought more and more dorsally. The reverse is the case for the boundary between the Syn— and Mesencephalon.

On a series of transverse sections of stage 5,0 cm. it is easy to see the one which cuts trough the ventral borundary-point. The fissura rhombo- mesencephalica (Fossa interpeduncularis) is namely so deep ventrally that

it even appears on transverse sections of the brain as an insented cavity»

(Fs. i. fig. 60). Further, the ependymal-fibres behind ‘the groove are considerably more vigorous and receive stains better than in the Mesencephalon. Lastly, a Stratum cellulare externum is lacking ventrally in the Metencephalon 1.

On this section there is in the medial column a relative paucity of cells (see the right side of the somewhat oblique section fig. 60 M. c.). A cell-agglomeration lying in the corresponding region, a couple of sections further caudad (see the left side of the section fig. 60 Mt. c.), is considerably richer in cells than the medial column in Subtectum.

Between this metencephalic cell-column and the mesencephalic lateral column, there lies a relatively cell—free zone, identical with that mentioned above as being the rudiment of the posterior mesencephalic segment (more distinct at stage 3,9 cm.). The rudiment of ‘the Sulcus intraencephalicus lies close to the ventricle. The nucleus of the trochlear nerve now emerges v-entro-medially to this zone and lies thus in the Metencephalon.

In following the series of sections caudad, we find the boundary-zone and the rudiment of the Sulcus intraencephalicus displaced upwards till the lateral column is replaced by a metencephalic cell-agglomeration or column. On certain caudal sections (fig. 61) the only remains of the Mesencephalon are the Tecturn (T. 0.) and the donsal part of the Subtectum (D. c.).


‘ Comp. Kmasnunv (1920): ’’It will be seen (fig. I-3) that the differentiated floor-plate characterized by the presence of the ependymal layer only and ncuroglial processes (neuroglia fibres), often grouped together in parallel radial bundles, terminates at the Fovea isthmi and cephalad of this point, in the floor of the mid-brain, the characteristic arrangement of the floor-plate is lacking, while ependymal, mantle and marginal layers appear." (Page 117.)


This latter extends right away to the clecussation of the troohlear nerve. Its boundary towards the Metencephalon is very pronounced. Stratum cellu-— lare internum i-s far richer in the Me»s— than in the Metencephalon. In Stratum oellulare externum the boundary-zone is distinct; ventral to it the cells lie also connsideraibly more closely packed than in the dorsal part of su-btectum, where, especially caudad, Stratum cellulare externum is formed of only a few cells (D. c. fig. 61). The dorsal part is thus distinguished from the Tectum formation by the cells of its Stratum cellulare externurn being considerably less closely packed, and by the claviform thickening of its Stratum cellulare internum (fig. 6I). Caudally the dorsal column disappears finally altogether in its capacity as a particularly closely packed ce11—oolumn.

At stage 15,0 cm. the Tor.us—formation can be followed through a series of tran-sverse—sections to sections which cut through the ventral part of the boundary-plane, between the Mes— and Metencephalon, where in other wonds, they are tangent to F-ovea isthmi, and where the rudiment of the Sulcus intraencephalicus is found again in the ”Haubenwulst”. The Torus formation becomes, further caudally, a fairly broad and closely packed cell- agglomeration (p. T. f. fig. 62), which like the Tor.us-formation lies imimeo diately to the boundary between Tectum and S/ubtectzum and is a differentiation in the Strat-u-m cellulare internunrn. This cellsagiglomeration is the same that at stage 5,0 cm. was described as a cvlavifonm thickening of the Stratum cellulare i-nternurn in the dorsal pant of the Susbtectum. (At stage 8,0 cm. it is again visible in like manner, but more clearly defined than at stage 5,0 cm.). Again at stage 15,0 cm. the characteristic rudiment of the ventral to this ce1l—agglomeratiorn, which fills a part projecting into the Sulcus intraencephalic-us (r. S. i. p. fig. 62) is to be found immediately

fig. 59. Transverse-section through the mid-brain of Accmthias at stage 15,0 cm. figs. 60, 6I. Transverse sections through the posterior part of the mid-brain of A canthia: at stage 5,0 cm. fig. 62. Transverse section through the posterior part of the mid- brain of Acanthias at stage 15,0 cm. figs. 63, 64. Transverse—sections through the anterior part of the mid-brain of Acanthias at stage 15,0 cm. (Microphotographs.)

b. M., the bundle of Meynert; C. p., Commissura posterior; D. c., the dorsal part and column; E. m., Eminentia medialis; F. l. d., Fasciculus longitudinalis dorsalis; Fs. i., Fossa interpeduncularis; G. l., Ganglion geniculatum laterale; L. c., the lateral part and column; M. c., the medial part and column; Mt. c., the metencephalic ventral column; N. III, Nervusioculomotorius; Nc. III, Nucleus oculomotorius; Nc. IV, Nucleus trochlearis; N. l. m.,_iNuc1eus.lateralis mesencephali; p. nc., the peripheral nucleus in the lateral part; r. s-mi, the boundary-ridge between the Syn— and Mesencephalon; r. S. i. p., the rudiment of the Sulcus intraencephalicus posterior; p. T. f., the posterior part of the Torus-formation; S. c. i., m., 1.,,the medial and lateral part of the Stratum cellulare internum; S. 1. v., Sulcus longitudinalis ventralis; T. f., the Torus-formation; V. c., the ventral part and column.

44 fig. 63.

ventricle (Torus semicircularis). The cell—nucleus in question constitutes a direct continuation of the Torus—fo.rmation caudad. It finishes a little before the caudal medial part of the Mesencephalon dorsally to the fissura rhombo-mesencephalica, i. e. is not connected caudally and medially with its vis-"a-viis on, the opposite side.

On sagrital sections of stages 8,0 and 15,0 cm. a part corresponding approximately i-n size to the dorsal Suubtectum part, can be distinguished between the Tecturm—formation and fissura nhombo-mesencephalica (figs. 40' 98). In the Stratum cellulare externum of this part the cells lie more scattered and are less numerous than otherwise in the Tectum. This part constitutes the posterior and medial portion of the dorsal part of the Subtectum.

By using the above described structural features as points of vantage, it becomes easy to determine the anterior boundary of the mid-brain in its relation to the nuclei on the series of transverse sections.

In front of the Eminentia medialis of KAPPERS the Stratum cellulare internum (S. c. i. fig. 63) is (up to stage 15,0 cm.) considerably broader in Teotum than in Diencephalon and ‘stops suddenly immediately dorsal to (behind) the Commissura posterior fibres (C. p. fig. 63), which like the blood-vessels, rows of cells and ependymal fibres run in a ventro-lateral direction towards the surface of the brain, and separate the ”Geniculatum laterale” (G. 1. fig. 63) from the Diencephalon. At any rate up to stage 8,0 cm. this nucleus lies clearly dorsal to (behind) t-he boundary—plane, but appears later to be displaced in a ventral direction.

On sections lying directly caudad of the primary boundary-ridge (r. s——m1 fig. 64) 1 the thickened part of the Stratum cellulare interrmum in the medial part of the Subtectum begins to appear (S. c. i. m. fig. 64). Dorsally to this part we find the foremost portion of the lateral part.

A few sections still further caudad, the Torus-formation (T. f. fig. 64) appears, which thus does not extend quite to the boundary—p1ane. Peripherally

‘ This ridge which at stage 15,0 cm. partly remains on the ventral side, where it enters into the formation of the Eminentia medialis, is characteristically vacuolized and can therefore be easily recognized on transverse sections of the brain.

figs. 65 and 66. Transverse-sections through the anterior part of the mid-brain of Acanthias at stages 3,9 and 5,0 cm. (Microphotographs.) fig. 67. Transverse-section through the Torus lateralis of Salmo at stage 0,9 cm. (sketch). fig. 68. Transverse section through the Torus lateralis of Salmo at stage 1,0 cm. (sketch). fig. 69. Transverse-section of the Torus lateralis of Salmo at stage C. fig. 70. Transverse—section through the Torus lateralis of Salmo at stage D. (Microphotographs.)

b. M., the bundle of Meynert; C. p., Commissura posterior; D. c., the dorsal part and column; G. 1., Ganglion geniculaturn laterale; L. c., the lateral part and column; S, Synencephalon; S. c. i., m., the medial part of the Stratum cellulare internum; S. 1. t., Sulcus lateralis tecti; T. 1., Torus lateralis; I. 2. 3., the grooves in the region of the Torus lateralis; T. 0., Tectum opticum.

46

48 AXEL PALM GREN

the ”Geniculatum laterale” (G. 1. fig. 64) is still visible, thus extending thrtougih three parts: the dorsal, the lateral and the medial. This nucleus

is in 0‘ er wor .s a eri eric 0 —mass «in e an erior or ions 0 .05 ' , th d , h ell th t t f th e

parts of whose Stratum cellulare internum it is formed (co-mp. figs. 65, 66', 64.).

The ventro—me~dial boundary-point i.s rather difficult to determine on transverse—sections. The bundle of M.EYNERT is however a useful guide, lying ventro—medial.ly in the boundary—p1ane, and so also is the Nervus III in accordance with what has been said above. It is evident that the ventral part of the Subtectuni coinmences when the boundary—plane is passed.

The major part of the nucleus of the dorsal longitudinal bundle lies in the Synencephalon, but a few cells also protrude into the Mesencephalon.

B. Tel e o s t ii (Salmo salvelinus).

Before passing on to a description of the embryonic development of the nuclei in Salmo, it would be as well to determine the position of the To-rus lateralis in relation to the fissura s_vn—mesence.pha1ica and thereby to the segments. The question is therefore, is the Torus lateralis to be assigned to the Mesencephalon or to the Diencephalon?

KAPPERS (1907) describes the Torus lateralis in Amia as a ventral correspondence to the Torus semicircularis and consequently as a mesence— pha-lwic part of the brain. HOLMGREN (1920), on the contrary, accepts HALLER’s interpuretation, according to which the Torus lateralis is a die:nce— phalic part detached from the Lobus lateralis hypothalami.

At stage 0,9 cm. of Salmo salzrelin-us. it is possible to distinguish three grooves on transv~erse—sections through the region where the Torus lateralis is beginning to develope (fig. 67). The central one is the fissrura synmesencephalica (2. fig. 67) which is clearly developed ventrally and ventro- laterally (comp. above page 32 and fig. 19 F. .s.m.). The nethermost groove (3. fig. 67) is the one which on the medial sagital section lies opposite the Tuberculum posterius (* figs. I9, 21). That this conception of the grooves is correct appears from a comparison between a series of sagital and transverse-sections of this and earlier stages. The uppermost groove (I. fig. 67) is not to be found at earlier stages, but appears first simultaneously with the peripheral cell-agglomeration, which at stage 0,9 cm. lies directly dorsal to the fissura syn-mesencephalica (fig. 67). This uppermost groove originates through the i-ntumlescence of the part in which this cell-agglomeration is found.

These three grooves (for the sake of brevity I call them I, 2 and 3) are found again at later stages (at least up to stage D) and appear to be found also in full—grown Osmerus. At stage 0,9 cm. thene lies as I have

48 49 STUDIES ON THE MID-BRAIN AND CEREBELLUM

said, in the region between grooves I and 2 a compact ce11—agglo.rneration (fig. 67). Between grooves 2 and 3 there are also some cells.

At stage 1,0 cm. (fig. 68) the number of cells between grooves 2 and 3 has considerably increased and become fused with the cell—agglomeration between grooves I and 2 to a great peripheral cel1—mass, which continues also in front of the part where the three grooves are found developed. The whole of this ce1l—ma:ss (T. 1. fig. 68) forms later the Torus lateralis.

At later stages the Toh‘us—oriment increases and becomes more and mo.re curved outwards. At stage C (fig. 69) the caudal part of the Torus is divided up by the grooves into two nearly equal-sized parts. If the series of sections are followed in a forward -direction the dorsal part is seen to diminish till it at last vanishes. Groove 2 does thus not run parallel with the Torus lateralis nor with groove I, but converges frontad with this. Before the point where groove 2 stops Tonus is undivided.

finally at stage D (fig. 70) groove I has, partly by the growth of the Tectuzrn, partly of the Torus lateralis become singularly deep and sharply defined. Groove 2, on the contrary, is at this stage rather faint.

Groove 2 being the ventro—la.teral part of the fissura syzmmesencephalica, Torus lateralis or the caudal part of the same, is thus formed of a mesencephalic part which on transverse sections lies dorsal to this groove and to a ventro—diencephalic part.

I shall return to this later, for the present I will content myself with stating a fact, which seems to support this view.

In the full—grown Osmerus eperlamts Torus lateralis (T. 1. figs. 76, 77) is divided up into two parts by a supe.rficial, rather shallow groove (2 figs. 76, 77), which is probably identical with the above described groove 2 (fissura syn—mesencephalica) in Salmo. In Osmems we find the point where grooves I and 2 converge is considerably further frontad, i. e. in the anterior part of the Torus lateralis.

Golgi and Haemalum preparations shew that specially vigorous ependymal-fiibres issue from a point in the floor of the Tectum ventricle in a downward direction towards this groove (e. f. fig. 76). Guided by the position of the boundary blood-vessel (b. v.) and of MEYN.ER'r’s bundle (b. M. fig. 76), it can be determined with certainty that the poingt from which th-e epermdymail-fibres issue, lies directly caudad of the bounda.ry—plane between the Syn- and Mesencephalon. If these ependymal-fibres emanate radially which appears probable, -the dorsal part of the Torus should be assigned to the mid—brain.

The form of the Mesencephalon in Acanthias -differs very greatly from that in Salmo. An imaginary line drawn on a transverse section between the Sulcus longitudinalis ventrralis and Sulcus lateralis tecti would in Awnthias run more o.r less vertically, in Salmo, on the contrary, already at stage 0,6 cm. horizontally. The Tectum is, in other words, broader in Salmo and projects continually more downwards in a ventral direction, so that the Subtectum becomes more and m-ore ”exverted” (comp. figs. 57, 71).

The embryonic development of the nuclei has been investigated chiefly at the following stages: 0,6, 0,7, 0,8, 0,9, 1,0, 1,1, 1,2 cm. and at stages C and D.

At stage 0,6 'cm. a fibre layer outside the neuroblastic stratum is already to be found. In Teotum on the contrary, the neuroblastic stratum extends to the periphery. The boundary between the Teotum and Subtectum runs, as in Acanthias, immediately ventral to the widest part of the ventricle (Sulcus lateralis tecti).

Not till stage 0,8 does the development of the Stratum cellulare exterrmm in Subtectum begin by cells wandering out i.nto the fibre layer from that part of the neuroblastic stratum, which lies directly medial (ventral) to the b0undary—line between the Tectnm and Subtectum. A cell-column (D. c. fig. 71) is thus formed, which in consequence of its position can be said to be homologous with the dorsal column of Acanthias. The wall of the Tectum has at this stage considerably thickened, this does however not refer to the cauldal part of the mid—brain roof, where the wall is still comparatively thin.

At stage 0,9 cm. single cells begin to wander out from the more medial parts of the neuroblastic stratum in Subtectum towards the periphery, there to form a compact cell—agglomeration in the region where the Torus lateralis will develope (fig. 67). In t-his manner is formed the oriment to the above mentioned dorso—caudal part of the Torus lateralis. No boundary exists between the T orus—oriment and the dorsal column 1.

It is to be noticed, however, that the ventral part of the cel1—agglome~ ration (dorso—caudal part of Torus) lies nearer the surface of the brain than does the dorsal column, which as in Acanthias lies approximately in the centre of the fibre layer.

The «dorso—caudal pant of the Torus-oriment is thus in continuity with the -dorsal columnni and can be said to form a frontal major part of this latter. As the dorsal column in Acanthias, goes over frontad into a large cell—nueleus (”Geniculatum 1aterale”), which lies in, and "is partly formed of the lateral and medial parts of the Stratum cellulare internum, wehave not far to look for the ”Geniculatum laterale” in Acanthias as homologous with the ‘dorsal part of the Torus lateralis in Salmo. The homology rests on the following”:

’ It is therefore possible although not probable that the entire cell-agglomeration

of the periphery is formed from the dorsal part of the Subtectum through the dorsal column being tilted down in a ventral direction.

‘ Comp. KAPPERS and CARPENTER (I911): ”Ich méichte hier auf der interressanten Tatsache hinweisen dass der von mir bei den Knochenganoiden: Amia und Lepidosteus, beschriebenen Torus lateralis vielleicht dem dorsalen Thalamus-Kern (the Geniculatum laterale) homolog ist.” (P. 148.)

50 51 STUDIES ON THE MID-BRAIN AND CEREBELLUM

I) The dorsal cel1—column is formed immediately ventral (medial) to the boundary between the Tectum and Subtectum, and lies in the fibre stratum outside the place where the Torus—formation is found (Acanthias) or will be formed (Salmo).

2) It extends further from the boundary-plane between the Mes— and Metencephalon frorntad, where it passes into a large cell—agglomeration: ”Genicu~latum laterale” (Acanthias), the dorso—caudal part of the Torus lateralis (Salmo).

3) In both cases this nucleus is formed from the dorsal as well as the lateral and medial parts of the neuroblastic stratum, and

4) lies nearer the surface in both Acanthias and Salmo than does the dorsal column.

A cell-agglomeration is also to be found between grooves 2 and 3 at stage 1,0 cm. as appears from fig. 68. This is the ventro—caudal part of the Torus lateralis. I cannot by reason of its position with certainty determine how this nucleus .is formed, lying, as it does, close beside the dorsal mesencephalic part. It may therefore possibly be formed of cells which protrude in a ventral direction from -this dorsal part. B.ut there is also another possibility, which I consider more probable, namely that the ventral pant of the Torus lateralis, which is, as mentioned, in the Synencephalon, is formed of cells migrating from the point * figs. 67, 68. Cells migrate from this point already at stage 0,9 cm. (fig. 67) when the ventral part of Torus begins to develope, i. e. from those parts of the neuroblastic layer, which lie ventral (on transverse sections) to the Commissura posterior (C. p. figs. 67, and about the bundle of MEYNERT (b. M.) (thus in the Syn-encephalon) towards the periphery or ventral part of the Torus 1.

At stage I,oi cm. (fig. 68) the cell-agglomerations forming the oriment of Toms lateralis has become still more elongated in a ventral direction. If the stnuotural features received at stages 0,9 and 1,0 cm. are to be interpretated as proving that the ventral part of the Torus lateralis is formed of the dorsal part, it should, in consequence and by the same right be assumed that the cell-agglomeration, lying ventrally to the Torus is of rnesen— cephalic origin. This cell—agglomeration continues caudal, however, into the hypothalamic region. I for my part, consider the formation of this nucleus (Nucleus diffusus lobi lateralis) from the mid-brain very im-probable, even if the continuity of these cell-agglome.rations make such a theo-ry possible.

The migration of cells in the Subtectum has at stage I,I cm. advanced so far, that it has become possible to distinguish veritable columns in the

' This migration of cells should not be mistaken for that, which at stage 1,0 cm. begins to appear from a point lying somewhat. more ventrally and is the origin of the Nucleus anterior thalami (according to as yet unpublished investigations by G. LINDIEN on the embryonic development of the nuclei in Salmo).

5I 5 2 AXEL PALM GREN

lateral and medial parts of the Subtectum. The Stratum cellulare internum is approximately in the centre of the Subtectum, somewhat thickened and forming a pointed projection into the fibre layer (fig. 72). A oolumn (L. c. fig. 72), relatively rich in cells, has developed from this point, which for reasons given below, has been pronounced homologo=us with the lateral column of Acanthias. More scattered cells lie medial to this column, radiating from the region round and lateral to the Fasciculus longitudinalis dorsalis. I consider these cells (M. c. fig. 72) to oorresponud with the medial column of Acanthias.

A peripheral cell-group (p. c. fig. 72) lies at this stage caudad to the Torus lateralis region, and extends from the tectum—subtectum boundary in a medial direction over both the lateral and medial pants. The cells [in this group are somewhat less numerous laterally i. «e. in the dorsal part, but I can discover no boundary between the dorsal column and [the rest of the part. Peripheral cell-nuclei are to be found, as above mentioned, in the lateral part of Acanthias, where the nucleus is comparatively distinct, as in the medial, where, however it is only possible to speak of a defined cell- condensation at certain stages. I consider probable that in Salmo parts homologous to these nuclei form part of the peripheral cell—group. The position and, as far as it is possible to judge, the embryonic development of these parts point to this.

The medial parts of the peripheral cellnsucleus are also in continuity with the oriment of the dorsal part of the Torus lat-eralis. A compact cell mantle of, as I believe, heterogeneous origin, thrus lies at the whole of the ventro-lateral surface of the Subtectum.

At stages 1,2 cm. and C, the columns are richer in cells, and at the same time more clearly defined than at previous stages.

At stage D (fig. 73) a compact small—celled cell—column is found ventral to the Nucleus III in the ventral part of the Subtectum. There is a density of cells (Nucleus ruber?) under the Fasciculus longitudinalis dorsalis i. e. in the lateral portion of the ventral part. Lateral to the Fasciculus longitudinalis dorsalis there is to be found in Salmo at this and earlier stages, as above mentioned, two fairly distinct columns: the medial and the lateral.


figs. 7I—73. Transverse-sections through the mid-brain of Salmo at stages 0,8 and 1,1 cm. and stage D. fig. 74, Transverse-section through the mid-brain of Lepidosteus. fig. 75. Transverse-section through the mid-brain of Rana at stage 3,6 cm. (Microphotographs.) .

D. c., the dorsal column; F. 1. d., Fasciculus longitudinalis dorsalis; H, the hypothalamic region; L. c., the lateral column; M. c., the medial column; N. III, Nervus oculomotorius; Nc. III, Nucleus oculomotorius; N. 1. m., Nucleus lateralis mesencephali; p. c., the peripheral cell-mantle; S. c. i., m., 1., the medial and lateral parts of the Stratum cellulare internum; S. l. t., Sulcus lateralis tecti; S. l. v. Sulcus longitudinalis ventralis; St, Subtectum ; T. f., the Torus-formation; T. 1.," Torus lateralis; T. 0., Tecturn opticum; T. 5., Torus semicircularis; V. c., the ventral column; V1. c., Valvula cerebelli.



In Salmo the structural features: Sulcus lateralis and the differentiation of the Stratum cellulare internum, used in Acanthias for determining the boundary between the medial and lateral parts, are indeed lacking; in Le1Ji- dosteus, on the contrary, the medial part of the Stratum cellulare internum (S. c. i. m. fig. 74) is composed of comparatively closely packed cells. It seems probable that this part corresponds with the medial thickened portion of the same cell—layer in Acanthias (S. c. i. m.. fig. 59). If this be correct, the difference between the topographical positions of the various parts in Lepidosteus and,$‘alm0 not being great, it may be considered that the medial of the two above mentioned columns in Salmo is homologous with «the medial column in Acanthias, and the lateral one with the lateral column of Acanthias. It should be noticed that a number of comparatively large cells are to be found in the medial part and close to the ventricle in both Osmems (fig. 77) and Labrus. These cells do not.however form any true cell—layer (Stratum cellulare internum).

The metencephalic oolumns, which in Salmo as in the other animal types, are better defined than the mesencephalic columns, form a direct continuation of these latter, a fact which also points to a correct designation of the columns in Salmo.

Salmo is an unsuitable subject for a study of the embryonic -development of the Nucleus ruber. The Nucleus ruber of GOLDSTEIN (1905) is very indistinctly developed in this species as in Osmerus, but in Labms, on the contrary it is well-defined, though ‘lying in quite the foremost part -of the

medial column on the boundary to the lateral. As the Nucleus ruber in.

Callus and M us is a differentiation in the ventral column, the Nucleus ruber of GOLDSTEIN can hardly be said to be homologous with the Nucleus ruber in Amniotes. HOLMGREN (1920) has come to the same conclusion, he writes: ”Die gekreuzten Bindearme des Cerebellums haben, soweit es aus hunderten Von Schnittserien hervorgeht, bei Osmerus keine intim-eren Verbirrdungen mit dem Kern, der somit schwerlioh mit dem Nucleus ruber der hoheren Vertebraten homolog sein kann. Die Birnclearme durchsetzen den Kern” (page 145).

The Stratum cellulare internum of the dorsal part is, at stage D in Salmo, hollowed out in a semicircular form and consists of only one unified layer of cells nearest to the tectum—subtectum boundary (T. f. fig. 73). This semicircular part of the Stratum cellulare internum corresponds in position and appearance with the Torus-formation in Acanthias. This Tor-us-formation constitutes the major part of the Torus semicircularis in full—grown teleosts.

Cells migrate from the Stratum cellulare internum in the caudal portion of the lateral part in a ventral and somewhat lateral direction till they occupy a position in the Torus semicircularis. A few cells are to be found in the Torus—formation proper already at stage D (N. l. m. fig. 73). These become more numerous later and form the Nucleus lateralis mesenoephali.

The above mentioned cell-mantle (p. c. figs. 72, 73), becomes more and more reduced and finally disappears completely. In Osmerus, however, a small part'0f the same remains dorsal to the caudal part of the Torus lateralis and lateral to the optic tract, even in‘ the fully developed brain. This nucleus (* fig. 77) is in continuity frontad with the dorsal part of the Torus lateralis.

The Torus lateralis proper has, in Lepidosteus, a caudal, undoubtedly mesencephalic part (T. 1. fig. 74), which extends about as far caudad as the Torus semicircularis (fig. 111). It appears to me conceivable that it is this caudal part which KAPPERS (1907) refers to when he says that the Torus lateralis in Amid: ”a-ls ein extraventriculéirer Auswuchs, man konnte sagen exvertienter Teil desselben (Tonus semicircularis), angesehen werden konnte” (page 470).

The Corpus geniculatum laterale in Teleosts is formed, according to LINDEN 1, by cell-migration from a frontal part of the mesencephalic neuroblastic layer. The point at which the Genic-ulatum is formed, lies partly anterior, partly dorsal to the point at which ”Geniculatum laterale” is formed in Acanthias. Geniculatum in Salmo is found primarily in the Tectum, in Acanlhias in the,Subtect~t1m.. Moreover, as Geniculatum in Salmo has not the slightest connection with the dorsal column, the nrucleus, which in Awnthias is designated Geniculatum laterale cannot be homologous with Geniculatum laterale in the Teleosts. I have not been able to discover a differen— tiated nucleus answering to the latter in Acanthias. In Chimaera, on the contrary, there is, according to KAPPERS AND CARPENTER (1911), a typical Geniculatum laterale frontad of and separated from a larger nucleus (Nucleus dorsalis thalami), which clearly is the same as ”Geniculatum laterale” (Geniculatum mediale of. WALLENBERG) in Acanthias.

finally, I am in complete agreement with I-IOLMGREN (I920) regarding Nucleus corticalis, when he says that this nucleus should be assigned to the Tectum opvticum. It is undoubtedly formed from Tectum parts of the neuroblastic layer.

C. Amphibia (Rana temporariq).

The development of the nuclei begins comparatively late in Rana. Not till the larva shews well-defined oriments of caudal extremities, is the cell migration from the neuroblastic layer at all distinct. At stage 3,6. cm. the differentiation of the nuclei is in the main concluded. In the ventral part

  • The studies on the embryological development of the nuclei in Salmo by G. LINDEN are not yet published.


(V. c. fig. 75) there is at this stage only a small number of cell-s in the fibre stratum ventral to the Nucleus III. This latter nucleus is, however, not clearly differentiated till after the larval stages are concluded.

In the fibre—laye.r of the medial part (M. c. fig. 75) the cells are companatively few in number. The nucleus described by GAUPP (1899) as Nucleus ectomammillari.s is found, at earl-ier stages, frontad of the boundary- plane between the Syn- and Mesencephalon. A peripheral cell-group is, at later stages, formed fromthe medial column. This cell-group is in continuity with the diencephalic part of the Nucleus ectomammillaris.

The lateral column (L. c. fig. 75) is fairly rich in cells, and is formed from a projecting pant of the neuroblastic layer, which strongly resembles the part from which the lateral column in Salmo is formed.

Outside the Torus-formation, we find a number of cells in the fibre layer, a dorsal column (D. c. fig. 75) continuous with the Stratum cellulazre extemum in the Tectum, but distinguishable from the Tectum layer by the greater compactness of the latter. I cannot discover in Rana any frontal part of this column corresponding to the ”Geniculatum _latera1e” in Acanthias. We also find diffusely -distnibuted cells, which form the Nucleus lateralis mesencephali, in the Torus-formlation itself. By the term Torus formation I define the semicircular part of the Stratum cellulare internum.

The hemispherical elevation in the floor of the mid-brain (Torus semicircu-

lanis), does not -coinci-de completely with the Torus-formation. The lateral part with its column will be found within the Torus sernlicirculari-s ventral to the Torus-formation proper.

I have, by reason of the lack of material, not been able to investigate the embryonic «development of the nuclei in Reptiles.

D. Av-es (Gallus domesticus).

The development of the nuclei in Gallus has been investigated at the following stages: 6," 7, 8, 8%, 9, IO and I 5 days.

figs. 76, 77. Transverse-sections (of the "same series) through the mid-brain of Osmerus. fig. 78. Transverse-section through the mid-brain of Callus at the 6 day stage. (Microphotographs.)

b. M., the bundle of‘ Meynert; b. v., the ”boundary—vessel”; D. c., the dorsal part; e. f., ependymal fibres; F. l. d., Fasciculus longitudinalis dorsalis; L. c., the lateral part; M. c., the medial part; m., 1., the medial, lateral portion of the part in question; N. III, Nervus oculomotorius; Nc. III, Nucleus oculomotorius; N. l. m., Nucleus

ilateralis mesencephali; O. t., the optic tract; S. 1., Sulcus lateralis; S. 1. v., Sulcus

longitudinalis ventralis; T. 1., Torus lateralis; I, 2, 3, the three grooves in the region of the Torus lateralis; T. s., Torus semicircularis; T. 0., Tectum opticum; V. c., the ventral part; vl., the ventro-lateral portion of the ventral part; V1. c., Valvula cerebelli.


The 6 day stage (fig. 78). The wall of the Tectum consists at this stage of a compact neruroblastic layer and outside that, a thin fibre—layer. Three cell-layers have, on the other hand, developed in the Subtectum: Stratum ependymale, Stratum cellulare internum and Stratum cellulare externum.

Lateral to the Sulcus longitudinalis ventralis (S. 1. V. fig. 78), we find in Subtectum two longitudinal ventricular grooves, of which the medial (* fig. 78) lies directly lateral to the Fasciculus longitudinalis dorsalis and bounds the ”Haubenwulst” on the lateral side. The lateral groove is Sulcus lateralis (S. 1. fig. 78).

Different parts can be distinguished in the Stratum. cellulare internum according to the compactness or paucity of cells in the layer. As these parts are in conformity with the corresponding parts in Acanthias, the same subdivision of the Subtectzum can be carried out in Gallus.

In the ventral part, which can be delimited in the same way as in Acamhias, and also by means of the above mentioned ventricular groove at the side of the ”Hatubenwulst” (* fig. 78), the Stratum cellulare internum is considerably thickened, but is not, as in Acanthias, thinner dorsal to the Fasciciulus longitudinalis dorsalis. The cells composing this layer are comparatively large and form the oriment of the Nucleus III. A ventral cell—column differentiated as follows, is found in the fibre-layer: A number of lesser cells lie close to the medial line; venitro-laterally the cells form a peripheral cell—group (vl. fig. 78). The cells in this group are fusiform, as in Acanthias, and arranged with their longitudinal axes in a lateral direction. As neuroblasts are generally bi-polar and arranged with their longitudinal axes in the direction of migration, it might be supposed that the arrangement of the cells in this peripheral cell—group was a proof of their being in the act of migrating in a lateral direction. This does not however, seem to be the case. At stage 6 days the lateral part of the cell-agglomeration in question lies slightly lateral to the Nervus III. In the same position it is also to be found at later stages, still with the characteristic cell arrangement.

Outside the Fasciculus longitudinalis doi-salis lies a cell-agglomeration (V. c., 1. fig. 78), to which there is a correspondance in Acanthias. In Actmthias these cells are formed from the part of the Stratum cellulare internum surrounding the Fasciculus longitudinalis dorsalis (stage 3,9 cm. [fig. 57]). At the 6 day stage this nucleus is, in Callus, connected with neuroblasts migrating from the corresponding point, but it is also in continuity with laterally directed bi-polar cells in more medial parts of the ventral column. In Acantlu'as (at e. g. stage 5,0 cm.) the cells are arranged in the same way in the corresponding part of the ventral column. It can be left an open question, however, whether a true cell—mig1ration takes place in a lateral direction from the medial part of the ventral column, or not.

58 59 STUDIES ON THE MID-BRAIN AND CEREBELLUM

The Stratum cellulare internum is likewise thickened between the Sulcus lateralis and the ”Haubenwulst”, but consi.sts of comparatively small cells. There are only a tolerably few scattered cells in the fibre-layer of this medial part (M. c. fig. 78), comp. Acanthias, stage 3.9 cm. (fig. 57). The cell- migration from the medial portion of the Stratum cellulare internum (M. c., m. fig. 78) appears to be somewhat more vigorous than in the lateral (M. c., l. fig. 78). The cel1—densi.ty i.s .sormewhat greater medially than laterally in the medial part (M. c. fig. 78). A

The Stratum cellulare internum is comparatively thin at that part of the Subtectum which lies dorsal to the Sulcus lateralis (S. 1. fig. 78), but becomes thicker towards the Tectum. The Stratum cellulare externum is composed of a compact mass of cells, which however becomes more sparse dorsally. Thus it is already possible at this stage to detect a differentiation into two parts, a dorsal (D. c. fig. 78) and a lateral (L. c. fig. 78).

The 7 day stage (fig. 79). The lateral part of the ventral column (lying under the Fascic.ulus longitudinalis dorsalis) has increased in size, so that a larger unified nucleus (* fig. 79) has formed, in which practically all the cells are fusiform and horizontally disposed. This nucleus has also enlarged in a lateral direction and is now found partly in the medial part, that is to say, the boundary between the ventral and medial parts has, by reason of the increase of cells, disappeared. However, the borundary—line between the ventral column and the medial is evidently imlrnediate-ly median of a condensation in the lateral portion of the cell—group * fig. 79, though it cannot be discerned in the Stratum cellulare externum.

No important change -has taken place in the medial part. In the lateral part the Stratum cellulare internum -has entirely vanished. In the dorsal part the oriment of the Torus-format-ion can henceforth be clearly distinguished.

The Stratum ependymale has become thicker, partly under the Sulcus longitudinalis ventralis (as at the previous stages), partly at Sulcus lateralis.

The 8 day stage (fig. 80). By reason of the increase of cells in the Stratum cellulare externum, the boiun-daities between the different columns have at this stage become rather difiicult to discern. The same parts in the Stratum cellulare internum can however be observed as at the preceeding stages. The thickening of the Stratum ependyrnale at the Sulcus lateralis can likewise still be used as a point of vantage, but the groove has almost been eliminated.

Ventral to the Fasciculus longitudinalis dorsali-s a cell-condensation is found, which corresponds to the medial portion (belonging to the ventral column) of the cell—group * fig. 79. The cells of this portion are somewhat larger than in the rest of the column. These cells form a nucleus, which becomes still more distinct later: Nucleus ruber.


The structure of the ventral part is otherwise in the main the same as at the preceeding stages.

In the medial part the Stratum cellulare internum is not so compact as at the pvreoeeding stage, but is instead broader (S. c. i. m. fig. 80). The number of cells is less in the lateral (M. c., 1. fig. 80) than in the medial portion (M. c., m. fig. 80) of the medial column. The Stratum cellulare internum is lacking in the lateral part as at the 7 day and following stages. A lateral column or condensation in the Stratum cellulare externum can be observed (L. c. fig. 80).

The Torus-formation (T. f. fig. 80) is more distinct than at the 7 day stage, and is composed of a differentiated part of the Stratum cellulare internum. A compact neuropil-substance has namely developed in this layer, surrounded by closely-packed cells, which are seen disposed in a semicircle on transverse-sections. The cells in the N-europilema (Nucleus lateralis mesencephali) do not lie so closely-packed as is otherwise the case in the Stratum cellulare internum. Outside the Torus-formation (between it and the optic tract [O. t. fig. 80]) lie a number of cells (D. c. fig. 80), which, later (stage 15 days), form a distinct nucleus. T-his nucleus lies in the same position as the dorsal column in Acanthias, but does not get merged frontad in any larger nucleus. I cannot discover in Gallus any equivalent to the ”Genioulatum laterale” in Acanthias.

A number of scattered cells are already visible at the 8 day stage in the periphery of the medial part, these cells have become more numerous at the 8% day stage, and form _a distinct peripheral nucleus in this part. This is the oriment of the Ganglion opticum ibasale (Ganglion ectomammillane) (EDINGER).

The 9 day stage (fig. 81). At this stage the Nucleus ruber (Nc. r.) like the Nucleus ectomammillaris can clearly be distinguished. The compact cellulated mantle in the Torus-formation has almost disappeared, there remains on the «other hand, the comparatively dark—coloured neuropil—substance with the Nucleus lateralis mesencephali.

The I0 day stage (fig. 82). The most striking change which has taken place at this stage, is the vigorous growth of the Torus semicircularis

figs. 79—8I. Transverse-sections through the mid-brain of Callus at the stages

7, 8 and 9 days. (Michrophotographs)

D. c., the dorsal part and column; F. l. d., Fasciculus longitudinalis dorsalis; L. c., the lateral part and column; M. c., the medial part and column; m., 1., the medial, lateral portion of the part in question; N. III, Nervus oculomotorius; Nc. III, Nucleus oculomotorius; Nc. e., Nucleus ectomammillaris; Nc. r., Nucleus ruber; O. t., the optic tract; S. c. i. m. 1., the medial, lateral part of the Stratum cellulare internum; S. 1., Sulcus lateralis; S. 1. v., Sulcus longitudinalis ventralis; T. f., the Torus-formation; T. s., Torus semicircularis; T. o., Tectum opticum; V. c., the ventral part and column; vl., the ventro-lateral portion of the ventral column.


(T. s. fig. 82). The Torus semicircularis appears, on transverse-sections, as a semilunar part sharply projecting into -the ventricle. It is quite clear that the Torus semicirculanis is composed of the lateral part as well as of the Torus-formation proper (comp. figs. 79, 80, 82). The ventral boundary of the Torus semicircularis coincides‘ with the boundary between the medial and lateral parts. The lateral (dorsal) portion only of the Torus semicircularis is filled by the Torus‘—formation (T. f. fig. 82), such as it has developed into since its first appearance in the Stratum cellulare internuni.

The 15 day stage (fig. 83). In the ventral part (V. c. fig. 83) we find the now very well-defined Nuclei III (Nc. III fig. 83) and the Nucleus ruber (N c. r. fig. 83), which is composed of similar, although not so tightly- packed cells. The nest of the ventral column consists of a mass of ver_v small cells. The above-mentioned ventro—medial portion (vl. figs. 78, 79, 80) can no longer be distinguished. (It vanishes more. and m-ore during the 8V2——-Io day stages). The cell—group m. figs. 8o—83 (the Ganglion profundum srnediale of EDINGER) remains. Ganglion opticum basale lies here

peripherally and the remainder of the section is filled with a diffuse mass of cells.

In the Torus semicircularis, medial to the Torus—formation, and therefore in the lateral part, we find a fairly compact cell—mass, indications of which are already noticed at preceeding stages. In the centre of the lateral part we also find a cell-condensation (the Ganglion profundum laterale of EDINGER).

The Torus-formation proper or Nucleus lateralis mesencephali (T. f. fig. 83) continues to occupy only the lateral portion of the Torus semicircularis.

Medial to the dorsal column there is at this stage another well—defined nucleus (Nc. i. fig. 83), whose embryological development points to its being correctly assigned to the Mertencephalon, i. e. the Isthmus region. Lateral to the Velu.m medullare anterius, dorsal to the lateral groove in the 4"‘ ventricle, and caudal to the boundary-plane between Mes- and Met-

encephalon (therefore in the Isthmus) a nucleus, Ganglion isthnii, is to be found in Acamfhias. At the 6 day stage of Gallus, a singularly well—defined and compactly cellulated nucleus lies in the same position as the Ganglion isthmi occupies in Acanthias. The lateral and frontal part of this nucleus increases in size so much that at the later stages it pnojects continually more frontad and finally crosses the boundary of the Mesencephalon and takes u-p its position medial to the dorsal column at the I5 day stage. That this nucleus should. properly be assigned to the Isthmus region, that is to say, that it is formed by the growth of the frontal and lateral parts of the Ganglion isthmi (Nc. i. fig. 99), I therefore consider proved by the following facts: I) that it has exactly the same very characteristic structure at the 6—Io day stages as'Ganglion isthmi, and is in continuity caudad with this nucleus, 2) that its growth can be followed step by step during its embryonic development.


figs. 82, 83. Transverse-sections through the mid-brain of Gallus at stages I0 and 15 days. fig. 84. Transverse-section through the mid-brain of Mus at stage 0,8 cm. (Mici-ophotographs.)

D. c., the dorsal part and column; F. l. d., Fasciculus longitudinalis dorsalis; L. c., the lateral part and column; M. c., the medial part and column; m., 1., the medial, lateral portion of thc.part in question; N. III, Nervus oculomotorius; Nc. III, Nucleus oculomotorius; Ne. e., Nucleus ectomammillaris; Nc. i., the frontal part of the Nucleus isthmi; Nc. r., Nucleus ruber; O. t., the optic tract; S. c. i., m., the medial part of the Stratum cellulare internum; S. 1., Sulcus lateralis; S. 1. v., Sulcus longi-

tudinalis ventralis; T. i., the ‘Torus-formation; T. 0., Tectum opticum; T. s. Torus semicircularis.



Thus I completely agree with BELLONCI (1888), when he says: ”Beim Huhn ist er (Corpus posterius, Nnucleus isthmi) schon am IO. Tage der Bebriitung deutlich zu sehen und senkt sioh unter allmahlicher Gréis-sen- zunahme in die eigentliche Substanz cles Lobus optieus ein” (p. 29). Under the name of Nidulus sub-pyriformis TURNER (I89I) describes this nucleus as belonging to the mid—brain.

I have been unable to discover any analogy to the cell-nucleus * fig. 83 so clearly developed in the anterior part of the mid—brain and in the optic tract at the I 5 day stage among the remaining animal species investigated by me.

The nucleus in question is not yet developed at the I0 .day stage; it developes therefore comlparatively late. It can «hardly be considered homologous with the dorsal column, partly by reason of its lying, as above mentioned, wedged right into the optic tract,‘ partly because it developes only in the anterior part of the Mesencephalon. The dorsal oolumn is found, as I have said, also in the caudal portion and extends towards the Velum region.

Neither can this nucleus be compared with the ”Gen»iculatum laterale" in Acanthias, as no proof can be given of any connection between it and the dorsal column. In addition to this, the ”Geniculatum laterale” developes already very early and in a different position in relation to the optic tract.


figs. 85, 86. Transverse-sections through the mid—brain of Mus at stages 1,0 and 1,3 cm. (Microphotographs)

D. c., the dorsal part; L. c., the lateral part; m., 1., the medial, lateral portion of the part in question; M. c., the medial part; N. III, Nervus oculomotorius; Nc. III, Nucleus oculomotorius; Nc. r., Nucleus ruber; S. 1. t., Sulcus lateralis tecti; S. n., Substantia nigra; S. l. v., Sulcus longitudinalis ventralis; T. 0., Tectum opticum; V. c., the ventral part; v1., the ventro-lateral portion of the ventral column.


E. Mammalia (Mus musculus var. albino).

I have examined Mus muscullus var. albino at stages: 0,8, 1,0, 1,3, 1,6, 1,9, 2,3 cm., young animals of 4,0 and 4,5 cm. in length, and the‘ brains of full—grown white mice.

A study of the embryonic development of the nuclei in mammals offers considerable difficulty. The reason of this is, -that the abundance of cells in the brain of mammals already at comparatively early embryonic stages, is so considerable, that it is not easy to draw up boundaries between the different parts.

At stage 0,8 cm. (fig. 84), where the number of cells is not yet too great. the Subtectum can however be divided into parts in the same manner as e. g. in Acanthias and Gallus. The Stratum cellulare internum is not, to be sure, as in those species, divided up into broader and thinner parts, but on the other hand, the Sulcus lorngitudinalis ventralis (S. l. v. fig. 84) and the Sulcus lateralis (S.1. fig. 84) are very well—defined, and the disposition of the cells in Stratum cellulare externum is, broadly speaking t-he same as in Acanthias (fig. 57) and Callus (fig. 78) at earlier stages (3,9 cm. and 6 days respectively).

In the ventral pant (V. c. fig. 84), the cell-mass in t-he fibre-layer (the ventral column) is composed of a medial part (m.) an-d a lateral (1.), which is situated ventrally to the Fasciculus longitudinalis dorsalis. Between these two parts in both of which there is a paucity of cells, the cells lie more closely-packed and are partly horizontally «disposed (vl.) as in Callus (comp. fig. 78, v1l.).

In the medial part can be distingruisherd in Stratum cellulare extemum a medial portion (M. c., m. fig. 84) with closely packed cells passing without any sharp boundary into the lateral portion of the ventral column (V. c., l. fig. 84), and a lateral portion (M. c., 1. fig. 84) consisting of rather larger ancl'not so closely packed cells.

A dense cell-mass, somewhat more closely packed ventrally, forms the lateral column (L. c. fig. 84), lying in the fibre-layer dorsal to the Sulcus lateralis or the boundary—line between the medial and lateral parts. Dorsally the cellular mass passes continuously into the corresponding Tectum layer.

The boundary between the Tectum and Subtectum, namely .the dorsal part, can at any rate approximately be ‘determined firstly because the Sulous lateralis tecti (S. 1. t. fig. 84) is clearlyideveloped and secondly because the Stratum cellulare internum in Tectum is considerably more vigorous than in Subtectum.

Stage 1,0 cm. (fig. 85). The medi'al,, more abundantly cellulated part of the medial column (M. c., m. fig. 85) has by the increase in the number of cells, become merged, together with the lateral part of the ventral column, into a large, fairly compact cellular mass.

Ventrally in the ventral part (V. c. fig. 85) a dense cell-group can be distinguished: the ventro—lateral portion of the ventral column.

As the Sulcus lateralis is -at this stage almost effaced, the boundary between the medial and lateral parts can only be determined by the guidance of the arrangement of the cells in Stratum cellulare externum. The lateral part of the medial column (M. c., 1. fig. 85) is still composed of comparatively few cells, which form a lesser cellular condensation.

The lateral column is on the contrary, a compact and more peripheral projecting cellular mass (compare Acanthias, stage 3,9 cm. [fig. 57]).

Stage I, 3 cm. (fig. 86). A considerable differentiation has taken place at this stage. In the ventral part (V. c. fig. 86) the Nucleuts III (Nc. III fig. 86) has become fully visible, and ventral and somewhat lateral to this nucleus, Nucleus ruber (Nc. r. fig. 86). Near the medial—line the cells lie more compactly than at the preceeding stages, and form a dorso— ventrally extended nucleus connected. wit-h the ventro—lateral part of the ventral column (V. c., vl. fig. 86). This latter part is vigorously developed.

In the medial part the cells nearest to the Stratum cellulare internum (”Centrales Hohlengrau”) lie comparatively far from each other and are, as at previous stages, disposed reticularly. Outside these cells is found a cell-condensation in almost the same position as the ce1l—group m fig. 83

‘in Gallus. There is likewise at stage 1,3 am. an already powerfully deve-

loped cell—agglomeration in the periphery of the medial part. This is the oriment of the Substantia nigra (S. n. fig. 86). A comparison with Gallm embryons at stages 9—Io days shews, that the correspondence, at least in appearance and position, between the rnesencephalic part of the Ganglion opticum basale (EDINGER) and Substantia nigra is very considerable.

In the lateral part I have been unable to observe any cell-condensation whatever. On the other hand such a condensation is to be found in the dorsal part (D. c. fig. 86).

As no important changes have taken place at stage 1,6 cm., I pass over to a description of stage 1,9 cm. (fig. 87). Substantia nigra (S. n. fig. 87) has grown and become covered with a distinct fibre-layer: the Pes pedunculi. The cell-condensation lateral to the Nucleus ruber (Nc. r. fig. 87) is decidedly less distinct.

Peripherally in the lateral part (L. c. fig. 87) the number of cells is certainly greater than at previous stages. The cells however do not lie so comrpactly as in the Substantia nigra and in the peripheral cell-consdem sation in the dorsal part (D. c. fig. 87).

Through the outward curving of the dorsal part (Brachium corp. quadr. post.) a groove (fissura interbrachialis) has appeared between it and the Tectum-formation. This same groove exists, as a matter of fact, already at stage 1,6 cm. and there are traces of it even earlier. In the centre of the dorsal part can be discerned a cell—mass, namely a column,‘ which together with the peripheral cellular mass, caudally merges into the nucleus of the posterior Colliculi.

I have not been able to discover a nucleus in Mus, any more than in Rana or Gallus, vwhich could /be considered homologous with the ”Geniculatum laterale” in Acanthias. The mammalian Corpus geniculatum mediale, which WALLENBERG (1907) has with some hesitation termed homologous with this Acanthias nucleus, clearly lies in and is formed from the Dience— phalon in Zllus.

4. The Corpus Quadrigeminum Posterius

According to KAPPERS (1906, 1907), the Tori semicirculares corresponds with the posterior Colliculi in mammals. He (I907) writes: ”Bei den Amphibien entwickeln sich die Tori sem:icirc.ulare»s in auffallencler Weise, denn, wiihrend sie in ihrem frontalen Teile dort dem Typus der Teleostomni beibehalten, verwachsen sie, weil keine Valvula sie trennt, in ihrem «mitvtleren Teile mit der medialen Wand untereinander und teilen so den Ventriculus opticus in zwei kleinere Ventrikel, wovon der obere die Fontseztzung des optischen Ventrikels ist und iiberdacht wird vom Tectum, wahrend der untere, der gleich mit der Sylvischen Wasserleitung korrumuniziert, hier sohon als Ventriculus corporis quad1'agemim' posterioris zu bezeiohnen ist” (page 470).

As Torus semicircularis is to be found in all the vertebrates except mammals, it is now generally considered homologous with the Corpus quadrigeminum posterius in works of reference on t-he subject. By a comparative embryological investigation of the caudal part of the Mesencephalon in Awnthias, Salmo, Rana, Gallus and Mus, I have oorne to a conclusion -differing from the above. I

In Mus at stage 0,8 cm. there has developed, caudally in the roof of the mid-brain two sacs or recesses (R. p. m. figs. 34, 88), the Recessi posteriores mesencephali of the authors. Along the lateral walls of each of these recesses, has formed a horizontal groove, which is connected neither with the Sulcus lateralis teoti nor with the Sulcus intraencephalicus. With the guidance of this groove it becomes possible to subdivide the caudal part of the Mesencephalon into a ventro—caudal and a dorsal portion. -Not until the development of the outer groove, which separates the anterior from the posterior Colliculi, has taken place, can the boundary of the dorsal pontion (C011-iCL1lJtlS inferior) be sharply defined from the Tectum opticum, viz. Colliculus superior. The ventro—cau~da1 portion extends down to the fissura rhombo-mesencephalica. The dorsal part of the Sulcus intraence— phalious posterior can also serve as boundary—rnark between this portion and the Metencephalon.


fig. 87. Transverse-section through the mid-brain of M M at stage I,9 cm. fig. 88. Horizontal section through the mid-brain of Mu: at stage 0,8 cm. fig. 89. Horizontal section through the mid-brain of Acanthias at stage 15,0 cm. fig. 90. Horizontal section through the mid-brain of Rana at stage 0.7 cm. fig. 9I. Horizontal section through the mid-brain of Salmo at stage 0,8 cm. (Microphotographs)

C, Cerebellum; C. p., Commissura posterior; D. c., the dorsal part; L. c., the lateral -part; M. c., the medial part; Nc. III, Nucleus oculomotorius; Nc. r., Nucleus ruber; R. p. m., Recessus posterior mesencephali; S. l. v., Sulcus longitudinalis ventralis; S. n., Substantia nigra; T. o., Tectum opticum; V. c., the ventral part; vl., the ventro- lateral portion of the ventral column.



Already at stage 0,8 cm. and later still more clearly defined, there is.

a histological difference between the dorsal roof-portion and the ventro— caudal portion that only consists of a compact neuroblastic layer (comp. fig. 94). The wall is, moreover, comparatively thin.

On comparing the Tectum with this roof—portion, it will be found that a histological difference is likewise to be noticed here. A peripherally compact cel1—layer has developed in the’ Tectum, but does not extend into the Colliculus inferior‘ (fig. 94). Further the Tectum (T. 0. fig. 94) is thicker than the roof of Collicul-us inferior (C. q. p. fig. 94).

In the fully developed brain (and even in young animals of 4,0 cm. in length) almost the whole -of the ventro—cau-dal portion is reduced to a thin membrane (fig. 95), which is non-nervous until the point where it passes into the roof—portion. Thus it is chiefly the dorsal, or that part of the caudal part of -the Mesencephalon, lying dorsal to the Ventriculus corporis quadrigemini posterioris (V. c. q. p. fig. 95), which developes into the so—called Colliculus inferior (C. q. 1). fig. 95). That portion that lies ventro-cautlal to the ventricle or ventricle—groove, becomes reduced‘, and forms, the anterior part of the Velum medullare anterius ”.

After this summary description of the development of the posterior colliculi in Mus, I will pass on to the lower vertebrates, commencing with:

Amphibia. Already early in «the embryonic development of the frog (stage 0,7 cm. [fig. 90]) the T-ecturn in horizontal sections has the form

’ At stage 2,3 cm. the absolute length of the ventro-caudal portion is about 2% times larger than the length of the nervous part of the same portion at stage 4,0 cm.

’ The term Velum medullare anterius should, by rights, refer to that part of the connecting part between the mid-brain and Cerebellum, which lies between the fissura rhombo-mesencephalica and the anterior border of the Cerebellum. I cannot determine where the boundary-line between Mes- and Metencephalon in Velum in the fully developed brain, should be drawn, since fissura rhombo-mesencephalica no longer exists. It is however probable, that it lies as at the embryonic stages, immediately frontacl of the dccussation of the trochlear nerve.

70 71 STUDIES ON THE MID-BRAIN AND CEREBELLUM

of a triangle with the angle pointing frontad. A comparatively thin wall forms the base of this triangle, at the angles of which the roof of the Mesencephalon forms a pair of recesses (R. p. m. fig. 90). The caudal wall of these recesses, which is comparatively thin, and has, moreover, no peripheral fibre stratum, extends down to the fissura rhombo—mesencephalica. The roof portion of the recesses is somewhat thicker, but thinner than the Tectum further frontad.

In consequence of the correspondence, as well in the structure of the

_walls, as in the general form, these posterior recesses can be considered

homologous with the two Recessi posterior-es mesencephali (Ventriculi corp. quadr. post.) in mammals.

Towards the end of the larva—stage, the Tori semicirculares begin‘ in consequence of their increase in size, to grow together caudally at the medial- line.

This does not cause, however, a division of the Ventriculus opticus into two parts, as the Torus (T. s. fig. 96) lies close to the boundary- plane between the Mes— and Metencephalon, directly dorsal of the Sulcus intraencephalicus (S. i. p. fig. 96) and fissura rhombo—n1esencephalica. The Torus semicircularis, on other words, lies in the caudal wall of the recesses, while an equivalent to -the Colliculus inferior is to be sought in the roof portion (comp. figs. 94, 95, 96).

HERRICK (1917) has newly published a work on the mid—brain and thalamus of Necturus, in which he divides the Tectum into two parts: Colliculus superior and Colliculus inferior. The former part ”is reached by the optic tracts and this part may therefore be termed tectum opticum and considered homologous with the mammalian colliculus superior” (page 283). This part ”does not extend backward to the caudal end of the tectum" (page 245). Behind the same lies the Colliculi inferiores. HERRICK is unable to draw a boundary—line between these parts. With regard to the caudal wall of the recesses, HERRICK says: ”In adult Necturus the recess is contracted except under the extreme caudal end of the tectum, where the roof is thin and non-nervous forming a small velu.m medullare anterius” (page 224). This Velum medullare anterius should_thus answer to the mesencephalic part of the Velum, i. e. the part that lies in (front of the decussation of the Nervus «trochlearis.

Salmo (fig. 91) corresponds with'Ran-a, although the Tori semicirculares have not, as is known, grown together caudad in the medial line. ' In Salmo, the caudal wall of the recesses soon becomes very thin and epithelial. Whether a part answering to the posterior Colliculi exists in the roof-portion of. these recesses could only be proved in the same way as HERRICK (1917) did in the case of Necturus, i. .e. by analysis of the fibre anatomy. "On haemalum-preparations I have not been able to ‘discover any caudal part diverging histologically from the Tectum opticum.

‘ Since the lateral column in Amphibia and Birds, which also belongs to the Torus semicircularis, ceases later at the boundary-plane between the Mes- and Metencephalon, it is clearly only the Torus-formation proper, that is to say, the dorsal part of the Subtectum that continues into the caudal part of the mid-brain which projects over the Velum.




In Callus at the 8 day stage (fig. 92) are to be found a pair of well- developed Recessi posteriores mesencephali (R. p. m.). The wall of these recesses is decidedly‘ thinner than in the rest of the Tectum. This is especially the case in the ventro—cauda.l wall. Already at the 8% day stage these recesses (R. p. m. fig. 93) become considerably smaller, at the 9 day stage can hardly any longer be traced, and disappear subsequently altogether. At the 8 day stage they lie in the caudal part of -the roof of the mid-brain. In consequence of the considerable modifications ingform, which take place at the following stages, they become displaced nearer the -medial line, which appears from a comparison between figs. 92 and 93. If Colliculi inferiores exist iniBirds, they sh-ould consequently be sought caudally and medially in the roof of the mid-brain.

The position of the Torus semiciroularis in relation to the Recessus posterior mesencephali and the .bounudary—plane between -the mid- and hind- brain is the same as in Rana (comp. figs. 96, 97). In Reptiles it seems to be so too.

' In Acanthias, on account of the lack of material at suitable stages (5-8 cm.), I have not been able to study closely the development of the part of the mid-brain in question. A faint indication of caudal recesses seems however to exist at stage 15,0 cm. (fig. 89). On the other hand, the caudal part of the Tectum is at least at this stage not thinner.

The Torus—formation in Aczmthias is, as above mentioned (p. 44), divided into: an anterior part in which the histological struc.ture is that characteristic for the Torus—formation, and a posterior part, more closely-cellulated, which extends towards the Velum medullare. The boundary between these parts coincides with the boundary between the recesses and the anterior part of the Tectum. On parasagital sections the caudal part of the Torus formation (p. T. f. fig. 98) can be proved to lie in the most caudal part of the mid—brain, i. e. close to, and dorsal to the fissura rhombo-mese.nce—


fig. 92. Horizontal section through the mid-brain of Gallus at stage 8 days. fig. 93. Horizontal section through the mid-brain of Gallus at stage 8% days. fig. 94. Parasagital section through the posterior part of the mid-brain of Mus at stage 2,3 cm. fig. 95. Parasagital section through the posterior part of the mid-brain of Mus at stage 4,0 cm. fig. 96. Parasagital section. of Rana at stage 3,6 cm. fig. 97.,Parasagita1 section through the posterior part of the mid-brain of Gallus at stage 8 days. (Microphotographs.)

C, Cerebellum; C. p., Commissura posterior; C. q. p., Corpus quadrigeminum posterius; F. rh. rn., fissura rhombo-mesencephalica; I, the infundibular region; M, Mesencephalon; P. dm., Prominentia dia-mesencephalica; R. p. m., Recessus posterior mesenoephali; S. i. p., Sulcus intraencephalicus posterior; T. o., Tectum opticum; T. s., Torus semicircularis; V.. c. q. p., Vcntriculus corporis quadrigemini posterioris; V. m. a., Velum medullare anterius.



Aphalica (F. rh. m. fig. 98) and Sulous intraencephalicus. The part of the

Tectum, where this Torus-formation is found and where t-he disposition of cells in Stratum cellulare exrternum is not that typical for the rest of the Tectum, can therefore be said to be homologous with the ventro—caudal wall of the Recessi posteriores mesencephali. Dorsal to it however, no differentiated indication of posterior Colliculi can be discovered, at least, not in cresyl—violet preparations.

A correspondence to this specially differentiated posterior part of the Torus-formation appears to exist in Gallus. The typical Torus-formation is here caudally in contact with a compact cellular mass (p. T. f. fig. 99), belonging to the Stratum cellulare internum. This cellular mass occupies the same morphological position as the caudal part of the Torus—formation in xlcanthias.

There is, as is well known, no developed Torus—f0rrnation in mammals, but if there were, it would assuredly lie at * figs. 94, 95 (comp. figs. 94-97).

The Nucleus colliculi' inferioris isian enlarged caudal pant of the cell- mass in what I have called the dorsalfpart, i. e. Braohium quadrigeminum inferius. No such larger caudal part of the Stratum cellulare exterinu-m of the dorsal part, namely the dorsal column, is to be found in the lower animal types examined by me. In all these the dorsal column is less prominent caudally than further frontad.

In Selachii (Acanthias) the caudal part of the Torus-formation presents, to be sure, a superficial likeness to the Nucleus collicvuli inferioris, but is certainly not to be compared with the latter. WALLENBERG (1907) describes and figures a nucleus (”Mittelhirnhaubenkern”) in Torpedo, which has: ”eine gr‘-osse Ahnlichkeit mit dem Corpus bigeminum posterius niederer Sauger” (page 389). The figure 32a is unfortunately so schematic, that it is not

possible for me to decide which portion of the dorsal part this nucleus would correspond with in Acanthias.

5. Conclusions

In earlier ontogenetic stages the Subtectum, i. e. the basal portion of the Mesencephalon, is, -on the whole, similarly constructed in all the animal types examined by me. A comparison made between) Actmthias, Gallus and Mus, most plainly proves this agreement.


In that part I call the ventral we find the Nuclei oculomotorii, which in all the types investigated form the Stratum cellulare internum. In Stratum cellulare externum a medial cell—group, a superficial ventro—lateral, as also a lateral ditto can be discerned in Acanthias at stage 5,0 cm. (fig. 58), in Gallu: in the 6 day stage (fig. 78), and in Mus at stage 0,8‘ cm. (fig. 84). In the ventro-lateral and lateral portions the cells are horizontally arranged. In Sa-Zmo and Rana this part i-s so devoid of cells that it is scarcely possible to discern any definite groups of cells. In Acanthias and Callus the medial group of cells becomes extremely diffuse in later stages. In M us the number of cells in this portion is relatively large. The ventro-lateral group -disappears as such in the later stages in Acanthias and Gallus. I-n Mus it is retained very evidently, at any rate up to the 1,9 cm. stage. finally the lateral group (in Gallus and Mus) is included in the formation of Nucleus ru-ber.


A Nucleus ruber has been described in fishes (Selachians [WALLENB1~:R(;, I907], Ganoids [_IoHNs'roN, I901], Teleosts [GOLDSTEIN, 1905]). De LANCE (1913) has written about a numb.er of large reticular cells in the ventral portion of the mid-brain in Selachians (and all groups of fishes) and Am-phibians (Rana), as ”prototype des Nucleus ruber”. He has also (1912) found a red Nucleus in Reptiles. In the ontogenetic .stages of fishes and Rana investigated by me these large cell-s could not be identified, and their embryological origin was therefore not studied.


In Gallus and Jlllus, where I could follow the development of the Nucleus ruber, there is at certain intermediate stages (in Gallus in the 7 day stage [fig. 79], in Mus at stage 1,0 cm. [fig 85]) a cel1—mass that appears to be formed partly from Stratum cellulare internum immediately dorsal to the Fasciculus longitudinalis dorsalis, partly from the lateral group of cells in the ventral part. (A similar group of cells is found also in Acanthias in the 5,0 cm. stage [fig. 58].) In Gallus this cel1—group projects somewhat into the medial part, i. e. the boundary between the ventral column and the medial is no longer discernible, or is indistinct. The boundary—line between the ventral column and the medial is, however, immediately median of a condensation in the lateral portion of the cell-group in question, though it cannot be discerned in the Stratum cellulare externum. The same boundary- line, even if not so very distinctly, is also noticeable in later stages (8—9 days). The Nucleus ruber is formed (in the stages 9-: 5 days) from the cell-mass median to the boundary—1ine. The Nucleus ruber in Callus is formed in the ventral part. In Mus it seems to be so too.


As belonging to the medial portion. I have assigned the very characteristic and rather -constantly occurring thickening-in the Stratum cellulare internum. In Acanthias, Gallus and M us, the medial column is more or less plainly divided into two portions.

In Acanthias the division is distinct only in the stage 3,9 cm. (fig. 57). In Gallus, on the contrary, up to the I 5 day stage. In both these forms the medial portion is more abounding in cells than the lateral. In Callus a nucleus (the Ganglion prof-unduin mediale [EDINGER, 1908]) is formed in the medial portion that appears to have a partial equivalent in M us. In M us, at stage 0,8 cm., the medial portion of the medial column abounds more in cells than the lateral, this being also the case in Acanthias and Callus. Already in stage 1,0 cm., however, the number of cells in the lateral portion has increased, so that this portion is almost as profuse in cells as the medial. By the coalescence of these two portions a nucleus (the Ganglion profundum laterale, EDINGER [I9II]) is formed, the position of which, as regards Nucleus ruber and Suhstantia nigra, is almost the same as that nucleus designates m in fig. 83 (Ganglion profundium mediale) in Gallus in relation to the Nucleus ruber (Nc. r.) and Nucleus .ectomammi'illaris (Nc. e.) 1.

In Salmo and Rana the medial column is undivided.-

In the periphery of the medial part of all .those animal types investigated there occuus a group of cells that in Mus forms the Substantia nigra. In Acanthias (fig. 58) the number of cells in the periphery of this part is relatively small. In Salmo (figs. 72, 73), but only in the embryos, there is a large peripheric nucleus, or cell mantle, in «the medial portion of which cells enter that formed ‘from the medial part. In Rana, as early as in stage 3,6 cm. (fig. 75), and later on‘ still more plainly, you discern how the medial column becomes more dense towards the periphery and forms the posterior part of a nucleus that GAUPP (1899) desigriated Nucleus ecto- mammillaris. The nucleus that in Callus (figs. 8I——83) appears in the periphery of the part in question has been described by EDINGER as Nucleus ect-omammillaris (Ganglion opticum basale, Nucleus ped-uncularis of BELLONCI [I888]). It is however evident that the real Nucleus -ectocma.mm.illaris belongs to the Diencephalon. Thus for instance in Teleosts (EDINGER [I9o8]), as also. in Reptiles and birds. The Nucleus ectomammillaris, figured by EDINGER in his work ”Studien iiber rdas Zwischenhirn der Reptilien” (1899), is evidently in the Diencephalon. Judging from EDINGI-:R’s fig. I 59 in his ”Vorlesungen, etc.” (1908) he has also assigned the mesencephalic nucleus in question as belonging to the Nucleus ectomamrnillaris. TURNER (I891) designates the Nucleus ectoanammillaris ”Nid»ulus inferius” and says: ”This Nidulus extends from the junction of the Mesencephalon with the Metencephalon cephalo-mesad about half—way to the optic chiasma” (p. I2I).

  • According to Edinger (I908, 1911) the Ganglion mesencephali profundum mediale and laterale are present in all Vertebrates. In fishes, Amphibians, birds (and Reptiles?) the lateral column seems to be what Edinger names Ganglion mesencephali profundum laterale. In fishes and Amphibians the medial column probably is synonymous with the Ganglion mesencephali profundum mediale. In birds (and Reptiles?), however, it is the medial part of the medial column that Edinger names the Ganglion mesence— phali profundum mediale. In man, at last, the Ganglion mesencephali profundum laterale lies immediately lateral and the Ganglion mesencephali profundum mediale medial to the Nucleus ruber. Thus Edinger has named very different nuclei with the same terms.


In older embryos, certainly, the dienoephal-ic and mesen_cephalic portions of the Nucleus ectomammillaris are‘ coherent with each other, but .in the earlier stages a boundary between them may easily be di.scerned.

As in Rana (fig. 75), during the larval stages, the number of cells in the nucleus is relatively small, I have been unable to discover any certain line of demarcation between the posterior mesencephalic and the anterior diencephalic portion.

The posterior mesencephalic portion of the Nucleus ectomammillaris in Rana and Gallus is formed in the medial part, and are, moreover, in the same position as the oriment of the Substantia nigra in mammals. These nuclei are probably homologous. The Substantia nigra merge, however, later with a cellgroup that belongs to the ventral part (fig. 87). The Substantia nigra has by BAUER (I909) previously been deemed homologous with the Nucleus entopeduncularis (EDINGER). This is, however, entirely a diencephalic nucleus 1.

Likewise, in the lateral part, an inner cell-mass and an outer peripheric nucleus may be distinguished. In the earlier stages the lateral column in all the forms (with the exception of Salmo and Rana) consists ‘of a dense mass of cells, rather large, and often’ projecting somewhat farther towards the periphery than the medial column. In the later stages in Acanthias a peripheric and very distinct nucleus forms in this part (fig. 59). From the lateral pant, even in Salmo (figs. 72, 73), cells proceed that enter into the construction of the above mentioned peripheric cell-mantle. In Gallus as in M us the number of cells in the periphery are considerably less in the lateral than in the medial part. It is however possible to ‘discern a slight cell-condensation in the periphery of the lateral part in Gallus in the I 5-day stage (fig. 83) and in Mus in the 1,9 cm. stage (fig. 87).


The mesencephalic part of the Substantia reticularis lies in the inner portion of the lateral as well as of the medial part, close to the Stratum cellulare intemum.


The dorsal part is at first indistinctly separated from the adjoining parts (Tectum opticum, and the lateral part). Soon, however, a difierentiation occurs. In the Stratum cellulare internum what I designate as the Torus formation appears, and to this I assign I) the Nucleus lateralis mesencephali, in accordance with the more limited definition of this nucleus I have introduced and 2) a denser, semicircular cell-mant.le, of the Stratum cellulare internunm, that in certain forms (’e. g. Acanthias and Gallus) is plainly visible in certain stages. In Acamfhias (fig. 59), this Torus-formation completely fills the Torus semicircularisl Already in the Ganoids and Teleosts the lateral column is partly in the Torus semicircularis. In Amphibians and still more plainly in birds, the lateral part lies entirely within the Torus semicircularis, which is therefore not fully homologous in the various groups of vertebrates.


  • Comp. also HERRICK '(I89I, 1892, a, c).


The cell—mass that in Acanthias (in the stages 5,0 to 15,0 cm. [F igs. 58, 59]) fills the intermediate space between the Torus—formation and the optic tract I name the dorsal column. In Salmo (fig. 71), from the neuroblastic stratum in the dorsal part, a cell-group is formed which I consider corre— sponds to the dorsal column in Acanthias. This cell-group may be traced frontad to the region where the Torus lateralis is formed later on (figs. 67, 68). In the dorsal part of the Torus lateralis cells enter formed from all the parts of the Subtectum, with the sole exception of the ventral. This dorsal portion of the Torus ‘lateralis, owing to the corresponding position and manner of forrnation, I have horn-ologized with the ”Corpus geniculaturn laterale” in Selachians. The nucleus last—mentioned is not homologous with the Geniculatum laterale in Teleosts. I have found no correspondence to the Selachian ”Geniculatum” in the higher animals (amphibians, birds, mammals).


In later stages we may distinguish in Salmo (figs. 72, 73) a peripheric part, disappearing subsequently, in the dorsal column. This column in Rana consists simply of a very small number of cells. In Gallus the dorsal column in younger stages (6 to 10 days [figs 80-—82]) is but little conspicuous. Owing to the cells therein increasing in size by growth later on, a visible nucleus (fig. 83 D. c.) is formed, however, which TURNER (1891) has described and figured under the IdCI101'l’]ilIl3tlOl’l Nidulus lenticularis.


The nucleus (* fig. 83) that in Gallus in the 15-day stage lies in the optic tract has scarcely any equivalent in the mid—brain of the other groups of animals. In Mus the dorsal column divides into a superficial condensation and a central nucleus that together form the grey matter of the Brachium colliculi inferioris. The posterior portion of the dorsal column in mammals has developed to the so-called Corpora quadrigemina posteriores (Nucleus colliculi inferioris).


In a suitably orientated series of cross sections of Acanthias in the stage 3,9 cm. it may be ascertained that the thickened Stratum cellulare internum of the medial part, as also the entire medial column continues in the Metencephalon and Medulla oblongata, lying there inside the Funiculus medialis. The medial column extends therefore into the ventral column in the Medulla and the spinal cord. To this we must also assign the ventral part in the mid-brain. In the Medulla this part is very incons-iderable and only in certain portions (e. g. at the Ganglion interpedvunculare) does it contain larger accumulations of cells. The medial and Ventral parts may therefore be said to correspond to the ventral column of the Medulla and spinal cord. So far as I can ascentain the lateral column in the Mesencephalon forms a continuation of the lateral column in the spinal cord. That the dorsal column apparently comes to an end in Mesencephalon, i. e. that it projects upwards dorsal to the fissura rhombo-mesencephalica, need evidently not necessarily signify that it is devoid of an equivalent in the Medull-a and the spinal cord. It simply show.s that a corresponding column is lacking in the isthmus region, i. e. beneath the Plica encephali dorsalis.


  • In Selachians (Galeux) a Torus semicircularis has previously been described by KAPPERS (I906).


The Mesencephalon may thus be stated to consist of four chief portions, viz. I) Tectum opticum; 2) The dorsal part of the Subtecturn with Corpus qua-drigeminum posterius; 3) _The lateral part, and 4) The medial part inclusive of the ventral. Naturally it is a difficult thing to definitely decide whether these four portions correspond to the four functional portions into which the spinal cord and the oblongata have been divided. Should this be the case, however, it would provide a good starting-point for the functional division of the Mesencephalon.


Next - II. The Cerebellum


List of Literature


Cite this page: Hill, M.A. (2019, July 23) Embryology Paper - Embryological and morphological studies on the mid-brain and cerebellum of vertebrates 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Embryological_and_morphological_studies_on_the_mid-brain_and_cerebellum_of_vertebrates_1

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