Paper - The development of the neural folds and cranial ganglia of the rat

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Adelman HB. The development of the neural folds and cranial ganglia of the rat. (1925) J. Comp. Neurol. 39(1): 19-171.

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This historic 1925 paper by Adelman described development of the early rat central nervous system and cranial ganglia.



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The Development of the Neural Folds and Cranial Ganglia of the Rat

Howard B. Adelmann

Department of Histology and Embryology, Cornell University, Ithaca, New Pork

Six Text Flgures and Twenty-Four Plates (Ninety-Five Figures) 1925.

It is a pleasure to acknowledge here the generous help of many friends. I am especially indebted to Prof. B. F. Kingsbury for constant advice and encouragement. It was he who suggested that I take up the work. Miss Janet A. Williamson kindly allowed me to use the models of the head of the rat which she constructed as well as a number of important series prepared by her. I also wish to thank Dr. Fred W. Stewart for the use of many excellent series of older embryos, and finally I should like to express my appreciation of the skill and unfailing courtesy of Miss F. Louise Duhring, who collected the material at The Wistar Institute. I am indebted to the Mrs. Dean Sage Research Fund, which bore the expenses of the investigation.

Introduction

The early development of the neural tube and cranial ganglia has been investigated with some degree of completeness in only a few mammalian forms. There are, it is true, numerous accounts based upon the study of very limited material and the literature abounds with observations on the development of the neural tube and cranial ganglia made incidentally in works dealiiig with the general anatomy of the embryo. Bartelmez ('22, '23) has recently called attention to the paucity of our knowledge of this subject and has himself contributed greatly to our understanding of the early development of the neural folds and sensory anlagen of the human embryo.


The reasons for the scarcity of work dealing with the development of these structures in the Mammalia are obvious, since the difficulties attending the collection of complete series of mammalian embryos are well known. In studying human embryos investigators have been especially handicapped both by the scarcity of the material at their command and the faulty preservation which such embryos frequently exhibit due to the circumstances under which they are obtained.


There are still many features of the development of the neural crest and cranial ganglia in mammals upon which additional observations are much needed, and many inter- esting questions are still at issue. Workers on the lower forms, especially the Ichthyopsida, seem to be quite generally agreed that the epibranchial and lateral-line placodes contribute liberally to the formation of the cranial ganglia, but the occurrence of such placodal contributions in Mammalia is still an open question. Several authors have described the formation of mesenchyme (mesectoderm) from neural-crest elements in the mammal, and this has been denied by others. The anterior limit of the neural crest, a point of the greatest theoretical importance, has not yet been clearly determined in the mammal. Further observations are needed on the exact mode of origin of the crest and its relation to the neural plate and ectoderm.


In addition, there are problems in connection with the individual cranial ganglia which have not been adequately solved. What, for instance, is the exact mode of origin of the ophthalmic ramus of the trigeminus in the mammal? Does it arise, as Belogolowy ('10) suggests for the bird, as a condensation of diffuse neural crest proliferated from the mid-brain; by placodal proliferation, as some have maintained; or by forward growth from the main ganglionic mass of the trigeminus? Is there a separate profundus anlage of the trigeminus, as Schulte and Tihey (’15) indicate in the cat? Does the acoustic ganglion split off from a common acoustico-facial mass? Is it increased by proliferation from the walls of the otic vesicle? Is it true that the ganglia petrosum and nodosum have an origin distinct from that of the rest of the IX-X anlage, as Streeter (’04) thought possible?


The answers to some of the questions proposed above depend upon a knowledge of the subdivisions of the early neural plate and tube.


The foregoing brief outline makes clear, I think, the desirability of further studies. The writer has taken advantage of the facilities bf The Wistar Institute for the collection of a close series of rat embryos upon which a study of the above problems was made. He has tried to keep constantly in mind the actual growth transformations of the whole head, in which the developing ganglia are involved. The early history of the neural plate and tube has been followed, inasmuch as an understanding of the growth and subdivisions of the early neural tube is necessary in studying the relations of the ganglia.


Material and Methods

This paper is based upon the study of more than 200 series of embryos of the white rat, Mus norvegicus albinus. The rats were obtained from The Wistar Institute and most of the uteri were removed and fixed there. The embryos were fixed at three-hour intervals from the nine-day-twelve-hour stage until thirteen days ; after that at less frequent intervals. Of many of the younger, more critical stages, two or even three litters were sectioned. Considerable variation was found in some uteri. Most of the material was fixed in Bouin’s fluid; a few were fixed in Carnoy’s and some of the older stages were fixed in Zenker’s, Helly’s or vom Rath’s picro-aceto-osmic-platinic chloride mixture by Doctor Stewart. All the material was cleared in xylene or toluene and embedded in paraffin. Most of the younger embryos were cut at 7.5 um, others at 10. Most of the embryos were stained in toto with Mayer's HC1 carmine, a few in Delafield's haematoxylin. A few were stained on the slide in iron haematosylin or in haematoxylin and orange G. Models were made of several stages by the Born method.


It was found best to section the younger stages in utero, after carefully cutting away the musculature. Since early embryos of the rat are oriented in a perfectly definite fashion with respect to the axes of the uterus, it is possible to secure favorable planes of sections in a large majority of cases, even of embryos sectioned in utero, up to a certain stage. The orientation of the embryos in the uterus has been found to be essentially as described by Widakowich ('09, '11). At eleven days and after, it is quite feasible to remove embryos from the uterus so that they may be oriented for cutting.


Throughout this paper the age will, so far as possible, be given in somites, since that method is most reliable. The Cornell collection now contains one or more embryos of every somite number from 1to 32, besides a large number of older embryos beginning with 34 somites. Of many stages trans- verse, sagittal, and frontal series are available, making it possible to check up carefully the interpretation of series.

The Development of the Cranial Neural Folds

The present work, begun primarily as a study of the devel- opmental history of the cranial neural crest, might naturally begin with a consideration of such questions as the origin of the neural crest, the site of its proliferation and the locus of its first appearance. However, the work had not progressed far before it became evident that a study of the growth and differentiation of the neural plate and tube of the rat was a necessary preliminary, so that important questions concerning the relation of the neural crest to the prospective sub- divisions of the neural plate and tube might be answered with some degree of certainty. Bartelmez's ('23) recent paper on the subdivisions of the neural folds of the human embryo has been very helpful in this connection.[1] As might be expected, the results are in general quite comparable. The developmental idibsyncrasies of the two forms must, however, be kept in mind in comparing them. It was found that the rat lends itself nicely to such a study, since certain landmarks are early established on, or in relation to, the early neural plate which allow one to follow accurately the differentiations of its regions.


In the hindbrain region of the neural folds Bartelmez ( ’23) recognizes three primary rhombomeres in young human em- bryos. These he designates rhombomeres A, B, and C. Rhombomere A divides first into two secondary rhombomeres- A, and 3- and still later, rhombomere A, divides into rhombomeres 1 and 2. Ultimately, therefore, rhombomere A gives rise to the first three rhombomeres. Rhombomere B (otic rhombomere) is a secondary as well as a primary rhombomere, undergoing no division.[2] It is the fourth of the series and is recognizable very early as a marked expansion of the neural plate related to the otic placode. Rhombomere C furnishes the last three rhombomeres -5, 6, and 7. For the sake of convenience, the nomenclature employed by Bartelmez will be used in this paper.


Like the human embryo, the rat ultimately develops seven typical rhombomeres. Figure 19 illustrates those of a 26- somite embryo. The first or cerebellar rhombomere is very shallow, sloping gradually to the isthmus ; the second marks the broadest region of the rhombencephalon and is related to the V nerve. The third rhombomere is narrow, separated from the second by a shallow groove, and free (externally at least) from nervous connections. The facial nerve is attached to the fourth or otic rhombomere, which is uniyue among the rhombomeres in that its swelling extends to the midventral line, producing there a prominent bulge. The others all ‘fade out’ before they reach the midventral line. It is somewhat wedge-shaped, being narrower dorsally than ventrally. The otic vesicle is related to the fifth rhombomere. In contrast to the preceding rhombomere, the fifth is wider dorsally than ventrally. The sixth calls for no special comment. The sev- enth is remarkable for its size, but it produces no prominmt swelling. Bradley ( ’04, p. 628) has already commented upon the large size of the seventh rhombomere of the pig embryo. The cristal proliferation for the IX-X ganglia is related to the two caudal rhombomeres. The above description is purposely brief and designed to show the essentially typical development of the rhombomeres in the rat. They seem to correspond fundamentally in their form and relations to those described by Broman ( ’95), Bartelmex ( ’23) in man and Bradley (’04) in the pig. Broman’s figure 2 (text and plate) is most serviceable for comparison, but his confusion of the cerebellum with the second rhombomere must be kept in mind. It is interesting to compare these results with the work on other rodents, which in spite of minor variations is essentially in agreement. Chiarugi (’97, pp. 33, 34) de- scribes six rhombomeres in the guinea-pig. The six described correspond perfectly with rhombomeres 1to 6 of the rat, his sixth rhombomere being related to the IX nerve. He states that the neural tube has a uniform contour posterior to the sixth rhombomere, but a model would possibly have revealed a seventh rhombomere between Chiarugi’s sixth and the first somite. Meek (’10) finds eight rhombomeres in the rabbit. He finds three rhombomeres related to the IX-X nerves. His rhombomeres 1to 5 are identical with the corresponding ones of the rat. Volker (’22) reports that eight are macroscopically observable in the ground-squirrel (Spermophilus citillus). His description disagrees with mine in assigning the trigeminus to the third instead of the second rhombomere. Valker states, however, that his rhombomeres 1 and 2 are recognizable in sections “nur bei einiger Aufmerksamkeit.” I have made no special study of the rhombomeres beyond the 26-somite stage.


At 26 somites the midbrain is fairly extensive, showing two faintly marked mesencephalic segments. Diencephalon and telencephalon are well marked and require no special description.


I now turn to a description of younger stages, in an attempt to show the gradual differentiation of the regions above outlined. A brief description of two young embryos, one of 1somite and another of 2 to 3 somites, will serve to introduce the description of a 5-somite embryo which marks the real starting-point of our study. In a 1-somite embryo (fig. 7) the neural folds have just begun to elevate. The folds diverge somewhat caudally. At both extremities they gradually subside to the level of the blastodisc. At the apices of the folds there is, as yet, no delimitation of neural from somatic material, although in the most elevated regions the arrangement of the cells suggests a future line of cleavage.


In a 2-to-3-somite embryo (fig. 8) marked expansion of the neural plate has occurred. Forward growth and expansion has resulted in the formation of a short head fold into which the foregut extends. The elevation of the anterior end of the embryo is unquestionably in the nature of an overgrowth of more ventrally lying parts, initiating a growth process which for a long time continues to characterize the development of the anterior end of the head. In certain sections through this embryo a definite cleft has appeared marking the boundary of neural plate and ectoderm, but as a rule this is not prominent until somewhat later. The two embryos just described should be compared with those figured by Parodi and Widakowich ( ’20).


The 5-somite embryo (figs. 10, 11) is most important for the purposes of this study. In this specimen the head region has grown enormously as contrasted with the preceding stage. The medullary plate is greatly expanded anteriorly, but the neural groove is relatively shallower than in the 2-to-3-somite embryo. The most interesting feature is the sharp ventral bend of the cephalic end of the medullary plate, so that the most anterior region of the plate is directed at right angles to the more caudal portion. Urith respect to this bending of the neural plate, the rat stands in marked contrast to the human embryo, where in early stages the neural plate shows no pronounced flexure (cf. figs. 3 and 4 of Bartelmez, '32). As might be expected, the spermophile (Volker, '22) seems similar to the rat in this respect, and the condition is approximated in the sheep (cf. Neumayer, '99, '06), and pig (Keibel, '96), being much more marked and occurring more preco- ciously in the rat, however. It is without doubt an expres- sion of great forward growth and expansion in restricted quarters. The anterior edge of the medullary plate is some- what recurved to form a rather prominent lip or ridge, which becomes less evident as development progresses.


The perpendicular anterior face of the neural plate has on each side a very shallow depression (fig. lo), probably the earliest indications of the optic foveae: This is the only one of many 5-somite embryos in which the optic foveae have started their development, and in spite of the fact that transverse sections are not in general favorable for the study of shallow depressions so situated, a careful study of this particular series has convinced me that the optic pits here shown are not artifacts. From the same aspect one can also see a marked deepening of the neural groove anteriorly, but not affecting the extreme cephalic end of the neural plate. It is bounded on each side by eminences which form the summit of the bend of the neural plate. This is the primitive infundibulum.


In the dorsal view of this embryo reproduced in figure 11, a well-marked groove is shown extending across the neural plate. I have designated this the preotic sulcus, since it lies just ahead of the otic expansion of' the neural plate. This preotic sulcus will be shown subsequently to mark the site of the third rhombomere. The grooves of the two sides end in a slight depression medially.


The medullary plate is widest anterior to the preotic sulcus. Posterior to it there is a conspicuous expansion marking the site of the otic rhombomere (Rh.4). Figure 11 shows the marked elevation of the neural plate anterior to the pre- otic sulcus and the sharp declivity posterior to it in the region of the otic rhombomere. A shallow postotic groove bounds the otic rhombomere caudally. The interval between the postotic groove and the anterior margin of the first somite is small (fig. 1). It is still smaller at 3 somites (fig. 22). There is some doubt in my mind as to whether one has the right to designate as rhombomere C this territory, showing no expansion, and as yet none of the characters of a rhombomere.


Sections through the region of the otic rhombomere (fig. 23) show a marked thickening of the somatic ectoderm which is probably the a d a g e of the otic placode. It is impossible to delimit it absolutely from the thickened ectoderm in the territory of the future first gill arch. Its approximate extent is shown in figure 1.


Between 5 and 6 somites a marked elevation of the cephalic regions has occurred (fig. 12). One notes, however, that while the anterior, perpendicular face of the neural plate has expanded somewhat and the optic pits have deepened, there has been no appreciable growth of that portion of the neural plate lying between the rostra1 flexure and the preotic sulcus. The ‘wave’ of expansion has now progressed caudally, affecting principally at this time the region lying between the preotic sulcus and the anterior boundary of the first somite, so that in the 6-somite embryo its extent is almost three times as great as in the 5-somite embryo.


The optic foveae are now broad, shallow depressions. The infundibular groove is somewhat more vertical in position than at 5 somitea, but it has not deepened especially. The pre-otic sulci meet medially in a shallow, but well-defined pre-otic pit, and, as previously, the neural plate returns rapidly to the general level of the blastoderm caudal to this point. The declivity affects almost the entire region between the preotic sulcus and the first somite. The expansion of the otic rhombomere is pronounced, but a postotic sulcus cannot be detected bounding it caudally. The plane of section (Sagittal) is, moreover, such as would not obscure such a sulcus if present. As far as the plane of section allows one to judge, the otic placode corresponds approximately with the otic rhombomere, extending somewhat ahead of its anterior boundary.


Fig. 1 A plotting, to scale (xloo), of tlie dorsal aspect of the neural plate of a 5-somite rat embryo (Ser. 110), showing the prospective subdivisions of the neural plate. The rostra1 neural crest is shown in stipple, extending along the margin of the prospective midbrain and rhombomere A,. The extent of the otic placode is indicated by a bracket. An arrow marks the level of the first somite.


The 8-somite (figs. 13 to 15) embryo shows marked ad- vance in the expansion and elevation of the head region. The optic pits have deepened considerably and the lateral borders of the neural plate in this region have begun to swing medially, initiating the closure of the neural tube. Forward and downward growth of the material of the anterior portion of the neural plate has resulted in a significant change. The two eminences previously spoken of as bounding the primitive infundibulum have now moved downward and lie slightly above the middle of the anterior face of the neural plate. As a result, the contour of the rostral flexure, viewed dorsally, has been changed markedly (cf. figs. 1 and 11 with 2 and 15).


Fig. 2 A plotting, to scale ( X l00), of the dorsal aspect of the neural plate of an 8-somite rat embryo (Ser. 85a), showing further differentiation of the prospective regions of the neural folds. The ganglionic anlagen are indicated in stipple. The extent of the otic placode is marked by a bracket and the level of the first somite is shown by the arrow. I n studying figures 1 and 2 it should be appreciated that the anterior perpendicular portion of the neural plate is not shown. See figures 10, 11, and 13 for comparison.

G.V., ganglionic crest of trigeminal ganglion; G.VII-VIII, acoustico-facial ganglion; G.IX-X, ganglionic crest of the glossopharyngeal and vagus ganglia ; Xe., mesencephalon; 0t.pZ., otic placode ; ah., rhombomere ; R.n.c., rostral neural crest, 8.1,level of first somite; Sp.n.c., spinal neural crest.


The preotic sulcus is prominent. Corresponding to it there is a ridge on the ventral surface of the neural plate (fig. 14), which runs slightly cephalad as one proceeds from the lateral margin of the neural plate toward the mid-line. Anterior to the sulcus lies the proliferation for the V ganglion, which has a much less extensive attachment to the neural plate than in the 5- or 6-somite embryo - a fact to which more attention will be devoted later. On the right side there is a shallow sulcus at the boundary between neural and somatic ectoderm, marking externally the site of the V anlage. Similar neuro-ectodermal sulci mark the position of the VTI-VIII and IX-X anlagen on the right side, but on the left only the IX-X anlage produces one.


The otic rhombomere is marked and there is a broad post- otic depression on the right. On the left side, however, while the expansion is more pronounced, no postotic sulcus can be detected and the otic rhombomere is not so clearly sepa- rable from the more caudal regions. Related to this rhombomere are the cristal proliferation for the VII-VIII ganglion and otic placode. The latter extends caudally beyond the territory of the otic rhombomere to the level of the IX-X anlage; it shows a slight invagination (fig. 27), which is, however, an artifact. There is an absolute break in the neural crest in the region of the preotic sulcus and another for eight sections caudal to the acoustico-facial anlage, probably mark- ing the site of rhombomere 5. The IX-X ganglionic anlage is directly continuous caudally with the spinal neural crest.


In the 9-somite embryo (fig.3) it is possible to establish the cephalic limit of the hindbrain. The embryo from which figure 3 was constructed is cut frontally - a favorable plane for a study of the subdivisions of the neural folds. Figure 20 is a photograph of a section through the hindbrain, showing the relations of rhombomeres 1 to 5. Just caudal to the midbrain there is a broad, shallow depression involving the entire extent of rhombomere A, and comprising secondary sulci related to external rhombomeric swellings. On the right there are two rhombomeres related to it, the smaller caudal one, free from neural crest, is rhombomere 3, while the broader cephalic swelling is rhombomere A,, which later divides into rhombomeres 1and 2. The double nature of this rhombomere is shown on the left, where two swellings are evident anterior to rhombomere 3, and there are indications of two shallow internal sulci corresponding to them. Bartelmez ( ’23) records evidence of subdivision of rhombomere A, in human embryos from the 14-somite stage on. The fifth anlage is attached to rhombomere A, on the right and to hombomeres 1 and 2 on the left. I n this embryo, then, the boundary between hindbrain and midbrain may be definitely determined at the anterior limit of rhombomere A.


Fig. 3 Drnwing from a model ( X 100) of a rat embryo of 9 somites (Ser. 223), to show the subdivisions of the neural folds. The extent of the V, VII-VIII ganglia, and the otic placode is indicated by broken lines. The plane of section (frontal) was not favorable for determining the exact anterior or ventral limits of the IX - X ganglionic anlage, hence it is not shown here. It begins a very short distance caudal to the otic placode and extends caudally to become con- tinuous with the spinal neural crest at the level of the first somite. A good idea of its ventral extent cnn he obtained from figure 71, which is a cross-section through the IX-X anlage of an embryo of the same age. The neural folds are entirely open anterior to the level of the fourth somite, gaping most widely in region of mesencephalon and rliombomere A,. The position of the ventral lip of the neuropore is marked by an asterisk. A section of this embryo is given in figure 20.

Di.,diencephalon ; Di-me., di-mesencephalic boundary; G., ganglion ; M., mesencephalic ‘segment’; Me.-Rh., boundary between mesencephalon and rhombencephalon; Op.zi., optic vesicle; Ot.pZ., otic placode; Pr.inf., primitive infundibulum; Rh., rhombomere; 8.1, first somite; Te., telencephalon.


While the external swelling for rhombomere 4 is promi- nent, there is practically no indication of an internal sulcus corresponding to it. The fifth subdivision is a gentle swelling free from neural crest (excepting, possibly, a small portion caudally), and not yet well defined caudally. From the region lying between its caudal border and the first somite the sixth and seventh rhombomeres will arise. The otic placode is still an extensive thickening covering the terri- tory of the fourth andmost of the fifth rhombomeres. Since the plane of section was not favorable for an accurate plotting, the IX-X proliferation is not shown in figure 3. It extends from the caudal border of the otic placode caudally to become continuous with the spinal neural crest.


The midbrain is interesting. The di-mesencephalic bound- ary lies along the line of the rostral flexure, and from this level the midbrain extends caudally for some distance. There are two mesencephalic 'segments' plainly visible, each marked internally by a broad, shallow sulcus. Chiarugi ( '22) finds a similar number of segments in the midbrain of the guinea-pig.


In this embryo closure of the neural folds has not occurred anterior to the level of the fourth somite, but in a 10-somite embryo the folds have approximated and are about to fuse as far cephalad as the anterior border of rhombomere 5. At 10 somites the nearal folds of the hindbrain anterior to this point and the entire midbrain are still widely open, gaping most markedly in the region of the midbrain. In the fore-brain region the folds approach, but have not fused.


There are few significant changes in the 10-somite embryo (fig. 16). The di-mesencephalic boundary which has ap- peared along the line of the rostral flexure of the 8-somite embryo (cf. figs. 3, 14, and 16) is unmistakable. Two mesencephalic segments are very prominent. Rhombomere A, marks the widest part of the hindbrain. Its doable nature is attested by the presence of two shallow sulci related to it internally (fig. 21). The otic rhombomere shows the mid-ventral swelling which is so characteristic of it. On the left side it possesses an atypical dorso-caudal swelling to which the VII-VIII anlage is attached, and this atypical swelling has an internal sulcus related to it. On the right side the rhombomere is typical. Rhombomere 5 is rather indistinct on the left side. The otic placode is invaginated slightly in the 10-somite embryo, the anterior lip of the pit overlying the VII-VIII ganglion. Its extent is indicated in the figure.


In the 14-somite embryo (fig. 17) the forebrain is distinctly differentiated into telencephalon and diencephalon, and the dimesencephalic boundary is so obvious as to require no comment. The midbrain shows no evidence of segments, possibly because the plane of section was not favorable for their demonstration in the model. I have examined sagittal and frontal sections of other 14-somite embryos where they also seem to be absent. Strangely enough, there are faint in- dications of two mesencephalic ‘segments’ in both the 18- and 26-somite embryos. In a 14-somite embryo sectioned sagittally the floor of the midbrain shows a marked bend which may possibly mark the inter-mesomeric boundary, but certainly there are no marked lateral swellings of the mid- brain in any of the 12-to-14-somite embryos I have examined.


It will be convenient to begin the description of the hindbrain with the preotic rhombomere 3 which is well marked, but which does not extend to the midventral line. Rhombomere 2, the trigeminal rhombomere, is narrow and also dis- appears before the mid-ventral line is reached. Rhombomere 1is difficult to delimit. At 14 somites it is only a slight swelling with a very shallow internal sulcus. While not prominent, it can be demonstrated in favorable frontal and sagittal sections. It will be remembered that it is split off from the anterior portion of rhombomere A,, which early shows its double character. In frontal section it is separated from the midbrain anteriorly and rhombomere 2 posteriorly by shallow grooves. Although it is inconspicuous at 14 somites, it rapidly expands until at 18 somites it is easily recognizable, and I have no doubt that if a fixer with greater shrinking properties than picro-aceto-formol were employed it would be more conspicuous at 14 somites. The trigeminal anlage extends along the side of rhombomeres 1 and 2, but has lost its attachment to rhombomere 1 and is closely applied to the neural tube only in the region of the second hindbrain ‘segment.’ The otic rhombomere to which the VII- VIIT anlage is attached shows a typical midventral swelling and is more extensive ventrally than dorsally. Postotic rhombomere 5 is wedge-shaped, more expanded dorsally than ventrally, and is free of nervous attachments. Rhombomeres G and 7 are apparently present, although a transverse plane is not favorable for the demonstration of such slight swell- ings. However, there are broad, shallow internal sulci cor- responding to the rhombomeres indicated in the figure. Rhombomere 6 (if I am correct in so calling it) is more extensive than 7. In later embryos the last rhombomere expands greatly and becomes most extensive of all. The pro- liferation of the neural crest for the IX-X nerves begins immediately caudal to rhombomere 5 and is continuous with the spinal neural crest. The otic pit has a prominent an- terior lip of thickened ectoderm which overlies the VTI-VIII ganglion. Figure 53, a frontal section of another 14-somite embryo, illustrates many of the above statements.


I n the 18-somite rat (fig. 18) the rhombomeres have reached their full development and are similar jn all essentials to those described in the %somite embryo. Rhombomere 1 is now distinct, having expanded considerably in the interval between 14 and 18 somites. The internal sulcus corresponding to this ‘segment’ is broad, but shallow even when best developed. It will be noted that the otic vesicle has not yet been completely constricted off from the overlying ectoderm.


The foregoing descriptions will perhaps serve to bring out the following facts :

  1. As Bartelmez (’23) has pointed out in the human embryo, so also in the rat, the expansion of the neural plate marking the site of the otic rhombomere is from early stages a prominent landmark with characteristic relations to the VII-VIII anlage and the otic placode. The proliferation of neural crest related to this rhombomere is first found in embryos of 8 somites, but the position of the otic rhombomere is indicated by an expansion of the neural plate at least as early as the 3-to-4-somite stage (fig. 22).
  2. A second landmark, appearing simultaneously with the otic rhombomeric expansion (fig. 22) is the preotic sulcus. The latter is situated between the otic rhombomere and the caudal edge of the V anlage and marks the position of rhom- bomere 3. From the beginning, the margin of the neural plate corresponding to the preotic sulcus is free from neural crest, so that the ‘crest-free’ condition of rhombomere 3 would thus seem to be a primary condition in the rat.
  3. At 9 somites, rhombomere A, (rhombomeres 1 and a), involving that portion of the neural folds related to the V anlage, has appeared anterior to rhombomere 3. This makes it possible to fix accurately the boundary between midbrain and hindbrain. The V anlage is related to the entire extent of rhombomere A, in embryos of 9 and 10 somites, but with the subdivision of this segment into rhombomeres 1 and 2 at fourteen somites, we find that the trigeminal anlage has lost its attachment to the first segment and is adherent only to the second rhombomere - its definitive relationship.
  4. The postotic region of the neural plate anterior to the first somite is extremely small at 5 somites and has expanded somewhat at 8 somites, but with subsequent growth furnishes the material for rhombomeres 5, 6, 7. Since the postotic sulcus is not a very definite structure in the rat, it is hard to determine accurately the anterior limit of this territory in young embryos.
  5. The di-mesencephalic boundary is clearly defined at 9 somites. Comparison of the 8-, 9-, and 10-somite embryos shows that the di-mesencephalic boundary of the 9- and 10- somite embryos lies approximately along the line of the rostra1 flexure of the 8-somite embryo.


The question oP the delimitation of the forebrain and mid- brain in the 5-and 8-somite embryos may now be considered. Figures 1and 2 are outline sketches of the dorsal aspects of models of 5- and 8-somite embryos, suggesting the probable subdivisions of the early neural plate in the rat. The positions of the ganglionic anlagen have been indicated by stippling and the extent of the otic placode is shown.


Let us first fix the anterior limit of the hindbrain in the 8-somite embryo. In this specimen rhombomere 3 is represented by the pre-otic sulcns. Its direction is oblique, running slightly cephalad as it passes from tlie lateral margin of the neural plate toward the midline. The region of the prospec- tive rhombomere A,, the anterior margin of which marks the rostral limit of the hindbrain, must lie anterior to it. Now since we have seen that the anlage of the trigeminus ganglion is coextensive with rhombomere A, in 9- and 10- somite embryos, and since the V adage is about equal in extent in 8- and 9-somite embryos, it is probably not far from correct to consider that the anterior limit of the V proliferation of the 8-somite rat marks the approximate anterior limit of the prospective rhomhomere 9,and hence the ceph- alic boundary of the hindbrain. There is a ridge on the ventral surface of the neural plate which marks the course of the preotic sulcus, and just ahead of it there is a groove (cf. fig. 14) about as wide as the V anlage, which runs parallel to the ridge just described, and like it runs forward as it approaches the mid-line. The line which I have drawn to mark the anterior limit of the prospective rhombomere A, of the 8-somite embryo runs along the anterior margin of this groove and parallels the course of the preotic sulcus.


One can be less sure of the anterior limit of the hindbrain in the 5-somite embryo. The preotic sulcus and otic rhombomere are definite and convenient landmarks. The rostral neural crest (V anlage) is much more extensive than at 8 somites, reaching from at point immediately anterior to the pre-otic sulcus to the rostral flexure of the neural plate.


Just how much of the territory anterior to the preotic sulcus represents rhombomere A1 is a question. There is a slight swelling of the neural plate just anterior to the preotic sulcus, as to the exact significance of which I am somewhat in doubt, because such an appearance might easily be caused by the uneven spreading of a few sections. Since it is such_ a slight swelling, I was unable to determine its validity by a study of sagittal sections of other 5-somite embryos. In determining the boundary line between future hindbrain and midbrain territories of the 5-somite embryo, I was influenced primarily by the fact that the distance between the rostral flexure of the neural plate and the preotic sulcus has not measurably increased between 5 and 8 somites, making it probable that the territory of rhombomere A1 would be of approximately the same size, and this conclusion is lent further weight by the fact that the extent of rhombomere A1 (rhombomeres 1 and 2) in the 14-somite embryo is only slightly greater than that of the 9- or 10-somite embryos. It was found that when a distance equal to the length of rhombomere A1 of the 8-somite embryo was measured off on the neural plate of the 5—somite embryo it coincided in size with the slight expansion noted above anterior to the preotic sulcus.


It is more difficult to fix the anterior limit of the midbrain. However, a comparison of figures of models of 8-, 9-, and 10somite embryos (figs. 3, 14, 15, 16) will make it clear that the di-mesencephalic boundary lies along the line of the flexure of the neural plate in the 8-somite embryo. The lateral margin of the neural plate is definitely indented on either side at the boundary between the two regions.


The line marking the di-mesencephalic boundary of the 5-somite embryo begins laterally at the anterior end of the neural-crest proliferation where there is an indentationof the lateral margin of the neural plate and proceeds medially to the mid-line just caudal to the primitive infundibular groove. In the 5-somite embryo there is thus visible in dorsal View anterior to the di-mesencephalic boundary a portion of the forebrain. As a result of the ‘overgrowth’ and expansion of the anterior regions of the neural plate, this material soon comes to form a part of the anterior face of the neural plate and is no longer visible in the dorsal View of the 8-somite embryo. It will be noted that the midbrain has a greater alar than basal extent, which is, of course, to be expected.


Attempts to carry the delimitation of prospective subdivisions of the neural folds into still earlier stages is deemed neither wise nor profitable, since it must be kept in mind that the brain is formed as the result of the differentiation and enormous expansion of a. relatively small portion of the anterior end of the neural plate, so that attempts to define regions before they are actually differentiated may easily lead to errors similar to those into which His (’74) fell in attempting to outline in mosaic fashion upon the early blastoderm all the parts of the future body.


Unfortunately, the mammal is an especially unfavorable form for the determination of the relation of the notochord to the developing brain, since by the time the anterior end of the notochord can be determined, the precocious elevation of the neural folds and the formation of ‘marked bendings of the neural plate and tube disturb the primitive relations of notochord and neural plate. For this reason it was impossible to determine the relation of the floor of the midbrain to the anterior end of the notochord in the rat. However, a study of the growth transformations of the neural plate in the rat seems to indicate that the forebrain and midbrain are differentiated from a very small portion of the anterior end of the neural plate, mainly by marked alar rather than basal expansion, and are therefore primitively much more intimately related than when they are first determinable. As evidence in support of the above statement might be cited the small extent of the medial as compared with the lateral regions in the forebrain and midbrain. Such a conception would aid in understanding the close functional relationships between the optic apparatus and the midbrain, for instance. NEURAL FOLDS AND CBANIAL GANGLIA OF RAT


This conclusion is in a general way in agreement with Work on other forms (cf. Kingsbury, ’24; Wilson and Hill, ’08; Johnston, ’24). Bartelmez (’23) concludes that the hindbrain is the dominant feature of the brain in early stages of development in human embryos. My studies of the rat embryo agree with his in general.


Bartelmez states that the midbrain “constitutes the knee of the cranial flexure. The latter can be recognized as the first abrupt bending of the neural axis which we encounter as We pass back from the rostral end.” His figures 3, 4, and 5 seem to indicate, however, that the midbrain really extends for some distance caudal to the ‘knee’ of the rostral flexure’, but perhaps I misinterpret his use of the term. At any rate, the midbrain does extend caudal to the rostral flexure of the neural plate in the rat. Bartelmez’s determination of the midbrain of a 2- and a 4—somite embryo is somewhat puzzling, since the forebrains of these young embryos are not only relatively, but absolutely, larger than they are in the 9—somite embryo figured by him———a condition hardly to be expected. Since the rostral neural crest is not present in the 4—somite human embryo, it is difiicult to make suggestions, but it would seem that in the 4-somite embryo the midbrain might be extended farther anteriorly, which would make it agree more nearly with the 9-somite embryo.“ It seems to me, also, that Bartelmez’s (’23) determination of the midbrain in Veit’s (’18) 8-somite embryo ignores natural boundaries, extending too far cephalad and not extending as far caudally as the anterior boundary of the first hindbrain rhombomere. It appears to the Writer that in Veit’s embryo the di-mesencephalic boundary clearly lies along the line of the rostral flexure just as it does in the rat and the mesencephalon extends caudally from this point as far as rhombomere A1. The summit of thee rostral flexure (knee?) seems to mark the boundary between di- and mesencephalon on the margin of the neural fold. Veit’s embryo then agrees closely with the rat; the mesencephalon lies caudal to the summit of the rostral flexure and has a greater alar than basal extent. It agrees, furthermore, in that the rostral neural crest (Veit’s Kopfganglienleiste) is related to the midbrain and rhombomere A, just as it is in the rat (cf. fig. 1). However, a comparison of Bartelmez’s figures 3, 4, and 5 (with the suggested slight change in fig. 4) reveals in general a happy agreement.

  • This difficulty has now been eliminated in Bartelmez’s (’25) latest paper. Before reading the above he had already come to the conclusion that the Eternod embryo, which he knew only from tracings, could not be satisfactorily analyzed, since there is so little to define the midbrain rostrally. He had accordingly omitted it from his final monograph.



The differentiation of the rhombomeres occurs so rapidly in the rat that it is diflicult to be certain of the sequence of their formation. Both the otic rhombomere and the preotic sulcus, marking the position of rhombomere 3, seem to have been formed simultaneously at 3 somites (fig. 22), since they are 11ot yet evident at 2 somites. The localized expansion of the otic rhombomeres probably is responsible for the formation of a sulcus anterior and posterior to it, the anterior (preotic sulcus) being further accentuated by the expansion of material lying ahead of it. Rhombomeres A1 and 3 attain definiteness shortly before rhombomere 5, which is still only imperfectly delimited when rhombomere A, has appeared, at 9 somites. At 8 somites the preotic groove marks the site of rhombomere 3, but rhombomere 5 is not recognizable.

The mesencephalic ‘segments’ are believed to be somewhat inconstant structures in the rat. They are clearly demonstrable at 9 to 10 somites, certainly not marked at 12 to 14 somites. They may, perhaps, disappear for a time. Chiarugi ( ’22) found that in embryos of the guinea-pig having a closed neural tube he was unable to detect ‘mesomeres’ until the embryos attained a length of 3.5 mm. They can be seen in the guinea-pig until the embryo reaches 7 mm. in length, after which time they disappear. Like Chiarugi, I regard them as being mechanically produced by the marked alar expansion of the neural folds.

The question of the morphological significance of neuromeres and rhombomeres has been often discussed. Neal’s (’98, ’18) admirable summary of the evidence for and against tl1e phylogenetic and mechanical theories of their origin obviates the need of an extended discussion here. While Neal recognizes clearly the validity of the arguments advanced to support a mechanical explanation of the origin of neuromeres, he nevertheless believes that the neuromeres are produced in early stages by local thickenings of the lateral walls of the neural tubc——a phenomenon not intelligible as the result of a passive bending or shoving of a tube already formed. Neal regards the rhombomeres as associated in their evolution with the branchial region. They are not, he believes, to be explained as caused by the anlagen of nerves, because rhombomere 3 is Well developed, but lacks nervous connections. Celestino da Costa (’23) has but recently suggested that

. on peut considérer la neuromerie comme une différenciation provoquée soit par l’émission de la créte ganglionnaire, soit par le voisinage d’une placode importante et précoce telle que l’auditive.” Rhombomere 3, related to neither neural crest nor placode, again comes to mind as an objection.

Zimmermann (’91, p. 113) noted the fact that blood vessels are dispersed inter-rhombomerically in the hindbrain and Graper (’13) found that such inter-rhombomeric vessels are regularly developed in mammals, but are inconstant in the bird. He doubts their segmental significance, “da man an Embryonen in jede Falte des Gehirns Gefasse einsenken sieht.” A similar arrangement of blood vessels has been observed in the rat, but it is questionable if they have any causal significance in the formation of the rhombomeres.

The Writer believes that in general it is correct to say that the rhombomeres are produced by the great growth of the neural tube in a confined space, but believes also that another factor must be taken into consideration. It is quite evident, I believe, that all parts of the neural plate or tube do not expand uniformly and that localized growth of portions of the neural folds are responsible in part for the production of rhombomeric folds. The hindbrain is predominantly a region of sensory outflow, and associated with this characteristic one might expect a more marked development of the alar regions, accounting for the fact that the rhombomeric grooves do not in general reach the midventral line except in the case of the otic rhombomere where localized expansion is so precocious and marked that the fold affects even the ventral regions. But it must be kept clearly in mind that the recognition of such localized expansion as a part of the developmental pattern of the hindbrain in no way implies that the rhombomeros have any phylogenetic or metameric significance.


Wilson and Hill (’08) find that in Ornithorynchus welldefined rhombomeres are formed while the neural plate is perfectly flat, and they believe that this condition excludes a mechanical explanation of the rhombomeres. To the writer’s mind, however, diife es in developmental tempo may account for differences ii a time of appearance of the rhombomeric folds. The anlagen of the cranial ganglia appear very precociously in the monotremes, and it is quite possible that localized expansions may occur early in the open neural-plate stage without having any phylogenetic or segmental significance.

The Neural Crest

In the rat the cranial neural crest is proliferated from a strip of ectoderm lying at the margin of the neural plate. In young embryos of 3 somites there is as yet no evidence of neural—crest proliferation and no indication of a differentiation of the neural plate from the ectoderm. At about 4 somites, however, the ectoderm at the summit of the neural folds shows signs of a separation of the neural plate from the ectoderm. The separation is not accomplished, in early embryos at least, by the formation of an obvious cleft between the two regions. The early changes occurring here are diificult to describe, but one gains the impression that a rearrangement of cells takes place, resulting finally in the differentiation of as superficial layer of cells covering the rounded lateral margin of the neural plate and continuous with the ectoderm. This layer of ectodermal cells is illustrated in figure 9. At this time there is no actual cleft separating the superficial layer of ectoderm from the neural ectoderm lying medial to it, but a potential separation is clearly evident from the arrangement of the cells. An actual cleft does exist in some embryos which have undergone marked shrinkage. There is a thickening of the ectoderm at the ventro-lateral margin of the neural plate which is doubtless identical with His’s (’79) ‘Zwischenstrang’ and Neumayer’s (’14) ‘Para-neural-leiste, but which, contrary to the opinion of His, takes no part in neural-crest formation.


Soon, a narrow strip of neural ectoderm, forming the lateral margin of the neural plate and lying just medial to the layer of ectoderm above described, begins to thicken and show signs of proliferation. As this material expands, giving rise to a loose mass of neural crest, it causes the elevation of the overlying layer of ectoderm so that a sulcus is formed where thelatter touches the neural plate. This is the condition in the 5-somite embryo (fig. 25). Celestino da Costa’s (’21) figure 1 illustrates identical conditions in the guinea-pig and Veit’s (’18) figures show very similar pictures for man.


In twelve 5-somite embryos studied, the proliferation of neural crest extends from a point just behind the rostral flexure of the neural plate to the anterior border of the preotic sulcus (cf. fig. 1). It is thus being proliferated from the territory of the future midbrain and rhombomere A1. It is evidently identical with the ‘rostral neural crest’ of Bartelmez (’23) and the ‘kraniale Kopfganglienleiste’ of Veit (’18, ’22). The Wave of proliferation passes over this territory extremely rapidly, so that it is difficult to follow its cephalocaudal progress. However, one embryo of 4 to 5 somites shows active proliferation of neural crest for a few sections caudal to the rostral flexure, while sections farther caudally show earlier stages of neuro-ectodermal differentiation, but no actual proliferation of neural crest.


Caudal to the anterior margin of preotic sulcus there is no evidence of neural-crest proliferation in the 5-somite embryo. At 5 somites the rostral neural crest forms a wedge-shaped mass of deeply staining, compacted cells which lies in close contact with the ectoderm (fig. 25). It is fairly easy to distinguish it from the loose mesenchyme, but unless one is careful one is apt to extend it too far ventrally, making it continuous with the more compact mesodermal stalk, lying lateral to the pharynx, which connects the paraxial and lateral mesoderm. Close observation under high power, however, shows that the neural crest is not really continuous with it.


In the 8-somite embryo there are present, in‘ addition to the rostral crest, the anlagen for the VII-VIII and IX-X ganglia. The rostral neural crest (V anlage) (figs. 14 and 26), situated just anterior to the preotic sulcus, has lost its attachment to the midbrain and is now related only to rhombomere A1. It forms a loose mass of deeply staining cells situated immediately beneath the ectoderm. It is easily distinguishable from the now greatly condensed mesoderm of the first visceral arch and from the looser paraxial mesoderm medial to it. There is a shallow neuro-ectodermal sulcus related to it on the right side (left in fig. 26) where the overlying ectoderm abuts upon the neural plate.


The VII-VIII anlage (cf. fig. 14 for its position) is less advanced in development. Sections through its anterior portion (cf. right side, fig. 27) show the general features described for the rostral neural crest of younger embryos. The section passes through the anterior limit of the otic placode against which the neural crest is closely applied.


As one traces the anlage caudally, however, it becomes a wedge-shaped mass, lying just beneath a thin overlying layer of ectoderm above the otic placode (see left side of fig. 27). The caudal part of the anlage is separated from the ectoderm by a distinct cleft, but a mucl1 less well-marked cleft separates it from the neural plate. While there is a slight neuroectodermal sulcus related to the VII-VIII anlage on the right side of the embryo, none is present on the left.


The IX-Y anlage is still less advanced in development than the VILVIII. It consists of a wedge-shaped mass of cells separated from the neural plate by a very indistinct cleft (fig. 28) and covered by a thin layer of ectoderm. There is a very shallow neuro—ectodermal sulcus related to it on each side.


From the foregoing description it is evident that there is a definite cephalo-caudal sequence inthe differentiation of the anlagen of the cranial ganglia.


The relation of the IX-X anlage to the spinal neural crest"’ is interesting and instructive. As the IX.-X anlage is traced caudally it becomes directly continuous with the spinal neural crest at the anterior edge of the first somite. Curiously enough, at the level of the first somite the spinal neural crest is actively proliferating (fig. 29). Its looser, wandering cells seem to be derived from at small, wedge-shaped mass lying at the summit of the neural folds. Tracing caudally into the regions where the neural folds approximate and finally fuse, one can follow the gradual approach of the neural-crest masses of each side until one finds them fused in the region where the neural folds have fused, at the level of the anterior end of the third somite. Figure 30 shows the mass of neural crest derived from the fusion of the crest of the two sides of the body, lying above the closed neural tube between it and the overlying ectoderm.


Veit’s (’22) human embryo of 8 somites seems quite comparable with respect to the relation of the IX-X anlage to the spinal neural crest. Veit and Esch (’22, p. 358) did not recognize the existence of the IX-X anlage in their human embryo, but they say: “ . am ikranialen Ende der Ganglienleiste Zellaustritte sich nicht finden, dass da11n eine langere Zone reichlicher Zellaustritte sich anschliesst noch vor dem ersten und in Hohe des 1. und 2. Somiten. . . . . ” The narrow anterior portion of this ganglionic ridge, anterior to the first somite, where no active proliferation is occurring, is evidently identical with what I have recognized as the IX-X anlage in the rat.


  • 5 The term spinal neural crest is used to include both spino—occipit-al and spinal neural crest. This is also the usage of Bartelmez and Evans (’25), justified for many reasons.


The spinal neural crest evidently undergoes a cephalocaudal ditferentiationf’ apparently independent of the cranial, since the anterior end of the spinal neural crest shows an advanced stage of proliferation in comparison with the IX-X. anlage immediately anterior to it and with which it is directly continuous. The material available does not allowme to determine at exactly what time the spinal neural crest begins to differentiate in the rat.


Judging by published figures, the process of neural-crest formation in the rat seems to be essentially the same as in the cat (Schulte and Tilney, ’15), in the guinea-pig (Chiarugi, ’94), and in man (Veit, ’18, ’22), agreeing also in general plan with the elasmobranch (Beard, ’88), the amphibian (Brachet, ’07), the reptile (Neumayer, ’14, p. 444), and the chick (Goronowitsch, ’93, pl. X, fig. 6).


The description of these early stages has been given in some detail, because they show that the prolonged and heated discussions of His ("(9, ’88), Beard (’87), and others as to whether the neural crest is to be thought of as derived from the neural plate, somatic ectoderm, or from an intermediate zone have really no justification, since the appearances obtained depend entirely upon the precocity of the proliferation of the crest with respect to the dilferentiation and closure of the neural tube. The writer is inclined to regard it as a structure associated primarily with the neural plate. If it differentiates very precociously it may appear as an independent element lying external to the neural plate. VVhen its differentiation is delayed, it may be carried along by the neural folds during their approximation and closure, and so appear to be split off from the neural folds or the roof of the closed neural tube. In the 8-somite rat one sees, in fact, all conditions.

The neuro—ectodermal sulci which have been described above as related to the anlagen of the cranial ganglia and which have been noted in tl1e cat by Schulte and Tilney and in man by Veit, have been thought by Schulte and Tilney ( ’15) to have a phylogenetic and morphological significance. Influenced, no doubt, by the fact that the cranial ganglionic anlagen are hollow in certain cyclostomes (Kupffer, ’95, ’00), they believe that the sulci in question indicate “some small degree of lateral movement of the cells at this point as though to form an evagination. The process is abortive but suggests that in the derivation of the ganglionic crest from the neural tube delamination may have been substituted for evagi~ nation and a solid anlage may have replaced a hollow one as elsewhere in the ontogeny of forms rich in cells. We see in these anlages a series of structures, passing by gradations from the delaminated ganglionic crest through the acoustico—facialis and quintus to the optic vesicle, which is formed by evagination alone.” Veit (’18, p. 312) very justly rejects such an explanation of them. In the rat I have found them to be inconstant, transient structures, not bilaterally symmetrical. They are mechanically produced by the expansion of the neural crest and the consequent elevation of the ectoderm where it adjoins the neural plate.


“Compare Brachet (’21), pp. 200 and 201. P""'


The anlagen of the cranial ganglia in the 8—somite rat are quite distinct from one another, a gap separating the V from the VII-VIII and another the VII—VIII from the IX-X an— lage. This differs somewhat from the condition described by Bartelmez (’23) in an 8—somite human embryoH87 in which the rostral neural-crest proliferation extends as far caudally as the acoustico-facial anlage. In a 14-somite human embryo the rostral division of the neural crest has been separated from the VII-VIII by the disappearance of the neural crest from the preotic rhombomere. In the rat no neural crest develops in the region of the preotic sulcus, and, so far as my material allows me to judge, there is from the beginning a gap between the acoustico-facial and IX-X anlagen. Martin (’90) reports that in the eat there is from the beginning a separation between the V and VII anlagen. He found that VIII and IX are continuous at first, but are later separated.


The occurrence of a blank rhombomere between the rostral and acoustico-facial neural crest has been recognized in reptiles, birds, mammals, fishes, and Amphibia (for citations cf. Johnston, ’05, p. 192, and Neal, ’98, pp. 212, ff.), but an interruption between the acoustico-facial and IX-X anlagen is present only in reptiles (see, however, Neumayer, ’14, Tafel fig. 12), birds, and mammals, being absent in the Amphibia. Johnston (’05) explains these blank rhombomeres as due to the shifting of the otic vesicle during phylogeny combined with a tendency to concentration of the centers of the V, VII, IX, and X nerves.


The question of the anterior limit of the neural crest is one of considerable morphological interest and importance. Theoretically, the neural plate might be thought of as possessing the potentialities of neural—crest formation along its entire perimeter. The neural crest, according to this conception, would be proliferated not only throughout the entire forebrain and completely surround the opening of the anterior neuropore; but such a conception is not in accord with the facts.


In man Bartelmez (’22) describes in an 8—somite embryo H87, lateral thickenings of the neural folds extending from the level of Rathke’s pouch (°.?) into the region of the hind brain. He divides this longitudinal thickening into two portions, separated by a constriction in the region of the midbrain. The anterior portion which extends into the forebrain is broader and converges toward its fellow of the opposite side. The posterior portion is narrower and extends caudally into the region of the hindbrain. Since this primordium gives rise to the optic vesicle, neural crest, and head mesenchyme, Bartelmez has termed it the ‘optic crest primordium.’ From its caudal portion there is a typical neural-crest proliferation in the 8—somite embryo. I

In a 12—somite human embryo H. 197, “The optic crest primordium shows several changes. Its rostral division is wider, the optic sulcus has deepened so that the optic anlage is delimited on all sides from the rest of the area. Mesial and rostral to it cells are migrating out from the primordium as mesectoderm. .H197 is the only embryo We have seen which clearly shows mesectoderm formation in the forebrain, nor has it been observed in any mammal.” 1


Celestino da Costa (’20, ’21, ’23) has recently described a proliferation of neural crest in the forebrain region of guinea—pig embryos of 6 and 7 somites. He describes a proliferation of neural crest from the external borders of the optic fossae at the point of neuro-ectodermal reflection. The cell cords so proliferated insinuate themselves between the optic fossa and the overlying ectoderm. The elements are stellate in shape and have a dark protoplasm resembling the elements of the neural plate. These cell cords, according to Celestino da Costa, can be traced caudally, always arising at the point of neuro-ectodermal reflection, but having their origin clearly from the neural plate. As soon as one arrives in the region of the foregut Where the mesenchyme becomes abundant, the cords in question form a typical neural crest such as has been found in other vertebrates. In guinea—pig embryos of 13 to 16 somites the borders of the anterior neuropore seem to be the point of departure of groups of cells identical in appearance with those of the ganglionic crest which extend around the optic vesicles. “Ce n’est plus une créte bien individualisée morphologiquement, mais des groups cellulaires dont les caracteres cytologiques tranchent sur ceux du mésenchyme.”


The fate of this ‘perioptic’ neural crest could not be determined by Celestino da Costa, but he seems to doubt its participation in mesenchyme formation. It will be noted that Chiarugi (’94), who also studied early guinea—pig embryos, did not recognize a perioptic crest, and that Bartelmez’s optic crest arises medial to the optic sulcus, while Celestino da Costa’s arises lateral to the optic fossa.


A careful study of a large number of rat embryos of 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, and 18 somites has failed to reveal the slightest evidence of neural—crest formation in the region of the forebrain. Neither from the neural plate lateral or medial to the optic fossa nor from tl1e borders of the anterior neuropore was there ever any indication of the proliferation of neural crest. It has been pointed out that the neural crest reaches its greatest anterior extent in embryos of 5 somites when its anterior limit lies at the rostral end of the prospective midbrain, but the extent of its attachment becomes very rapidly reduced, due to factors to be discussed later. Figure 31, which is a transection through the region of the optic vesicles of an 8-somite embryo, shows how ‘clean-cut’ the boundaries of the neural plate are in this region. Sections of 5-, 6-, 7-, and 9-somite embryos give similar pictures in this respect. The mesenchym_e around the expanding optic vesicles becomes compacted as time goes on, and the cells as a natural consequence stain more deeply, due to the loss of their stellate form. They resemble the compact mesenchymal material of the visceral arches far more than the neural-crest cells. There is no evidence of their neural-crest origin. Appearances identical to figure 2 of Celestino da Costa (’22) are found in rat embryos from 12 somites on, but the cells surrounding the optic vesicles, though indeed compact, are purely mesenchymal. Celestine da Costa (’21) himself admits that in 13-somite embryos this perioptic tissue is no longer ‘bien individualisée morphologiquement’ and that the cells ‘tranchent sur ceux du mesenchyme.’


The writer believes that the crista neuralis prosencephali of Bartelmez (’25) should be regarded with extreme caution, as he himself clearly appreciates. As the above description shows, no trace of such crest could be found in at large number of young rat embryos. VVith the exception of this single, possibly abnormal embryo (H197), in which Bartelmez describes the above crest, his studies on the extent of the crest agree with the writer’s.


It seems very probable that the cells surrounding the optic vesicle of a 6-somite guinea-pig embryo which Celestine da Costa (’23, fig. 2) has called perioptic neural crest are merely mesenchymal cells. At least, that is the impression received from the figure.


Observations on the rat are supported by Veit’s (’22) study of an 8—somite human embryo, where the rostral neural crest extends along the midbrain, ending a short distance behind its anterior border. As Bartelmez (’23) points out, Veit was obviously in error in assigning this rostral neural crest to the forebrain. Veit’s identification of the eye anlage as related to this crest (cf. his figure 11) is also erroneous, as I believe he would now agree. Schulte and Tilney’s »( ’15) figures show clearly that there is no proliferation of neural crest in the region of the optic vesicles in the cat. In the 4somite cat, the youngest in which a neural crest has appeared, the crest ends some distance caudal to the anterior end of the neural plate.

Martin’s (’90) statement that the neural crest in the cat begins “dicht hinter der Ausbuchtung der Augenblasen (beim Embryo von 4 mm. Gesammtlange),” is very indefinite, but excludes a forebrain proliferation, since it must be kept in mind that the early optic vesicles seem to involve practically the entire lateral walls of the forebrain not only in the mammal, but in lower forms as well.

The absence of a neural-crest proliferation in the forebrain is also generally the rule in lower forms; Neal’s (’98) figures (pl. 3) show no forebrain neural crest in Squalus, but van Wijhe’s (’82) statements are a little hard to understand. On page 18 he states, “ . . . . Milncs Marshall zeigte darauf, dass sie beim Hiinchenembryo bis in den vordersten Theil des Kopfes vorhanden ist. . . . fig. 27 zeigt dass Letzteres auch fiir die Salachier gilt.” His figure 27 is a frontal section which does not support his contention, since the upper part of the section lies far caudal to the optic vesicles cut in the lower part of the section.


Harrison ( ’01) describes the anterior division of the neural crest in Salmo salar as extending from the caudal half of the eye region to shortly in front of the ear pit, but Boeke (’04) found in Muraena that it arose “dicht hinter die Augenblase.” Landacre (’10) finds that in Ameiurus the neural crest begins five or six sections posterior to the optic stalk.


In Amphibia Brachet (’07) found the V anlage extending as far forward as the ‘repli cerebral transverse’ which gives rise, according to him, to the prechordal region of the brain. Stone’s (’22) and Landacre’s (’21) plottings seem to indicate that the proliferation of neural crest does not extend over the forebrain in the amphibian forms they studied. In the chick, however, Marshall (’78), Goronowitsch (’93), and Belogolowy (’10) describe and figure neural-crest prolifera tion in the region of the optic Vesicle. So far as the writer is aware, no one except Celestine da Costa (’21) has ever described a proliferation of neural crest from the lips of the anterior neuropore.


The absence of neural crest from at least the major portion of the forebrain seems, then, to be a very general condition. It has been commented upon by Schulte and Tilney (’15), who suggest that in the region of the forebrain the neural crest is retained, constituting the dorsal region of the neural plates, forming a zone of material lying ectal to the optic sulcus. According to them, the forebrain must be analyzed not only in terms of alar and basal laminae, but also in terms of this neural-crest element. Such a suggestion is perhaps not without value, but it would perhaps be more correct to say that in the region of the forebrain the neural crest is not formed as such during the process of differential growth and expansion of the forebrain. To say that the ectopic struc~ tures are of ganglionic equivalency cannot be sustained by a study of the structure of the cerebrum, and it is extremely doubtful if one can establish the existence of a definite marginal strip of ganglionic material in the forebrain region. There is some evidence, however, that the olfactory sense cells and the bipolar elements of the retina and pineal eye are the equivalent of neural-crest elements. (See discussion in Kingsbury and Adelmann, ’24, pp. 255-268.)


The Development of the Cranial Ganglia

In studying the development of the cranial ganglia,there are three questions of fundamental importance which must be kept in mind. The first is concerned with the part the neural crest plays in their formation. The two extremes of opinion are represented by Goronowitsch (’93), who believes that the neural crest yields only a ‘nervenfiihrendes Gewebe,’ and those writers who believe the neural crest to be the sole source of ganglion cells. The second question deals with the relation of the ganglion to the overlying ectoderm and the part which proliferations from localized thickenings of it (placodes) play in the formation of the cranial ganglia in the mammal. The third question is one which has been largely neglected. It has to do with what one might term the gross morphology of the ganglion at different periods of development and its explanation in terms of the growth of the embryo.

1. The Trigeminus

As has been previously described, the neural-crest proliferation which gives rise to the trigeminal ganglion begins its development in the 5—somite embryo as a proliferation from the border of the neural plate throughout the territory of the prospective midbrain and rhombomere A1. The mass of neural-crest cells (fig. 25) lies between the paraxial mesoderm and the ectoderm to which it is closely applied. The paraxial mesoderm is more diffuse than the ganglionic mass and is connected with the pericardial mesoderm by a rather compact column of cells passing lateral to the pharynx. This compact mass of cells connecting the paraxial and lateral mesoderm is the material which upon further condensation becomes the core of the mandibular arch. Upon first sight, it might appear that the ganglion is continuous with the arch mesoderm, but a careful examination of the photograph submitted will show that such is not really the case. An arrow marks the ventral limit of the ganglionic mass at about the level of the dorsum of the pharynx; it is clear that the crest has not migrated ventrally into the arch, but ends at the upper limit of the thickened ectoderm of the arch. Celestine da Costa (’23) describes a similar thickening of the branchial investment of a 6—somite guinea-pig embryo (his fig. 3) which he regards as the epibranchial placode of the mandibular arch. The writer, however, is inclined to regard it simply as branchial ectoderm thickened diffusely, as is usually the case. Tl1e ventral limit of the rostral crest makes contact with, but does not fuse with the dorsal extremity of the thickened branchial ectoderm, but there seems to be no reason for considering it placodal in nature. The inner surface of the branchial ectoderm is perfectly ‘clean’; no proliferation from it could be observed.


In an 8-somite embryo (figs. 14, '26) the trigeminal anlage has a much less extensive attachment to the neural plate than is the case at 5 somites. It is now confined to the territory of rhombomere A1. And this change has come about with startling rapidity. A 6-somite embryo shows that the rostral neural crest has already lost its attachment to the neural plate anteriorly, extending forward from the preotic sulcus for a distance of only 96 u. A 7—somite specimen is very similar to the 8-somite embryo in the development of the trigeminal anlage. A sagittal section (fig. 24) (somewhat oblique) of an 8-somite embryo is here submitted to Sl10W the form and relations of the ‘V anlage. The blood vessels lying ahead of it should not be mistaken for neural crest. The anterior margin of the anlage seems to be absolutely ‘clean-cut,’ but its ventral limit cannot be made out in this plane. A crosssection of the V anlage at 8 somites (fig. 26) reveals it as a mass of deeply staining cells extending from the margin of the neural plate to the mesenchymal condensation of the mandibular arch, from which it can easily be distinguished. The marked condensation of the arch mesoderm between 5 and 8 somites has occurred independent of any neural—crest contribution, I believe, since I was unable to trace any migration of the crest into the arch in 6- and 7-somite embryos. The ventral limit of the ganglion lies at the upper end of the compacted arch mesoderm. The ectoderm is intact, thickened somewhat over the mesenchymal condensation of the arch, but thin where it comes into contact with the ganglion. Although contact between crest cells and the overlying ectoderm is seen in places, there is clearly no proliferation of ganglionic or mesenchymal elements by the ectoderm.


The rostral neural crest not only loses its attachment to the midbrain between the 5-and—6-somite stage, but, as far as one is able to judge, it disappears without a trace in that region. Obviously, a change occurring with such rapidity is most difficult to analyze or to follow without a large number of embryos covering the transition period. The possibilities as to the fate of the anterior portion of the rostral neural crest are as follows: 1) The neural crest degenerates, 2) The anterior portion of the rostral neural crest loses its attachment to the midbrain and becomes so diffuse that for a time it is unrecognizable as such, but retaining its identity and specificity, it later recondenses or ‘nebulizes’ to form the ophthalmic branch of the trigeminus. This was the interpretation of Belogolowy (’10), who studied the subject in the chick. 3) The anterior neural crestbecomes diffuse; the cells become indistinguishable from typical mesenchymal elements and, losing their identity and specificity, share the fate of the mesenchymal cells with which they have become indiscriminately intermingled. 4) The anterior portion of the rostral neural crest loses its attachment to the edge of the neural plate as a result of the expansion of the neural plate away from it so that it becomes ‘left behind’ as the strip of neural crest forming the V anlage shown in figure 24.


The first possibility, namely, that the anterior portion of the neural crest degenerates, may, I think, be discarded as untrue. The examination of several embryos covering the period in question reveals not the slightest evidence of degeneration of the neural crest in the region of the midbrain. Neither could I find any support for the second possibility.


Up to and including the 14—somite embryo there is absolutely no trace of an ophthalmic branch of the trigeminus ganglion. Belogolowy (’10) observed that in the chick the ophthalmicus profundus first arose as an intensely staining nebula extending throughout the region of the mesencephalon, but he says (p. 169) that he was unable to follow the process of its formation in detail. However, the fact that the ophthalmicus profundus of the chick arises coincidently with the disappearance of the crest in this region seemed to indicate to him a connection between the two processes. No such gradual ‘nebulization’ could be observed in the rat. There is no trace of an ophthalmic ramus of the trigeminus until long after the anterior portion of the rostral neural crest has disappeared. Figure 24 shows clearly that no forward extension of the V anlage can be recognized at 8 somites, and the model of the 14-somite embryo (fig. 17) shows that the ophthalmic branch is not yet formed. The problem of the origin of the ophthalmicus will be returned to later.


The third possibility must be more seriously entertained. It is quite possible that there is an extremely rapid transformation of these anterior crest cells into mesenchyme, so rapid as to make impossible its detection without a large number of embryos at the age of transition. I was unable to determine from the study of severalembryos of 6 to 9 somites in age whether such a transformation of neural crest i11to mesenchyme actually occurs. The possibility must be admitted. Celestino da Costa (’21) also reserves judgment upon this point.


In the writer’s opinion, however, the fourth-mentioned possibility expresses the real condition, although the actual growth transformations are somewhat diflicult to follow. A comparison of figures 11 and 12 shows that between 5 and 6 somites there has been considerable expansion and elevation of the head region, so that it is easily conceivable that during this time the margin of the prospective midbrain area of the neural plate has expanded away from the neural crest, leaving it behind as a band of crest material extending from the anterior margin of the prospective hindbrain ventrally toward the first arch. According to this conception, the anterior edge of the V anlage of the 8-somite embryo (fig. 24) would be thought of as once having been attached to the neural plate. Neal’s (’98) figures 7 to 17 indicate a comparable growth transformation in the shark. In the shark the rostral neural crest retains its attachment to the midbrain until a relatively much later stage than in the rat. Growth and expansion of the midbrain soon result in the loss of this attachment so that neural crest once attached along the midbrain comes to lie along the antero~dorsal margin of the V anlage. The process is complicated in the shark by simultaneous forward growth of the ophthalmicus profundus. In the rat the forward growth effecting the forward extension of the ophthalmic ramus is not coincident with the loss of the attachment of the rostral crest to the neural plate, but occurs somewhat later, as I shall have occasion to show subsequently.


Neumayer’s (’14) Tafel figure 12 seems to the writer to indicate a similar state of affairs in the reptile.


The gross form of the V anlage remains much the same from 9 to 14 somites. During this period it remains a roughly oblong mass attached to rhombomere A1. It lies slightly ahead of the hyomandibular cleft and extends ventrally to the mesenchymal condensation of the mandibular arch (see models of 8-, 9-, 10-, 14-somite embryos). Due to the flexure of the neural tube, it willbe appreciated from a study of the models that transverse sections of the mandibular arch passing through the anterior margin of the ganglion would lie immediately behind the eye. Transections passing through the optic vesicle and the trigeminus anlage might, unless a model were made, suggest the existence of an ophthalmic ramus, when really none is present.

A cross—section of an 11-somite embryo through the trigeminal anlage is given in figure 32. The ganglion is wedgeshaped with the apex directed dorsally. It extends from the margin of the neural plate ventrally where it comes in contact with the condensed mesenchyme of the mandibular arch. In this specimen it is quite possible to determine the boundary between the ganglion and the arch mesoderm. The mesenchymal cells are much more compact. Medially, there is a distinct notch at the boundary between the two, due to the fact that the long axis of the ganglion is oblique to the arch, and by tracing the series cephalad and caudad of the ganglion one finds that the compact arch mesenchyme extends dorsally , to the level indicated by the notch. The integument is, as is usually the case, much thickened over the branchial arch. The epithelium overlying the ganglion is composed of a single layer of cuboidal cells. It becomes much flatter as it passes dorsally. Although the ganglion is in contact with the overlying epithelium, a careful study failed to show any evidence of proliferation on the part of the epithelium. The ganglionic cells are more intimately apposed to the epithelium ventrally than dorsally. The medial boundary of the ganglion is distinct and there seems to be no intermingling of the ganglionic elements with the mesenchyme lying medial to it. The section does not pass through the site of attachment of the V anlage to the neural plate. Conditions are fundamentally the same in 10-somite embryos.

The trigeminal anlage shows little change between 12 and 14 somites. During this time the neural tube closes in the region of rhombomere A1, resulting in a shifting of the contact between ganglion and neural tube. In earlier embryos the ganglion tapers somewhat dorsally and its upper extremity is wedged between the lateral margin of the closing neural tube and the overlying ectoderm. As the neural tube closes the ganglionic anlage maintains contact with its lateral surface, so that at 14 somites, after closure of the neural tube has been effected, the trigeminal anlage has an extensive contact medially with the surface of the hindbrain (fig. 33). Its wedge-shaped dorsal extremity does not quite reach the middorsal line.

In the 14-somite embryo (fig. 33) the lower limit of the ganglionic mass is still marked by a notch medially at the boundary between the compact arch mesoderm and the ganglion. It is separated from the ectoderm by a small amount of looser tissue, presumablymesenchymal in origin.


It will be observed that the ectoderm related to the ganglion has not thickened appreciably, but that the ectoderm gradually decreases in height as it is followed dorsally. The ganglion makes no contact with it except possibly at its ventral ex tremity and there are no indications of its proliferative activity. The position of the primitive head vein medial to the ganglion should be noted, no venous channel is formed lateral to the trigeminus as in the case of the VII-VIII and IX-X anlagen.


Figure 34 is a transection through the body of the trigeminal ganglion of an 18-somite rat embryo. The expansion of the neural tube has resulted in a ventral shifting of the area of contact between ganglion and hindbrain. The area of contact extends dorsoventrally for a little less than the middle third of the lateral surface of the neural tube, its ventral limit lying just above the groove between rhombomeres 2 and 3. The -ganglionic mass forms an obtuse angle with the mesoderm of the arch. It is separated from the ectoderm dorsally by a small amount of mesenchyme, but it comes into contact with the ectoderm ventrally. In later stages (fig. 37), as the ganglionic mass expands, the area of contact between ganglion and ectoderm becomes more extensive. In the 18-somite embryo the ectoderm is not specially thickened, being intermediate in height between the thickened integument of the gill arch and the thinner epithelium dorsally. It is composed of a single layer of cuboidal cells. A careful study of the ectoderm in contact with the ganglion revealed no mitoses in this particular embryo and no cells were seen in process of being pinched off from the ectoderm. It is apparently simply a case of intimate contact or adhesion of the compacted ganglionic cells and the adjacent integument.


The ophthalmic ramus is a direct prolongation of the main ganglionic mass. Caudally it lies just above and in contact with the compact mesoderm of the mandibular arch, and as it is traced forward it bears a similar relation to the mesenchymal condensation in the maxillary region (fig. 35). Externally, the course of the ophthalmicus is marked by a distinct ridge beneath which it lies. The ophthalmic ramus recedes from the ectoderm as it is traced forward and no contacts between it and the adjacent ectoderm could be noted. Figure 35 shows that the thickened ectoderm over the maxillary region is prolonged dorsally i’,,;,,a very short distance, but the ectoderm is not appreciablgmthickencd along the course of the ophthalmicus. The ram» fiophthalmicus is entirely cellular in nature, more or less_;;‘;,,§tened from side to side and becomes very diffuse as it nears the eye.


A more extensive area of intimate contact between the main mass of the trig'ef_(o;_3.¢1al ganglion and the ectoderm above the mandibular arch Wit-5;..?vJOt€d in embryos of 19, 20, 21 (fig. 37), and 23 to 24 somites. In all of these stages the characteristics of the overlying epithelium are, first, that it consists of a single layer of cuboidal cells, transitional in height between the integument of the arch and the dorsum; it appears to be a dorsal extension of the branchial investment; secondly, in its antero-posterior extent it coincides with the main ganglionic mass; finally, there seems to be no certain evidence of proliferative activity. F Very few mitotic figures were found in this area in the embryos examined and when the plane of the spindle could be determined it was always, so far as observed, parallel with the surface and therefore not in a position suggesting the proliferation of cells into the subjacent tissue, but providing merely for the expansion of the ectoderm. Furthermore, I was never able to find at cell being pinched off from the ectoderm. I believe it is simply a condition of close contact between an epithelium having no welldefined basement membrane and the ganglionic mass.


In the interpretation of appearances, the effect of plane of section must always be kept in mind. Figure 36 is an oblique frontal section of an 18-somite embryo which illustrates an especially deceiving condition. Due to the obliquity of the section, the thickness of the integument over the entire surface of the head is exaggerated. Note, for example, the exaggerated thickness of epibranchial placode I. When a closely compacted mass comes into contact witl1 such an obliquely cut surface, as in the case of the V ganglion illustrated here, it becomes more dilficult than ever to decide whether proliferation is occurring. I could find no direct evidence of it and believe that it is simply a case of contact between the ganglion .1 the epidermis. On the other side of the same embryo 9" l in other 18—somite embryos cut transversely it seems cle: that no proliferation is occurring. The relation of the op ralmicus profundus to the ectoderm is never as intimate as that of the main ganglionic mass; a layer of mesodermal tissue always intervenes (cf. fig. 40).


At 26 to 27 somites (figs. 38 and '7 the ganglionic mass is well circumscribed and no longer in contact with the ectoderm, separated from it by a slight interval. The integument over the ganglion is thi11, showing no thickening. The ophthalmic ramus is more compact and is well separated from the ectoderm. I could find no special thickening of the ectoderm along the course of the ophtlialmic ramus; the epithelium along its course is intermediate in height between that clothing the maxillary region and the flattened epithelium of the dorsum.


So far as the author is aware, only four investigators definitely commit themselves to the placodal contribution to the trigeminus in mammals. Chiarugi ( ’94) described the intimate relation of the trigeminus to the ectoderm in the guineapig and noticed the occurrence of ectodermal papillae from which he believed cells were detached to be added to the ganglion. Giglio-Tos (’02), Volker (’22), and Bartelmez (’24) have also described ectodermal contributions to the trigeminus in man and the rat. Celestino da Costa (’23) does not definitely commit himself. He states that the relation of the gasserian ganglion to the ectoderm in the guinea-pig is intimate enough “a permettre une contribution placodiale a la formation du ganglion de Gasser,” but decides that “cette collaboration doit étre peu importante et bien plus réduite . . . ” Froriep (’85), VVeigner (’01), and Neumayer (’06) find no placodes related to the V ganglion in the mammalian forms studied by them. Davis (’23) thus describes the ganglion of the V nerve in a human embryo of 20 somites, “Laterally the mass is continuous with the condensed mesenchyme of the first branchial arch, the two tissues being c.lewrly differentiatcd ftromeach other. (The italics are mine.) The ectoderm covering this arch exhibits a distinct general thickening, but anything approaching a true placodal structure is wanting.”

It has been mentioned that the growth of the ophthalmicus occurs in the rat between 14 and 18 somites. The question of its origin may now be considered. In many forms the ophthalmicus profundus has been described as arising in part or entirely by placodal proliferation. Miss Platt (’96) believed both the gasserian ganglion and the ramus ophthalmicus profundus of Necturus to be derived from cells migrating from the primitive supraorbital ridge. Landacre (’12, p. 10) found a ‘pronounced contact’ of the ophthalmicus profundus with the epidermis in Lepidosteus, but did not determine its placodal nature. Stone (’22) believes that in Amblystoma punctatum the ophthalmicus profundus is composed entirely from cells proliferated from a supraorbital placode. Wlien the supraorbital placode is experimentally removed, the ophthalmicus fails to develop. In Chrysemys Brachet (’14) found that the maxillo-mandibular ganglion is entirely of neural—crest origin, but that the ophthalmic placode is transformed entirely and directly into the ophthalmic ganglion

Belogolowy (’10) concludes (p. 169) that the formation of the ophthalmicus profundus of the chick must be ascribed exclusively to the condensation of ganglionic-crest elements. He could recognize no supplementary factors of any importance. However, he did find on each side, in relation to the ophthalmicus, one or two ectodermal papillae which were very different from the epibranchial placodes, but which resembled in their structure ectodermal papillae which he found ‘iiberall gleichm‘.ei.ssig verstreuten.’ The papillae associated with the ophthalmicus were two to three times as large as those found elsewhere, and he found that they constantly invaded the layer of mesenchyme lying between the ectoderm and the ganglion, and at their summit proliferated cells. These papillae are seen for only a short time and soon disappear without trace. He concludes that, since they yield so small an amount of material, their importance in ganglion formation must be minimal. Chiarugi (’94) and Griglio-Tos (’02) describe an ectodermal contribution to the ophthalmicus in the guinea-pig and man. Chiarugi noted the presence of ectodermal papillae from which he believed cells were detached to be added to the ganglion. Celestine da Costa (’23, p. 518) found in the guinea-pig that “Avant ou aprés la courte période entre 16 et 18 myotomes l’épibla’ste qui recouvre la créte du trijumeau et son long prolongement antérieur qui va jusqu’auX vésicules optiques réagit a ce voisinage, en 's’épaississant d’une facon diffuse. La possibilité n’est pas exclue d’une contribution de ce genre, telle que Chiarugi l’avait supposée.” It is my impression, however, that Celestino da Costa has misinterpreted the sections presented as evidence (see his figs. 7, 10). They do not show the ophthalmic ramus of the trigeminus, but pass through the caudal wall of the optic vesicle and the main trigeminal mass (cf. my 14-somite model). The right side of his figure 10 shows the thickening of the branchial integument with subjacent mesenchyme, not a dorsolateral placode and the ophthalmic ramus.

7 In this connection, I desire to call attention to a recent paper by de Beer ( ’2-1).



In my study of the rat several places were noted where the ectoderm showed slight papillary thickenings to the inner surfaces of which cells adhered in a manner to suggest a possible proliferation. Then again, where the ganglion approaches the ectoderm closely, ganglion cells were found adherent to the overlying ectoderm which was not especially thickened at the place of contact. In places where the ganglionic mass was separated from the ectoderm some of the intervening mesenchymal cells might appear to adhere to the overlying ectoderm. These appearances, however, are not confined to the ectoderm overlying the trigeminal ganglion or to the i11tegument of the head, but may be observed all over the body, as botl1 Belogolowy (’10) and Veit (’22) have already observed. None of Veit’s illustrations show a cell actually being pinched off from the ectoderm, but merely fully formed cells adhering to it. I interpret these appearances as cases of adhesion. VV. H. Lewis (’22) has commented upon the marked adhesive properties of mesenchymal cells, and of cells in tissue cultures, such cells frequently becoming so adherent to glass that they may even be centrifugalized without dislodgment. He also points out the important role which this adhesive property plays in early stages of development. Cells in culture were found to migrate out from the explant along certain solid objects (coverglasses, fibrin filaments, etc.) for which they seem to display a special adhesive tendency. I think that this adhesive property of embryonic cells is responsible for the fact that the V and VII (and the early IX-X) ganglionic proliferations grow ventrally in such close approximation to the overlying ectoderrn which possibly exercises a directive influence, and is responsible also for the fact that the vagus (cf. figs. 83, 88) hugs so closely the outer wall of the vena capitis lateralis as it grows ventrally. It explains why it is so difficult to draw a boundary line between branchial ectoderm and brancl1ial mesoderm and accounts for numerous descriptions of mesectoderm formation by the branchial integument. Brachet (’21, p. 385) has noted this peculiar and intimate relationship between the ganglionic anlagen and the deep face of the ectoderm; “il semble que ce dernier (the epiblast) exerce, sur ses couche superficielles, un tactisme particulier,” he says. If the proliferation of mesenchymal or ganglionic elements is so general a phenomenon, one would expect to find abundant examples of cells being actually pinched off from the ectoderm into the subjacent tissue.

The only constant ectodermal thickening related to the eye is illustrated in figure 41. It lies just caudal to and above the eye and the underlying mesenchymal condensation comes intoclose contact with it. Its under surface is intact. This is, however, not a placode. It is simply a thickening of the ectoderm above the cleft separating the mandibular arch from the anterior part of the head and it is continuous with the thickened integument of the mandibular arch and a less marked diffuse thickening of the ectoderm extending forward over the eye. It can be observed for a long period, but never comes into contact with the ophthalmic ramus of V. This thickening was, I believe, noticed by Froriep (’85, p. 44) in older embryos of the calf, but he says “ . . . . dieselbe steht aber in keiner Beziehung zum Trigeminus. Sie ist vielleicht ein Rudiment der Seitenorgananlage, welche bei Selachierembryonen an den vorderen Dorsalasten des Facialis sich bildet.” A placodal origin for the ramus ophthalmicus profundus trigemini in the mammal is, I believe, untenable.


The possibility of a placodal origin of the ophthalmicus being excluded, how, then, does the ramus ophthalmicus of the V ganglion arise in the rat between the ages of 14 and 18 somites? The answer lies in a study of the growth transformations of the region during that period. Up to the 14somite stage, growth has proceeded but slowly, since the embryo requires some time to becomeproperly established in the uterus. At about this time, however, environmental conditions more favorable to rapid growth have apparently been established and the embryo consequently ‘speeds up’ in growth, the increase in bulk between 14 and 18 somites being almost incredible. Coincidently with this enormous growth, significant shiftings of parts have occurred which account, I believe, for the growth of the ophthalmic ramus of the V ganglion.


Externally, one notes the following changes. In embryos of 14 somites and younger, the mandibular arch lies imme diately caudal to that part of the head composed mainly of

forebrain. Since in early embryos the optic vesicle is relatively larger than subsequently, constituting almost the entire lateral wall of the forebrain, the core of the mandibular arch lies just caudal to the eye. Consequently, no maxillary region can be identified. The above facts are most strikingly illustrated in embryos of 8 to 14 somites (figs. 14, 4, 17). The external aspect of the 10—somite embryo shows that between 8 and 10 somites some forward growth of dorsally lying parts has occurred, and this is reflected in the oblique direction of the long axis of the mandibular arch, the dorsal extremity of which lies considerably cranial to its Ventral end, which is more or less fixed. A forward extension ofthe mesenchymal condensation of the first arch actually rests against the postero-superior border of the optic vesicle. The position of the trigeminus with respect to the optic Vesicle has already been commented upon.



Figs. 4: t.o 6 A series of reconstructions from models of 14-, 18-, and 26-somite rats, respectively (X 50), to illustrate the formation of the ophthalmic ramus of the trigeminus, coincident with the growth shiftings involved in the establishment of the maxillary region. The figures also illustrate the relation of the ganglia to the branchial arches. See text, pages 65-69, for discussion. In figure 4 the extent of the thickened ectoderm surrounding the otic pit is indicated by a broken line. In figure 5 the otic vesicle has not yet been completely closed off from the ectoderm (fig. 18).

A.br., branchial arch; D.c.c., dorsal cornmunicating cord of ganglionic material between the IX and X ganglia; G., ganglion; 013.19., optic vesicle; 0t.pl., otic placode; Ot.'u., otic vesicle; R.op.V., ophthalmic ramus of the trigeminus; SJ, the first somite; V.c.c., ventral communicating cord of ganglionic material between the IX and X ganglia.



At 12 and 14 somites (figs. 4 and 17) the eye is still in intimate contact with the mandibular-arch territory. The mesenchymal condensation above and behind the eye shown i11 figure 41 is a continuation forward of the mesenchymal condensation of the mandibular arch. The long axis of the arch is still oblique. During the interval between 14 and 18 somites at significant change has occurred. The maxillary region has been established by the forward growth of material at thedorsal end of the mandibular arch so that the optic vesicle is now some distance removed from the mandibular arch proper, the maxillary region intervening. The mai11 mass of the trigeminal ganglion lies over the caudal region of the first arch and the ophthalmic ramus extends forward from it, traversing the maxillary region to reach the optic vesicle (cf. figs. 4 to 6).

An analysis of the growth shiftings of internal structures will perhaps allow us to understand more fully what has occurred. Between 10 and 14 somites considerable expansion has occurred in the ectopic region of the telencephalon, and the diencephalon has enlarged a11d become more clearly separable from it. The midbrain has elongated somewhat and there has been some further bending of the neural tube. So far as the relation of the optic vesicle to the trigeminal ganglion is concerned, however, the growth shiftings occur ring during this period have been, in the main, compensatory, so that no significant change has occurred in the relation of the two structures, the trigeminal ganglionic mass lying in the transverse plane immediately dorsal to the caudal half of the optic vesicle.

Between 14 and 18 somites, the cranial neural tube has grown enormously. The prospective cerebral region has expanded greatly. The optic vesicle has grown slightly, but its relative size with respect to the telencephalon has been much reduced. The diencephalon, too, has expanded considerably, but by far the most marked expansion has occurred in the region of the midbrain which has practically doubled i11 length from the di-mesencephalic boundary to the anterior margin of the cerebellar rhombomere. The le11gthening and accentuation of the isthmus is especially noticeable. The cerebellar rhombomere has also lengthened. Now, while in the 14-somite embryo the compensatory flexing of the neural tube is suflicient to retain the earlier relation of optic vesicle and trigeminal anlage, in the period between 14: and 18 somites flexure of the neural tube has not kept pace with its rapid expansion, and as a result the eye suffers considerable forward displacement with respect to the main mass of the trigeminus and the dorsal portion of the mandibular arch. The lengthening of this interval is accompanied by the shifting forward of the material dorsal to the mandibular arch to establish the maxillary region.


The important fact from the standpoint of this discussion is the fact that the trigeminal anlage has taken part in the growth shiftings above analyzed and as the optic vesicle has been displaced forward the trigeminal anlage has kept pace with that shifting, the ophthalmic ramus representing material which has grown forward from the main ganglionic mass simultaneously with the elongation of the maxillary region. A study of 16- and 17-somite embryos is confirmatory. The growth of the ophthalmic ramus forward keeps pace with the shifting of the optic vesicle, which occurs with startling suddenness between 14 and 18 somites. There are no placodes to account for its formation and there is no evidence from a study of the rat to support Be1ogolowy’s (’10) supposition that it arises by the condensation of crest elements formed along the extent of the midbrain. Neal (’98, pl. 3) illustrates a comparable forward growth of the ophthalmicus profundus from the main mass of the trigeminus ganglion in the shark, and I believe that both Landacre’s (’21, figs. 7 to 10) and Stone’s (’22, figs. 1 to 8) plottings of the neural crest in Amphibia may be similarly interpreted.

N 0 great change in the gross morphology of the ganglionic anlage of the V nerve has occurred in the 26—somite embryo (cf. figs. 6 and 19). The cephalic fiexure is more acute than in the 18-somite embryo and the optic vesicle has shifted posteriorly for a short distance so that its caudal margin lies opposite the posterior limit of rhombomere 2 in a plane transverse to the long axis of the neural tube. In the 18somite embryo the caudal margin of the optic vesicle lies relatively much farther forward, in the plane of the anterior border of the first rhombomere. The ophthalmic ramus is still entirely cellular in nature, but much less diffuse than at 18 somites, extending now for some distance over the optic vesicle, but too tenuous to model. There is as yet no evidence of a maxillary ramus and only a short ventro-caudal projection represents the mandibular branch. Tho ganglion is now separated somewhat from the condensed mesenchyme of the arch. In earlier embryos (figs. 34, 37) the ventral extremity of the V ganglion makes a verylbroad contact with the condensed mesenchyme of the mandibular arch, and the two tissues are sometimes difiicult to distinguish. However, after careful study, it is not impossible to differentiate the ganglionic mass from the mesoderm, the two tissues differing somewhat in staining properties. Davis (’23) found in a 20-somite human embryo “the mass is continuous with the condensed mesenchyme of the first branchial arch, the two tissues being clearly differentiated from each other.” I find that there is a distinct medial notch constantly present at the boundary between ganglion and arch mesenchyme in embryos of 10 to 24 somites, and when the ganglion separates from the mesenchyme at 26 to 27 somites, the line of separation lies at the level of the notch in younger embryos (cf. figs. 37, 38). Another feature which serves to indicate the boundary between ganglion and mesenchyme is the presence of considerable numbers of degenerating cells at the boundary between the two, commencing with embryos about 17 somites in age. All stages of degeneration may be observed, and there are in addition some cells having irregular nuclei and containing a large number of coarse acid-staining granules. These cells apparently act as phagocytes. Some degenerating cells and some phagocytes may be found among the ganglion cells, but they occur predominantly in the arch mesenchyme, becoming very abundant in later stages. Such cells were found also in relation to the developing VII, IX, and X nerves, a few scattered among the ganglion cells, but mostly confined to the mesenchyme. They occur first in the mandibular, next in the hyoid, and finally in the mesenchyme of the third and fourth arches. Davis (’23) has described a large number of small polymorphonuclear cells, many of which exhibit vacuolization, indicating “possibly a degeneration of neuroblasts or an invasion of the ganglia by macrophages.” Since I have found these cells to be constantly present in the rat in connection with the developing ganglia, I can hardly regard them as abnormal, but I am inclined to consider them as cells degenerating as a result of growth pressure or tension at the junction of the two tissues. Similar cells have been found in other situations of the body. A more complete discussion of them is reserved for a later communication.


At 29 somites the ophthalmic ramus is becoming more fibrous distally. The mandibular branch is short and a few motor fibers accompany it. The maxillary ramus is represented by a few scattered fiber bundles projecting into the maxillary region.


In a 34-somite embryo the three branches of the ganglion are well developed. The ophthalmicus is very cellular proximally, but becomes more fibrous distally. The mandibular division can be traced into the mandibular arch for a short distance. It also consists of a cellular extension from the main ganglionic mass, sending a few fibers into the arch accompanied by the fibers of the motor root. The maxillary division is most diffuse, consisting of a few scattered fiber bundles springing directly from the ganglion into the mesen— chyme of the maxillary region. Since the further history of the trigeminal ganglion is so well known, it is omitted here.

Returning now to the study of the main ganglionic mass of the trigeminus of the 18-somite embryo, we find at that time a spur-like projection from the antero-ventral aspect of the base of the ganglionic mass. The cells resemble those of the ganglion in their histological appearance and are without doubt derived from the main ganglionic mass, growing ventrally in close contact with the wall of the neural tube. In later stages there is evidenced the separation of a medial mass of ganglionic cells, of which the spur is a continuation, from at more lateral group which makes contact with the neural tube just caudal to the medial mass so separated. It is well shown in a 21—somite embryo (fig. 37).


The spur is well defined at 27 somites (fig. 38); its cells are still very similar in appearance to those of the main ganglionic mass, but there is a distinct difference in their arrangement. At 31 somites (fig. 42) there is a great difference in the appearance of the cells of the spur and those of the main ganglionic mass. Those of the latter are spindleshaped with cloudy, deeply staining cytoplasm; the nucleus is oval and the cells are compactly arranged with their long axes parallel with the long axis of the ganglionic mass. The cells of the spur, on the other hand, are irregular in shape; the nuclei are round or slightly oval and the arrangement of the cells is irregular. At 34 somites (fig. 43) the mass is invaded by the outgrowth of motor fibers from the neural tube, and finally the mass is obliterated when the motor root is well established. Even at 31 somites there are delicate fibrils to be detected extending from the neural tube into the spur. A similar spur with an identical fate has been observed in the case of the acoustico-facial anlage. It will be described, and a discussion of its significance will be given later.


Other than the slight temporary cleavage of the ganglion noted above, there is no evidence, in the rat, supporting the view that the trigeminal ganglion of the mammal is a composite structure. Giglio-Tos (’02) concludes from the study of a single 15-somite human embryo that there is a complete correspondence between the development of the trigeminus in man and the lamprey. He identifies nine parts in the trigeminal ganglion of man--three primitive neural proganglia (ophthalmic, maxillary, a11d mandibular), three mesocephalic (epibranchial) proganglia, and three branchial pronerves. Giglio-Tos’ figures show clearly that he has confused mcsenchymal condensations with ganglionic anlagen.


A careful search was made for evidence of the existence of a separate profundus division of the trigeminus, but none was found. Schulte and Tilney have described a profundus anlage in the cat and have figured it in 10-, 12-, 14-, 16-, 17-, 19-, and 21-somite embryos. I have become convinced that the structure consistently termed by them the profundus ganglion in the figures given on plates 32 to 38 is in reality the quintal anlage and that the structure termed by them the quintal anlage is really the facial. What they have called the facial is really the IX-X anlage continuous with the spinal neural crest. In a 12-somite cat which I have examined the trigeminal ganglion is quite small, as Schulte and Tilney’s model of the so-called profundus anlage shows, and this may be the reason for the confusion. A study of their figures (pl. 34, for instance) shows the so—called quintal anlage attached to the otic rhombomere which may be identified by its ventral swelling. Ahead of it lies rhombomere 3, free of ganglionic attachment, and then their so-called profundus anlage attached to the caudal portion of rhombomere A1. Caudal to the otic rhombomere lies rhombomere V, and still farther caudally is situated their so-called acoustico-facial continuous with the spinal crest. This figure alone quite plainly indicates that their profundus equals the trigeminus, their quintal anlage is really the facial, and that their acoustico-facial is the IX-X anlage with its characteristic relation to the spinal crest. Schulte and Tilney have apparently ignored the rostral neural crest of younger embryos. The cat is apparently not alone in having a quintal anlage of smaller size than the acoustico-facial, since Davis (’23, p. 19) mentions the same condition in man. Celestine da Costa (’23) and I have independently come to the same conclusion regarding Schulte and Tilney’s profundus anlage. 74 HOWARD B. ADELMANN

2. The Acoustico-Facial Ganglion

The position and relations of the acoustico-facial anlage of the 8-somite embryo have been briefly described above and are illustrated in figure 27. It is shown on both sides of the figure. The plane is very slightly oblique, so that the section passes through the anterior end of the ganglion on the right and through the caudal portion of the left ganglion. The proliferation is more advanced in the anterior than in the caudal part of the ganglion, but only two or three sections on each side show an active ventral migration of cells. The anlage extends through thirteen 10 u sections on the right and twelve on the left. The relations of the mesoderm should be observed. The loose mesenchymal material forming the paraxial strand is connected with the pericardial mesoderm by an intermediate column of cells which passes lateral to the pharynx and which in subsequent stages becomes greatly condensed to form the mesodermal core of the second gill arch when that structure is delimited by the development of the second pouch. One has no difiiculty in defining the boundaries of the anlage even in sections where proliferation is most advanced.


The otic placode is extensive at 8 somites, and it is impossible to determine its exact boundaries. It is about 260 u in length on the right and somewhat less extensive on the left side. It is probable that the slight invagination of the placode illustrated in figure 27 is an artifact. The acoustico—facial anlage rests closely against the medial surface of the cephalic extremity of the placode; there is no intervening mesenchyme.


A very early stage of the acoustico-facial anlage can be recognized in a 7-somite embryo. Its position with respect to the neuralplate is as described. for the 8-somite embryo.


Figure 44 is a section through the acoustico-facial anlage of a 9-somite embryo. The section is somewhat horizontal in direction, the lower portions of the section being caudal to the upper. The left side of the section is anterior to the right.


The ganglion has grown considerably in size since the 8-somite stage. It consists of a column of densely packed rounded or oval cells rich in cytoplasm, which adheres closely to the inner surface of the otic placode. The latter, however, takes no part in the formation of the ganglion;absolutely no indication of proliferation could be observed. The ganglion can be traced ventrally to the dorsal level of the first gill pouch where it is in intimate contact with the mesodermal material of the second arch, but the two tissues can be differentiated. In transverse sectionsithe ganglionic mass is fusiform in shape; in frontal sections (fig. 20) it is round or slightly oval. One notes that the ganglionic mass is well, even sharply delimited from the mesenchyme medial to it. As in the previous stage, the primitive head vein lies just medial to it. The mesodermal stalk lateral to the pharynx has not grown appreciably. Neither is it more compactthan previously.


The extent of the otic placode and its relations to the acoustico-facial ganglion are well shown in figure 20. The thickening of the placode is most pronounced opposite the fourth and fifth rhombomeres. Its invagination commences first caudal to the acoustico-facial anlage, but for some time the latter lies beneath a pronounced anteriorlip of the otic pit (fig. 53).


The most noticeable change in the 11-somite embryo (fig. 45) is the great growth of the mesoderm of the second visceral arch. It has not only increased considerably in thickness, but has become much compacted. The individual cells have consequently lost their stellate shape, and as a result resemble very closely the cells of the ganglionic anlage. Ten-somite embryos show the arch mesoderm in stages of compaction intermediate between the 9- and 11-somite embryos. The acoustico-facial anlage has changed but slightly. It is still a fusiform mass extending ventrally to the level of the dorsum of the first gill pouch, which extends dorsally to the level of the ventral surface of the neural tube. In spite of the similarity between the compacted mesodermal cells of the hyoid arch and those of the ganglionic mass, the ventral limit of the acoustico—facial anlage can be approximately fixed. The long axis of the ganglion is oblique to the axis of the hyoid arch, so that there is a medial notch where the ganglion comes into contact with the arch mesoderm. The notch lies at the upper level of the dorsum of the first gill pouch, to which height the mesoderm ca11 be seen to extend in sections just in front of or caudal to the ganglion (fig. 46), and it will be recalled that the ganglion extends ventrally to that level in 9- and 10—somite embryos. A study of 10somite embryos yields convincing evidence that the ganglionic cells take no part in the formation of the mesodermal core of the branchial arch.


A very thin lamina of looser cells (probably mesenchymal) separates the upper portion of the ganglion from the ectoderm in the 11-somite embryo. It can be recognized by its more diffuse character and tl1e presence of a few blood cells. The blood cells have deeply staining nuclei in the figure.


The otic placode becomes thicker, but the invagination has progressed but slightly between 9 and 11 somites.


In the interval between 12 and 14 somites (figs. 47 to 50), the acoustico—facial anlago keeps pace in growth with the embryo and its relations to surrounding parts remain much the same as in earlier stages. During this interval the neural tube closes in the region of the otic rhombomere and the figures show the attendant ventral shifting of the attachment of the ganglion to the neural tube. The fusion of the lips of the neural folds is not very intimate, so that they separate somewhat in the 14-somite embryo as a result of handling. At 12 somites the dorsal end of the ganglion is in contact medially with the dorsal surface of the closing neural tube, lying between it and the overlying ectoderm (fig. 47), but at 14 somites it has shifted ventrally, so that it is now in contact with the middle third of the neural tube.


The compact mesoderm of the hyoid arch still extends dorsally to the upper level of the first gill pouch, i.e., approximately to the level of the ventral surface of the neural tube (fig. 47 and cf. the right and left sides of figs. 48, 50), where it comes into contact with the ventral extremity of the ganglion. The long axis of the acoustico-facial ganglionic mass is even more markedly oblique to the axis of the hyoid arch than previously, and consequently there is a more pronounced medial notch at the boundary between the two structures than in younger embryos. Furthermore, one notices a distinct difference in the arrangement of the cells of the mesoderm and the ganglionic anlage, more pronounced in some specimens than in others. In some embryos the differentiation of ganglionic and mesodermal tissues is particularly striking (for instance, figs. 48, 49). ‘In embryos of 12 to 14 somites, therefore, one is justified, I believe, in placing the ventral limit of the ganglionic anlage at a point corresponding approximately to the dorsal level of the first pharyngeal pouch, which is situated just anterior to the acoustico-facial anlage. The exact level varies but slightly. It is impossible to determine whether there is an intermingling of ganglionic elements with the mesoderm at the place of contact.

Figures 47 to 50 also show the gradual separation of the acoustico—facial anlage from the overlying ectoderm by the invasion of mesenchyme during this period. The separation of the ganglion from the ectoderm is more marked dorsally than ventrally where the ganglion approaches the surface more closely. In figures 47, 48, the separation appears more marked on the right than on the left, but that is because the plane passes more caudally on the left, passing immediately (fig. 48) in front of the otic-pit against which the ganglion rests. The integrity of the inner surface of the epithelium both in relation to the ganglion and the branchial arch is unquestionable. Frontal sections of the ganglion in the 14somite embryo show it as a perfectly discrete cell column (fig. 53) separated widely from the ectoderm dorsally, but approaching it ventrally. A new venous channel, the vena capitis lateralis, has been formed in the mesenchyme lateral to the ganglion between 12 and 14,somites. In the 12-somite embryo sprouts from the primitive head vein are beginning to work their way around the ganglionic mass, and the process is completed at 13 somites.


Figure 51 is a section through the ventral tip of the acoustico-facial anlage of an 18-somite embryo. Its attachme11t to the neural tube is shown in more caudal sections. Figure 51 is the third section caudal to the first branchial pouch. A study of it shows that while the embryo has grown markedly there has been no change in the relative positions of the parts concerned. The ventral extremity of the ganglion can be easily determined in figure 51, resting directly upon the mesodermal core of the hyoid arch. It can be traced forward to the section immediately caudal to the first branchial pouch where it makes contact with the epibranchial placode, but there is no continuity between the two structures. A line drawn laterally from the ventral surface of the neural tube coincides approximately with the ventral end of the anlage and serves as a convenient reference line for comparison with younger embryos. Figure 52, passing through the attachment of the VILVIII ganglion to the brain, three sections caudal to figure 531, shows that the compact core of the hyoid arch has the same relative dorsal extent as in younger embryos. Study of the series shows that the upper limit of the mesodermal core of the hyoid arch lies at the level of the dorsal surface of the first branchial pouch— a fact well illustrated also in figure 67, a sagittal section of an embryo of 19 somites. It will be appreciated from the above description that the ventral extremity of the acousticofacial anlage of the 18-somite embryo lies in the same relative position as in the younger stages—namely, immediately caudal to the dorsal end of the first pouch.


The most important development of this’ stage is the placodal thickening of the ectoderm at the upper level of the arch. ‘It is illustrated in figures 51 and 52, and no doubt is identical with Froriep’s epibranchial sense organ. The thickening begins immediately caudal a11d dorsal to the first cleft and extends caudally and dorsally as far as the anterior border of the otic vesicle. The extreme ventral end of the acoustico—facial anlage rests lightly against it, but there is no cellular continuity between the placode and the ganglion; the inner surface of the placode is everywhere intact. Four mitoses, tl1e planes of which were not determinable, were found in the entire left placode which may be traced through fifteen sections. It is evidently not growing very actively. A few cells could be found in it which showed unmistakable evidence of degeneration, and these degenerating cells increase greatly in number as time goes on.


From the base of the ganglion where it comes into contact with the neural tube, one finds a spur of cells which seems to be growing ventrally from the base of the ganglion, keeping in close contact with the neural tube. 'Tl1is spur of ganglionic cells is small at 18 somites, but is rather prominent at 21 somites (fig. 54). Its possible fate will be discussed later.


The changes are slight in 19- and 20-somite embryos. In specimens of these ages the epibranchial placode is still extensive, reaching caudally as far as the otic vesicle. The acoustico-facial anlage extends laterally as it proceeds ventrally, its ventral extremity just touching the epibranchial placode, but not fusing with it.


A series of sections through the acoustieo—facial anlage of a 21-somite embryo are given in figures 54 to 56. Figure 56 is a section through the caudal edge of the ganglion. It shows the anterior surface of the otic vesicle. The attachment of the ganglion to the brain is shown, but its ventral extremity is not included in the section. The figure illustrates the dorsal extent of the arch mesoderm, which is relatively the same as in younger embryos. Figure 55 is five sections anterior, revealing the full extent of the ganglion, whose ventral extremity approaches the placode. Three sections farther anteriorly (fig. 54) the ventral tip of the ganglion comes to lie in the angle between the epibranchial placode a11d the first branchial pouch, and in this embryo there is only a very restricted contact, but no fusion of ganglion and placode at this point. The ventralspur extending from the base of the ganglion along the side of the neural tube is well shown.

In this,21-somite embryo the epibranchial placode is no less extensive than in the 18- and 19-somite embryos described.


It has not thickened appreciably and no evidences of proliferative activity could be discerned. Its inner surface seems intact, and while mesenchymal cells lie close against it, there seemed to be no continuity between the two. Sagittal and frontal sections of 21-somite embryos confirm the evidence of the transverse sections as to the extent and relations of the acoustico-facial anlage. At 21 somites a few degenerating cells and phagocytes may be observed in the dorsal part of the mesoderm of the second arch. They are not numerous and seem to lie chiefly caudal to the ganglion.


Three sections through the acoustico-facial anlage of a 26» somite embryo are presented in figures 57 to 59. Figure 57 is a section through the tip of the ganglion which extends into the angle between the dorsal surface of the first branchial pouch and the epibranchial placode, where it makes contact with the placode, but does not fuse with it. The relation of ganglion to placode is well shown in figure 59—a section through the VII-VIII ganglion on the opposite side of the same embryo. The contact of ganglion and placode extends through only two sections on both sides of the embryo. Elsewhere ganglion and placode are separated by mesenchyme which is closely applied to the placode. In sections immediately caudal to the pouch where the curvature of the arch is steepest and in which consequently the placodes are most obliquely cut, this close adhesion of mesenchyme and placode may be interpreted as proliferative activity on the part of the placode. Figures 57, 58 may serve as an illustration of this. It will be observed that the obliquely cut wall of the adjacent pouch shows an epithelio—mesenchymal relationship similar to the placode, so that one must grant an entodermal proliferation of mesenchyme if the placode is interpreted as proliferating. In this connection, one recalls Weigner’s comment: “ . . . . die Beurtheilung eines Zusammenhanges gerade an dieser Stelle mit fast uniiberwindlichen Schwierigkeiten zu kampfen hat. Es ist eben jeder der Schlundbogen nach allen Richtungen convex und wird deshalb immer seine Epithel schief getroffen. Liegt nun eine Zellgruppe diesem Epithel nahe so kiinnen sehr leicht solche Schiefschnitte eine Verbindung vortéiuschen auch an Stellen, an denen keine existiert.” However, the difficulty is solved when frontal sections of embryos of the same age are consulted. These show no evidence of proliferation from the placodes, although mesenchyme may be in close contact with them.


In some of the eight 26-somite embryos examined, a number of degenerating cells were found in the placodal epithelium, giving evidence of early retrogressive change. Mitoses are relatively few, those found are superficial in position, with the spindle axis parallel to the free surface when determinable. Considerable numbers of degenerating cells and phagocytes are found in the mesenchyme near the dorsal end of the second arch in these embryos.


The acoustico-facial anlage shows no essential change in eight 27 —somite embryos studied. In 29- and 30-somite embryos the anlage is somewhat larger, and for the first time a few fibers can be seen extending from the ganglion for a short distance into the hyoid arch.


A 30-somite embryo, favorably sectioned, shows interesting developments. The ganglion has been still further separated from the ectoderm by the increase of the intervening mesenchyme. The ventral extremity of the ganglionic anlage is now perfectly distinct, and from it two branches may be followed. A short cellular cord proceeds from the ventral extremity of the ganglion laterally to the placode to which it adheres, but there still seems to be no intimate fusion of the two tissues. This placodal ramus is formed, I believe, during the separation of the ganglion from the ectoderm by the expansion of the hyoid arch. The original contact of the ventral extremity of the ganglion and the placode persisting, the recession of the ganglionic mass from the ectoderm results in the ‘drawing out’ of a cord of cells leading from the ventral extremity of the ganglion to the site of original contact with the placode. The second branch of the ganglion can be followed into the hyoid arch for some distance.


A similar but somewhat stouter placodal ramus may be identified in embryos of 31 a11d 34 somites (figs. 60, 61). Its caliber varies considerably, but its point of attachment to the placode is remarkably constant in position, namely, just above and behind the first branchial pouch in the angle between pouch and placode. This point, it will be recalled, has been described as marking the ventral limit of the ganglion in earlier stages. The relations just described are well shown in figure 61, which is a frontal section of a 34-somite embryo. In this embryo three terminal branches of the ganglion may be followed—one passes antero—medially between the dorsal aorta a11d the pharynx (the greater superficial petrosal nerve), one continues ventrally into the hyoid arch, and the third is the placodal ramus above described.


The placodal branch of the geniculate ganglion is still well developed in some embryos of thirteen days, but at this time it seems to be in process of disappearance, since in some specimens of this age it can be recognized upon one side only and in others it can be recognized with difficulty. In some it is still prominent.


In an embryo of thirteen days four hours there is no longer any trace of an epibranchial connection of the genicu— late ganglion. On one side the placodal region takes part in the formation of a short ductus branchialis I with a very small lumen. On the other side the adjacent surfaces of the first cleft have fused, forming a short cord of cells leading from the pouch to the surface, but there is no lumen.


One could never be surer of the perfect integrity of the inner surface of the epibranchial placode of the VII ganglion than i11 these embryos of 30 to 34 somites and beyond. Mesenchymal cells are at times found closely applied to the placode, but there is no indication of their being proliferated from it. At the place of fusion of ganglionic ramus and placode there is, of course, some doubt. The branch itself is, I am confident, of ganglionic origin. VVhether cells pass into the ganglion from the placode by way of this branch it is impossible to say. There is, however, no direct evidence of it. Surprisingly few mitoses are present in the placode, and these are superficial with spindle axes parallel to the surface when determinable. After 31 somites the epibranchial placodal of the facial rapidly thins out and is soon not to be separated from the surrounding ectoderm.


In the case of the acoustico-facial, as in the case of the trigeminus, there is a well-defined, spur—like downgrowth from the base of the ganglion along the side of the neural tube. It is, however, more pronounced in the case of the acousticofacial than the trigeminus. This spur from the acousticofacial ganglion first becomes well defined in embryos of 17 and 18 somites. Like that of the fifth nerve, it seems to be derived from the main ganglionic mass by a ventral growth of cells along the side of the neural tube, and one soon recognizes a median ganglionic mass of which the spur is a projection as distinguished from the more lateral mass. It is well marked at 27 somites in figure 62, and is still well defined at 31 somites. At 34 somites (fig. 63) it constitutes a well~ defined compact mass lying just ventral to the place of attachment of the acoustico-facial ganglion to the neural tube. At its ventral extremity one now finds a well-developed tuft of motor fibers growing out from the neural tube. As time goes on, the spur becomes more and more completely invaded, until at thirteen days (fig. 64) it is only a tiny mass crowded between the now greatly enlarged motor root and the base of the ganglion. At thirteen days and four hours (fig. 65) it has disappeared, its place having been taken by the enlarged motor root.


In the case of both the trigeminus and the acoustico-facial, then, the motor fibers seem to grow out into the substance of the spur, finally obliterating it. The fate of its cells is a matter of some interest. Harrison (’06, ’24) has shown very clearly for the spinal nerves that the sheath cells of the motor root are derived from the ganglionic crest in Amphibia, and he has also found (’24) that “after removal of the ganglion crest and placodes of the head, cranial motor nerves without sheath cells may develop, but iii the case of these medullary cells evidently begin to migrate along the ventral roots at a relatively early period, giving rise to some sheath cells, even though the other sources are removed.”


According to Neal (’14), the neural crest seems to be the chief source of origin for the sheath cells in reptiles, birds, and mammals. In Selachii, however, the sheath cells have been recorded as being exclusively medullary in origin.


In the spinal region of the rat (fig. 66) the ganglionic crest forms a spindle-shaped mass closely adherent to the sides of the neural tube. The apex of the ganglionic mass comes in contact with the developing motor fibers as soon as they leave the neural tube, probably, according to the most reliable evidence of Harrison, supplying these fibers with sheath cells. The ganglionic spur described in connection with the anlagen of the trigeminus and acoustico-facial ganglia bears a relationship to the developing motor roots of those ganglia which is entirely comparable to the relation of the ganglionic crest to the motor root in the case of the spinal nerve, and the suggestion is strong that the spurs, invaded by the developing motor fibers, furnish the neurilemma cells of the motor portions of these cranial nerves. It is significant that the anlagen of the IX-X nerves develop no such spurs.


The author does 11ot wish to infer the homology of the spur with the spinal ganglionic crest in its entirety, nor does he consider the sp11r and the main ganglionic crest as representing different ‘orders’ of crest. The entire cranial ganglion plus the spur are probably the equivalent of the spinal neural crest.


The acoustic gawnglion. The present study was directed mainly toward a solution of the problem of the origin of the acoustic ganglionic mass, and no special attention was paid to its subsequent differentiation-—-a subject to which Streeter (’06) has already devoted a special paper. Concerning the origin of the ganglion cells of the acoustic nerve in man, Streeter (’12) says: “They are evidently not derived from neural crest; but Whether they migrate out from the brain wall or the walls of the developing ear vesicle, or are derived from the ectoderm immediately adjacent to the auditory pit, remains to be determined.”


In the rat the last possibility is ruled out by a study of frontal sections of the acoustico-facial mass. Until the V11somite stage the acoustico-facial anlage does lie in close contact with the otic placode, but at this time one can find no evidence of ectodermal proliferation. After .12 somites the ganglion is a perfectly discrete cell column separated from the prominent anterior lip of the otic pit and from the anterior wall of the pit itself by considerable mesenchyme. It never subsequently comes into contact with the epithelium overlying the otic vesicle.


There was no evidence in the rat of a proliferation of acoustic ganglion cells from the wall of the otic vesicle. From 12 to 18 somites the acoustico-facial ganglion is distinctly separated from the anterior wall of the otic pit or vesicle by mesenchyme, but shortly thereafter there is evidence of the migration of cells from the caudal face of the ganglion, giving rise to a mass of cells closely applied to the anterior wall of the otic vesicle and not at first separated from the facial ganglion. A series of photographs is presented to show the formation of the acusticus. At 19 somites (fig. 67) there is still no evidence of the presence of an acoustic ganglion. In the 21-somite embryo there is only a small caudal projection of the ganglion toward the otic vesicle, but in a 24—somite embryo (fig. 68) there is a well-developed mass of cells between the facial mass proper and the otic vesicle. To what extent this compact mass is composed of mesenchymal cells could not be determined. The photograph does not give an adequate idea of its size, since in other sections it could be followed along the anterior face of the otic vesicle and to a slight extent along its ventral surface.


At 27 somites (fig. 69) the proliferation of ganglion cells from its caudal face has brought the acoustico-facial mass into close contact with the anterior face of the otic vesicle. At 28 to 29 somites (fig. 70) the two components of the ganglion are distinct. A proliferation of cells of the acoustic ganglion from the walls of the otic vesicle could not be observed, since all through the period of formation of the acoustic mass the walls of the otic vesicle are absolutely intact. I therefore conclude that the acoustic mass is a derivative of the main ganglionic mass and hence derived from the neural crest.


In summarizing the development of the acoustico-facial anlage, the following facts may be emphasized. The anlage, a mass of neural crest attached to the otic rhombomere, grows ventrally between the paraxial mesoderm and the overlying thickened ectoderm of the otic placode with which it is in intimate contact, but from which it receives no contribution of cells. The ganglionic mass quickly reacl1es its ventral limit (10 to 11 somites) at the upper level of the first pouch where it rests against the condensed Inesenchyme of the second arch. The condensation of arch mesenchyme occurs independently of neural crest or other ectodermal contributions and no migration of neural-crest elements into the arch could be observed. The ganglion gradually becomes separated from the overlying ectoderm by the invasion of mesenchyme lateral to it, but its ventral extremity maintains contact with the ectoderm just behind the dorsal extremity of the first branchial pouch, where at 16 to 18 somites a pronounced thiel<ening marks the site of the epibranchial. placode.


The contact of the ganglion with the ectoderm above and behind the first pouch is, then, of long standing, existing from the time the ganglion reaches its ventral limit at 9 to 10 somites, and the relative position of the ventral extremity of the acoustico-facial anlage remains the same until by growth the anlage comes to be separated from the mesenchymal condensation of the arch (of. figs. 67 to 70). The ganglionic and mesodermal tissues are always tolerably well differentiated, so that it is not difiicult to ascertain the boundary between them.


At about 29 to 31 somites the expansion of the hyoid arch results in the separation of even the ventral extremity of the ganglion from the ectoderm, but, as the ganglion recedes, it maintains its placodal contact by the drawing out of a cord of ganglionic cells extending from the lower pole of the ganglion to the ectoderm which persists until shortly after the thirteen—day stage. There is no doubt in the writer’s mind that this placodal ramus is ganglionic in origin.


In determining the relation of ganglion and placode one must rely on good transections through the long axis of the ganglion or on frontal sections. The latter, however, are not favorable for determining the ventral extremity of the ganglion, but allow one to judge of the integrity of the inner surface of the placode. Oblique sections of any kind are apt to be misleading and the curvature of the arch surface must be kept in mind even in the interpretation of good transections. While the lower extremity of the ganglionic mass is almost from the beginning in co11tact with the pla codal ectoderm, no intimate fusion of parts appears to occur until about 31 somites. It is impossible to determine whether there is any migration of cells from the placode to the ganglion by way of the placodal ramus, but appearances do not favor such an interpretation.

3. The Glossopharyngeal and Vagus Ganglia

So far as the writer is aware, the early history of the glossopharyngeal and vagus ganglia has never been completely followed in the mammal. The work of Martin (’91) on the IX-XII nerves of the cat and the excellent studies of Chiarugi (’89, ’90, ’94) on the rabbit and guinea—pig, and of Streeter (’O4) on the human embryo deal with later phases of their development.


The IX—X anlage is first discernible i11 the rat of 8 somites. It has been described previously as being at that stage the least advanced in development of the anlagen of the cranial ganglia. Attention may again be called to figure 28, which is a section through the IX-X anlage of the 8—somite embryo. At this early period, active proliferation has not yet begun and the anlage consists simply of a conical mass of cells fitted between the lateral edge of the neural plate and the ectoderm.


Externally, there is a shallow groove on each side marking the boundary between the neural plate and the ganglionic anlage. As yet the ectoderm cannot be traced over the ganglionic anlage to the edge of the neural plate. The IX-X anlarge may be traced caudally to the anterior margin of the first somite, where it becomes directly continuous with the actively proliferating spinal neural crest. The relations of the vas primitivum rhombencephali (Sabin) should be noted. It lies just below the IX-X anlage in close proximity to the neural fold.

At 9 somites (fig. 71) the proliferation of the IX—X anlage has begun, the cells migrating laterally and ventrally to form a compact mass lying lateral to the vas primitivum rhombencephali and in contact with the overlying ectoderm which extends over the neural crest to the edge of the neural fold. The thickness of the ectoderm overlying the anlage is exaggerated in figure 28, due to the curvature of the embryo. It is impossible to fix the exact ventral limit of the crest. In some sections it appears to extend to the ventral border of the primitive rhombencephalic vessel. The extent of the IX-X anlage of the 9-somite embryo is somewhat greater than at 8 somites, due to the expansion of the territory between the anterior margin of the first somite and the otic placode. As one traces the IX-X crest caudally it becomes somewhat more diffuse, but maintains its position lateral to the primitive rhombencephalic vessel. At the anterior boundary of the first somite the vas primitivum rhombencephali divides into two slenderer branches; one passes medial to the somite and the other ventro-laterally into the somatopleure. The crest proliferation for the IX-X ganglia accompanies the medial branch of the vessel and becomes directly continuous with the spinal neural crest medial to the somite. A


In figures 72 to 74 a series of photographs of an embryo in which the twelfth somite is just forming is presented to illustrate the essential features of the IX-X anlage at this stage. Figure 72 lies three sections caudal to the otic pit on the left. At this level the crest is a definite compact mass lying lateral to the Vas primitivum rhombencephali. The primitive rhombencephalic vein makes an abrupt Ventro— lateral bend just in front of the first somite to make its way into the somatopleure. A branch passing medial to the somite could not be found. Part of its course may be followed in the figures. As the crest is traced caudally it becomes somewhat more diffuse (fig. 73) and, passing around the caudal border of the Vas primitivum rhombencephali (anterior cardinal vein?) as the latter bends ventrally, becomes continuous with the spinal neural crest. Figure 74 is a section just caudal to the upper end of the anterior cardinal Vein showing the place of transition between the IX-X and the spinal neural crest. The diffuse character of the IX-X crest at most levels makes it impossible to determine accurately its ventral limit.


The IX—X anlage of '13—somite embryos is Very similar to that of 11- and 12-somite specimens, but one finds some changes in the relation of the crest to the venous channels of the head. In the l3—somite embryo the anterior portion of the postotic neural crest in the region of the future IX proliferation is just beginning to be separated from the overlying ectoderm by the establishment of a new Venous channel lateral to it, the beginnings of which are illustrated in figure 75. One finds many places where the neural crest seems to have migrated ventrally medial to the Vas primitivum rhombencephali as well as lateral to it, so that in some sections (of. the left side of fig. 76) that Vessel appears to be surrounded by neural-crest cells. The relations of the posterior portion of the IX-Y crest remainas described for the IX-X crest of earlier embryos. The anterior cardinal Vein is cut longitudinally on the right side of figure 76, which shows the neural crest lateral to that Vessel. The neural crest passes around the caudal wall of the Vein to become continuous with the neural crest related to the first somite.


Sections just caudal to the otic pit of the 14-somite embryo (fig. 77) show that the new Venous channel lateral to the anterior portion of the IX—X anlage is now well established; the old vessel is probably represented by a more or less collapsed channel embedded in the substance of the neural crest, but which communicates (fig. 78) frequently by anastomoses with the new vena capitis lateralis. The latter tends to lie more and more ventral to the crest (fig. 79) as it is traced caudally. I was unable to determine whether a new venous channel has been established in the caudal region of the IX-X anlage or whether there has simply been some ventral and lateral shifting of the old vas primitivum rhombencephali in this region. The limits of the neural crest constituting the IX—X anlage at this stage are better defined for a short distance caudal to the ear vesicle in the region of the prospective IX downgrowth where the crest is more compact and abundant than in the more caudal portion. In the territory of the future IX proliferation the crest lies medial to the vena capitis lateralis, but as the sections approach the first somite the crest becomes Very diffuse and shifts dorsally somewhat so that it tends to lie above the lateral head vein and closer to the overlying ectoderm. In this region, on account of its diffuse character, it becomes impossible to accurately delimit the crest from the mesenchyme (fig. 79), but careful study shows it to be continuous with the spinal crest at the level of the first somite (fig. 80). In embryos of this age the relations of the IX-X anlage to the early venous channels of the head are extremely variable, as one would expect from the plastic nature of the neural crest and the endothelial tubes, so that the description given above applies in detail only to the right side of the 14:—somite embryo here illustrated.


In a 16-somite embryo the IX-X anlage is widely separated from the ectoderm by diffuse mesenchyme in which the now rather capacious vena capitis lateralis courses. Beginning immediately caudal to the otic vesicle, the IX-X crest forms a lens—shaped mass closely applied to the lateral surface of the neural tube. It is largest in its anterior portion and thins out, becoming more diffuse as it proceeds caudally. An» teriorly, the anlage extends ventrally to the middle of the lateral wall of the neural tube, but does not reach the midline dorsally. It shifts dorsally, however, as it is traced caudally, and in its more posterior regions a thin li11e of crest cells overlies the dorsal surface of the neural tube. In some sections through the caudal portion of the IX-X anlage the crest is so diffuse that it is impossible to be positive of its presence. However, such appearances are confined to single sections and one has no difficulty in determining the continuity of the crest of the IX and X ganglia with the spinal neural crest. It is quite probable that such isolated sections where the crest appears to be absent are due to localized rarefactions of the crest of no significance. In later stages the caudal portion of the IX-X crest is more compact and there is never any doubt of its continuity with the spinal crest.


Wliile the IX—X crest is more extensive, more compact, and more prominent just caudal to the otic pit than in more posterior portions, there is as yet no more definite indication of the position of the anlage of the glossopharyngeus. A definite ventral extension of the IX-X crest marking the position of the ninth nerve is first observed in embryos of 17 and 18 somites. In the 18-somite embryo (fig. 18), three sections caudal to the ear, a slender cord of cells, visible in two sections 7.5 u thick, extends ventrally from the crest as far as the ventral border of the vena capitis lateralis, where it comes into contact with the mcsenchymal condensation of the third branchial arch. It lies medial to the vena capitis lateralis, a position which it retains.


A series of sections through the IX-X anlage of a 21-somite embryo are presented in figures 81 and 82. The section illustrated in figure 81 lies eight sections caudal to the second gill pouch and five sections caudal to the otic vesicle. Figure 82 passes through the region of the ultimobranchial body. The IX anlage shown in figure 81 is a compact cord of crest cells extending through four 7.5 p sections. It lies medial to the vena capitis lateralis, to the lower border of which the ganglionic anlage extends. The anlage comes intocontact with the mesenchymal condensation of the third arch, where it seems to be insinuating itself between the compact arch mesenchyme and the lower border of the lateral head vein. There is a slight thickening of the ectoderm above and behind the second gill pouch, which probably represents the epibranchial placode of the ninth nerve, but the thickening is not marked nor is it extensive. The IX anlage does not come into contact with it. The anlage of the glossopl1aryngeus is continuous caudally with the lens-shaped mass of neural-crest cells (fig. 82) that finally becomes continuous with the spinal crest. 8 It should be noted that at this stage the condensed arch mesenchyme caudal to the third pouch extends dorsally as far as the ventral border of the lateral head vein, and in more caudal sections it extends still farther dorsally around the lateral border of the vena capitis lateralis.


Four sections caudal to figure 82 as few neural-crest cells appeared to be moving ventrally over the lateral surface of the vena capitis lateralis, probably the earliest indication of the vagus anlage. Twenty—four—somite embryos were the first to show a definite vagal proliferation. In the 24-somite embryo there are two well-defined downgrowths from the postotic neural crest—-—the glossopharyngeal and the vagal. The former lies medial to the lateral head vein, while the latter is situated lateral to it. The glossopharyngeal proliferation has increased considerably in bulk, its ventral extremity is somewhat enlarged and seems to be insinuating itself between the lower surface of the lateral head vein and the compact arch mesoderm. The mesenchyme above the level of the dorsal aorta shows numerous degenerating cells, and subsequent stages show that the compact mesenchyme disappears in large part from this situation (cf. figs. 83, 89). The vagus (fig. 83), proliferating lateral to the vena capitis lateralis, also seems to be insinuating itself between the lateral head vein and the compacted arch mesenchyme, but in this region no degenerating cells are found in the mesenchyme at this stage. It is almost impossible to determine the exact limits of the vagal anlage. I11 the figure of the 24-somite embryo here reproduced, it seems to be easily distinguishable from the mesenchyme, but such is not the case in other embryos of the same age. It is also difficult to determine the caudal extent of the ventral extremity of the vagus, because just caudal to the vagal proliferation the compact mesenchyme extends dorsally lateral to the vena capitis lateralis. In the interval between the IX and X proliferations there is a short extent of neural crest where the proliferation is less active, but nevertheless occurs with sufficient intensity to allow one to trace crest cells ventrally along the inner side of the vena capitis lateralis; the cells so proliferated form in later stages a definite cord of ganglionic cells extending from the glossopharyngeal to the vagal anlage.


About twenty-five embryos cut in various planes were available for the study of the neural crest of the glossepharyngeus a11d vagus in the period between l.'W€11t}"—five and thirty—one somites. Great variatiionwas noted in embryos of the same age and many features which could be clearly observed in some embryos were not so obvious in others. For instance, in some embryos it was possible to delimit the ganglionic anlage easily from the adjacent mesenchyme, in others such a delimitation was not possible, due to individual variations in the compactness of the surrounding mesenchyme or of the ganglionic anlage itself. The following descriptions must therefore apply in fine detail only to the embryos illustrated; attention will be called to important individual deviations.


Five pliotograplis of sections of a 26—somite embryo are reproduced in figures 84 to 88. The first section passes through the caudal wall of the second branchial pouch and is the most anterior section through the glossopharyngeal anlage. A stout cord of ganglionic crest cells constituting the IX anlage could be traced through nine sections of 7.5 u. Caudal to that point the neural crest becomes very attenuate in its middle portion, so that extending between the glossopharyngeal and vagal proliferations there are two cords of neu1‘al-crest cells (fig. 87), a dorsal one lying close against the lateral surface of the neural tube and a ventral less compact cord of crest cells which lies just medial to the vena capitis lateralis and just dorsal to the dorsal aorta. As this latter cord is traced caudally it passes laterally beneath the vena capitis lateralis to join the ventral extremity of the vagus anlage. There are thus two communicating strands, a dorsal and a ventral, between the ganglionic anlagen of the IX and X nerves at this time (of. fig. 19). The dorsal and ventral communicating cords owe their origin to a continuation of crest proliferation caudal to the IX a11lage. A study of embryos of 24 to 27 somites confirms fully this statement. In some embryos there is a profuse proliferation of crest cells medial to the vena capitis lateralis extending as far caudally as the vagal proliferation, while in others one finds the two communicating cords as described above with a delicate strand of crest cells extending between them. I have therefore been drawn to the conclusion that this ventral communicating ramus is formed by the piling up of crest cells medial to the vena capitis lateralis in the region between the IX and X proliferations. As time goes on, the dorsal communicating ramus becomes somewhat reduced in size. In this embryo many degenerating cells were found in it, possibly some degeneration being a factor in its reduction in size. The cells of the ventral communicating ramus are somewhat diffuse and in some embryos of 26 to 27 somites it could not be traced with certainty as far caudally as the vagus.


In this particular embryo it is somewhat difficult to fix the ventral limit of the glossopharyngeal anlage in the first few sections caudal to the second pouch. However, careful study of several embryos of the same period as well as the evidence presented by this embryo makes me feel fairly certain in fixing its ventral limit at about the dorsal boundary of the dorsal aorta. In the first place, reference to figures of 21~ somite embryos will show that the dorsal limit of the compact arch mesoderm of the third arch lies slightly above the dorsal border of the dorsal aorta. In some embryos in which the ventral end of the IX anlage is less compact than the arch mesoderm one l1as no difficulty in distinguishing the limits of the two tissues. The same is true also of embryo figured in sections through slightly more caudal portions of the glossopharyngeal anlage. Secondly, one finds in embryos of 25 to 27 somites a large number of degenerating cells at the boundary between the two tissues, the degeneration affecting the compact mesoderm lying just lateral to the dorsal aorta both in the region of the IX anlage proper and extending caudal to this point beneath the ventral communicating strand. The degeneration tends to become slightly less marked in this embryo as the vagus is approached, but there is considerable individual variation in the amount of degeneration observed. In some degree, however, it can be found in all embryos of this age (24 to 27 somites). It is probable that this degeneration of the mesenchyme may be induced by the growth pressure of the ganglionic anlage, but while appearances indicate that the degeneration affects the mesoderm most profoundly some degenerating cells could also be found among the cells of the ganglionic anlage itself. The vagus anlage at 26 somites lies lateral to the vena capitis lateralis (fig. 88). It extends ventrally to the lower margin of the vein and then curves medially to join the ventral communicating strand. Due to the compactness of the mesoderm of the embryo figured (figs. 84 to 87), these relations of the vagus could not be well made out, but another embryo of the same age (fig. 88) as well as older embryos (fig. 89, 29 somites) show clearly that the vagal proliferation curves medially around the ventro-lateral margin of the vena capitis lateralis to join the ventral communicating ramus. In embryos of 26 and 27 somites it is impossible to follow the vagus farther ventrally and caudally. Posterior to and continuous with the vagal anlage a strip of diffuse neural crest extends along the side of the neural tube to join t-he spinal crest. In a 27—somite embryo, i11 sections immediately caudal to the second gill pouch, one finds that the glossopharyngeal anlage extends somewhat farther ventral than the upper border of the dorsal aorta which seemed to mark its lower limit in younger embryos. In this specimen one finds clear evidence of the ventral extension of the IX ganglionic anlage lateral to the dorsal aorta. The advancing mass of ganglionic cells seems to be pushing ahead of it a mass of degenerating mesodermal cells. It is quite impossible, however, to determine the ventral limit of the IX anlage in a 27-somite embryo sectioned frontally; one must accept the evidence afforded by favorable transections as more reliable. The ventral extension of the IX anlage lateral t.o the dorsal aorta and third aortic arch never progresses far, however. The ganglionic aulage reaches its definitive ventral limit in embryos of 29 and 31 somites when it reaches (figs. 90, 91) a point slightly ventral to the dorsal level of the second pouch, so that it is ‘tucked’ into the angle between the caudal wall of the second gill pouch and the ectodermal thickening which constitutes the epibranchial placode. In figures 90, 91 the thickness of the epibranchial placode of the glossopharyngeal is exaggerated, due to the curvature of the caudal wall of the second cleft. The ganglionic anlage rests lightly against the placode and there is no fusion of the two structures nor is there any evidence of placodal proliferation. At 31 somites the glossepliaryngcal anlage still shows no evidence of division into superior and inferior ganglionic masses. It has remained up to this time a purely cellular structure and, while the cells of which it is composed are much more elongated or spindleshaped than inprevious stages, exhibiting delicate fibrous processes, no fibrous processes could be traced into the substance of the third arch. Reference to the figures shows that a ‘spur’ of ganglionic cells such as was described for the V and VII ganglia cannot be detected in the case of the IX or the X ganglia.


The glossopliaryngeal anlage of 29- and 31—somite embryos is continued caudally by a lamina of neural-crest cells lying medial to the vena capitis lateralis; as this is traced caudally it divides into two communicating cords, one passing laterally along the lower surface of the vena capitis lateralis to join. the nodosal region of the vagus anlage and the other continuing caudally along the side of the neural tube to join the root of the vagus anlage (cf. fig. 92). The vagus anlage of -the 31-somite embryo (fig. 93) still lies lateral to the old vena capitis lateralis, but a new venous channel has become established lateral to it. Three or four small venous channels may be recognized lateral to the vagus anlage in the 31-somite embryo, but in most embryos this new channel is single. In slightly older embryos (fig. 96, 34: somites), the venous channels lateral and medial to the vagus have enlarged so that the nerve is surrounded by a lake of venous blood, the vascular endothelium being closely applied to the entire circumference of the nerve between tl1e ganglion radicis and the ganglion nodosum.


In sections of the 31-somite embryo through the region of the attachment of the vagus to the neural tube one finds the beginning of an enlargement of the anlage lateral to the vena capitis lateralis, constituting the nodosal swelling, but there is still no well—defined division into root and nodosal ganglia. In the 26- and 27-somite embryo no caudal extension of the vagus anlage could be traced. Now, however (in 29- and 31somite embryos), as prominent cellular extension of the ganglionic mass lying lateral and slightly ventral to the vena capitis lateralis can be tracedlfor some distance caudally (fig. 94). It curves ventrally and medially a few sections posterior to the ultimobranchial body and soon ends in the mesenchyme lateral to the pharynx.


This caudal extension of the vagus anlage between the ages of 27, 29, and 31 somites is without doubt associated with the growth of parts to which the vagus is related. In the 26-somite embryo (fig. 19) the vagus anlage lies directly above the third (the last true pouch in the rat) pharyngeal pouch and the ultimobranchial body which arises as a caudal extension of the posterior wall of the third pouch. In 29and 31-somite embryos, however, great growth and expansion of the caudal region of the pharynx has occurred, resulting in the separation of the ultimobrancl1ial body from the third pouch, so that it comes to open independently into the cavity of the pharynx. In the 31-somite embryo the ultimobranchial body lies about twelve sections caudal to the third pouch. Now the vagus apparently takes part in the growth shiftings of this region, growing caudally as the territory of the pharynx to which it is related expands, and in keeping pace with the growth shiftings the ganglion nodosum becomes extended caudally and slightly ventrally along the territory of the fourth arch (and more caudally in some cases). This caudal extension of the vagus is much slenderer and farther removed from the ectoderm in the 29-somite embryo than in the 31-somite specimen. In the latter it has attained some size and lies nearer the surface ectoderm, but, so far as could be determined, makes no contact with the extensive area of thickened ectoderm caudal to thethird cleft.


There is a caudal extension of the neural crest back of the X anlage of the 31—somite embryo, representing a prolongation of the ganglion radicis nervi vagi which becomes continuous with the neural crest related to the occipital myotomes, of which there seem to be three. Among its cells numerous fibers of the spinal accessory nerve can be recognized, extending as far caudally as the sixth myotome (probably the third cervical, since the fourth myotome is the first to which a definite ganglionic swelling is related). The fibers of the spinal accessory form a well-defined bundle along the caudal border of the vagus anlage. In the 29-somite embryo a few fibers of the spinal accessory can be found among the cells of the ganglion radicis vagi, but a distinct bundle of accessory nerve fibers could not be identified associated with the main ganglionic mass———a condition similar to what Streeter has described in a 4-mm. human embryo.


Superior and inferior ganglionic masses could be recognized in both the glossopharyngeal and vagal anlagen of a 34-somite embryo (figs. 95, 96). In the case of the IX nerve, however, the division into the ganglion of the root (Ehrenritter’s) and the petrosal ganglion was not nearly so distinct as the separation of the vagus into a ganglion of the root and the massive ganglion nodosum. The root ganglia are connected by a wide cellular lamina and the petrosal and nodosal ganglia are joined by a cellular cord passing under the vena capitis lateralis. The venous channel lateral to the vagus should be noted.


The petrosal ganglion extends to the angle between the caudal wall of the dorsal end of the second gill pouch and the ectoderm of the second cleft where its lower pole comes into contact withthe tip of the short ductus branchialis II, the caudal wall of which, slightly thickened, represents the epibranchial placode of the IX nerve. This relationship is best shown in a frontal section of an embryo of the same age (fig. 97). Study of the figure reveals that the fusion of the IX nerve with the ectoderm is neither very intimate nor extensive. The ductus branchialis II becomes much elongated in older embryos. Its medial end occupies a characteristic position dorsal to the third pouch and it extends laterally and somewhat caudally, its lumen communicating with the cervical sinus. The tip of ductus branchialis II remains in contact with the lower pole of the ganglion petrosum (fig. 97) until the thirteen—day stage. In embryos of thirteen days four hours it is represented by a vesicle connected with the ectoderm by a cord of cells, the lumen of its outer portion having been obliterated. The vesicle, however, is no longer in contact with the lower pole of the ganglion, but rests against the nerve a short distance below the ganglion petrosum. In thirteen—day-twelve-hour embryos the cord connecting the vesicle with ectoderm is degenerating and in a fourteen-day embryo only a solid mass fused with the dorsal side of the second pouch remains to mark its presence. There is never the slightest evidence, in the writer’s opinion, that the ectoderm contributes any cells to the substance of the petrosal ganglion.


The ganglion nodosumof the 34—somite embryo extends caudally over the territory of the fourth arch and bends ventrally back of the ultimobranchial body. A cellular extension of it can be traced to the root of the lung buds. There is an extensive area of thickened ectoderm lying caudal to the third gill cleft, representing a continuation of the branchial ectoderm caudal to the last cleft and including in its dorsal portio11 the vagus placode, which is 11ot separable from it. The ganglion nodosum is now a bulky structure which skirts along the upper margin of the thickened epithelium back of the third cleft. In the 34-somite embryo of figure 96, in one or two sections the ganglion lies closely against the ectoderm, but there was no fusion of the two structures, the inner surface of the ectoderm being everywhere quite ‘clean.’ On the other hand, another 34—somite embryo (fig. 98), evidently somewhat farther advanced in development, shows two well-defined contacts of the vagus with the ectoderm. A cellular extension of the ganglion running closely against the caudal wall of the third pouch is probably the superior laryngeal nerve, first recognizable in this embryo. It comes into contact and fuses with the thickened ectoderm just candal to the third cleft. Somewhat farther dorsally and caudally the ganglion nodosum itself is fused with the ectoderm. The ectoderm with which the vagus is fused now forms a part of the caudal and dorsal wall of the cervical sinus. More extensive contacts were noted in twelve—day~eighteen-hour (fig. 99) and »tl1irteen—day embryos. At thirteen days it becomes included in the cervical vesicle which becomes cut off from the exterior mainly by the expansion of the hyoid arch and the postbranehial region. Figure 100 illustrates the formation of the cervical vesicle by the expansion of postbranchial region and the second arch. The material of the third arch, not undergoing expansion, is becoming buried and overgrown by the rapidly expanding material lying on either side of it. The vagus thus maintains its contact with the cervical vesicle which lies at the lower pole of the ganglion at the level of the superior laryngeal nerve. Wit-l'1 the continued expansion of the postbranchial and hyoid-arch material and the consequent obliteration of the cervical sinus, the cervical vesicle becomes more deeply buried a11d for a time is connected with the ectoderm by a cellular cord which soon ruptures. Its attachment to the lower pole of the ganglion nodosum at the level of the superior laryngeal nerve persists for a long time, but in embryos of fourteen days twelve hours, it seems to have separated from the lower pole of the ganglion.


The relation of the vesicle to the ganglion nodosum is nicely shown in a thirteen-day—four-hour embryo (fig. 101). Careful study of the vagus placode and the cervical vesicle failed to convince me that any contribution is made by it to the vagus ganglion. It is, I believe, a case of adhesion, which, I think, is borne out by a study of the relations of the vagus to the vesicle in embryos of thirteen days, thirteen days four hours (fig. 101), thirteen days twelve hours, and fourteen days.


The caudal prolongation of the ganglion radicis vagi joining the spinal neural crest is still prominent in the 34.somite embryo. It has been almost entirely replaced by fibers of the accessory nerve in thirteen—day embryos.


Attention l1as already been called to the fact that beginning with embryos of 29 somites, a new venous channel is established lateral to the vagus nerve, the latter thereby coming to lie in the midst of a venous circle. The ninth and tenth nerves are thus separated by the medial branch of this venous ring, but connected with one another by dorsal and ventral communicating rami which form a ring of ganglionic material around the medial vein. Later, with the expansion of the caudal pharyngeal and postpharyngeal regions, the descent of the heart and the attendant descent of the caudal pharyngeal complexes related to these two nerves, onefinds that the two come to approach one another closely as the logical result of longitudinal tension. VVith the approximation of the two nerves the medial half of the venous ring around the vagus becomes compressed, attenuate, and finally ruptures. In a fourteen~day embryo there is an interval between the place of emergence from the brain of the root fibers of the IX and X-‘H nerves, but there is an intimate fusion between the root ganglia of the two nerves and a fusion also between the inferior part of the petrosal and the superior portion of the nodosal ganglia which lie side by side.


Streeter (’04) found a communication between the ganglion petrosum and nodosum in a human embryo of 7 mm., but “in other embryos of this stage, and younger, they are completely separated. Later, following the relative change in position of adjacent parts which succeeds their unequal growth, these structures are gradually brought together, and secondary communications are established between them.”


Ohiarugi (’90) studied the development of the vagus in rabbit embryos. In 4.5-mm. embryos the vagal and glossopharyngeal “semblent avoir une origine independante.” His description is not detailed enough to allow me to correlate his 4.5-mm embryos with rat embryos, but it would appear that they are fairly advanced in development. Chiarugi, however, apparently recognized the continuity of the neural crest, because he says of embryos of 6.5 mm.: “ . . . sur les points 011 elle (the neural crest) s’est conservé, elle est plus nettement accentuée et a pris la forme de cordon commissural entre les productions ganglionnaires que se sont formée a ses dépens. ” He also describes the continuity of the neural crest of the vagus with that related to the myotomes.


In the rat the fusion of the ganglia of the roots and the ganglion petrosum and nodosum of the ninth and tenth nerves was found to be constant. Moreover, the connections were found not to be secondary, but primary connections, since the ninth and tenth anlagen are derived from an essentially continuous postotic proliferation of neural crest which is molded more or less by the mechanical influence of related structures. There are two especially well-developed downgrowths of the postotic neural crest; one just caudal to the otic vesicle back of the second pouch, situated medial to the vena capitis lateralis, is the glossopharyngeal anlage and another occurring just above and behind the third pouch and lying lateral to the vena capitis lateralis is the vagus anlage. Between these two prominent proliferations, however, there is a continuous if less-marked proliferation of neural crest giving rise to dorsal and ventral communicating cords between the ninth and tenth ganglia. This agrees with findings in lower groups where the IX-X are ‘einheitlich’ (cf. Neumayer, ’06). With the approximation of the ganglia due to the growth shiftings described on page 101, the communicating cords become less prominent.


Streeter ( ’04, p. 102) thought that the petrosal and nodosal ganglia of the nint:h and tenth nerves might possibly have an origin different from that of the ganglia of the roots of these two nerves, and suggests that the former may arise in situ rather than by a subdivision from the rest of the anlage, but if I interpret him correctly, advances the idea purely as a. supposition.


The question raised by Streeter (’04) can be answered only by a study of these ganglia in embryos younger than were available for his study. In the rat, due to the ease of collecting material, I have been fortunate enough to be able to follow completely their early development. The youngest specimen studied by Streeter, a 4-mm. human embryo, corresponds approximately to a 34-somite rat embryo in its stage of development, preceding which stage both nerves have had long and interesting histories. Examination of the complete series of rat embryos available for this study shows convincingly, I believe, that the petrosal and nodosal ganglia do not arise in situ, but that they attain their positions relative to the gill arches as a result of the migration ventrally of the ganglionic crest from which they are derived. The migration can be followed in detail in a close series of embryos. An examination of the series of photographs presented shows clearly that the division of the anlagen of the IX and X nerves into ganglia of the root and ganglion petrosum and nodosum, respectively, occurs relatively late in the development of these ganglia, i.e., not until about 34 somites.

Discussion

In the foregoing pages the development of the sensory ganglia of the V, VII-VIII, IX, and X nerves has been described. They have been found to be derived from proliferations of the cranial neural crest which move ventrally in each case approximately to the level of the corresponding branchial pouch. The growth of the ganglia and the nerves derived from them has been found to be influenced largely by the growth of parts to which they are related. In the case of the trigeminus ganglion, for instance, we have found that the growth of the ophthalmic ramus accompanies the shifting in the position of the optic vesicle which occurs during the forward growth of material in the anterior region of the head (of. discussion, pp. 65-69). No placodes were found which could possibly account for its formation. The growth of the ma.xilla1*y and mandibular rami undoubtedly accompanies the downgrowth of the maxillary process and the downgrowth of the mandibular arches to form the lower jaw (of. Streeter, ’22, figs. 3 and 4).


The growth of the vagus nerve affords a striking illustration of the same principle, showing a marked response to the growth of related parts. It was found (of. p. 101) to be influenced in its growth by the rapid growth transformations and shiftings of the caudal pharyngeal and postpharyngeal regions. During the rapid expansion of the caudal region of the pharynx, and the growth shiftings of the caudal pharyngeal complexes attending the descent of the heart, the ganglion nodosum becomes greatly elongated caudally and ventrally posterior to the ultimobranchial body, and it might be suggested that the problem of the distribution of the vagus to the viscera might be explained upon the basis of the expansion and shifting of parts, relatively very restricted in extent inthe early embryo. One has only to note in the models here reproduced the relatively short extent of the digestive tract between the last pharyngeal pouch and the liver diverticulum to appreciate the tremendous elongation which the region must subsequently undergo. Jackson (’09) and Carey (’20) have already commented on the great elongation of the esophagus in the embryo.


The VII nerve has the least interesting growth history, since it is related to a territory situated at the boundary between two opposing growth tendencies, namely, the forward growth and expansion of the anterior part of the head and pharynx and the relative caudal movement of the posterior regions accompanying the descent of the heart (cf. rKingsbury’s, ’15, analysis of the growth of the pharynx). At first, therefore, the seventh nerve merely grows into the substance of the hyoid arch and has a somewhat negative history. Its subsequent migrations with the forward spreading of the facial musculature are of course well known.


The early history of the growth of the glossopharyngeal nerve is also simple; it undergoes at first but little caudal shifting, due, no doubt, to the more or less negative role of the material of the III arch, which has been found to undergo but little growth, and to be buried during the obliteration of the cervical sinus by the growth and expansion of the material of the hyoid arch and postbranchial regions.


The most interesting question relative to the development of the cranial ganglia concerns the r6le which the epibranchial placodes play in their formation. In lower forms——cyclostomes, elasmobranchs, teleosts, and Amphibia-——two series of placodes are described, a dorso—lateral series and an epi branchial group of placodes. Tl1e former are concerned with the development of the lateral-line nerves. VVith regard to the lower forms, it has now come to be quite generally agreed that cells proliferated from both series of placodes are incorporated in the cranial ganglia. There is, of course, considerable variation in the extent to which proliferation from the two series of placodes participate in the formation of a particular ganglion in the different forms. A review of the literature of the subject relating to non-mammalian forms may be foregone here, since many excellent summaries have already been presented.


With regard to the placodes of the mammal, it would seem that there are several questions at issue: 1. To which cranial ganglia are placodes related’? 2. Are dorso—lateral placodes present in the mammal?

Froriep (’85), who made the pioneer observations on the epibranchial placodes, described them in relation to the VII, IX, and X ganglia in cow and sheep embryos, but was unable to find a placode for the V nerve.

Chiarugi (’94) studied the development of the trigeminal ganglion in the guinea-pig, and described clearly the broad and intimate contact of the V ganglion with the epidermis in embryos of 3 to 3.6 mm. He believes that cells proliferated from the epidermis take part in its formation.

Weigner (’01), who studied the development of the V and VII ganglia of the ground-squirrel (a few pig and human embryos were also studied), was unable to confirm Chiarugi’s observations on the fusion of the trigeminus with the ectoderm.

Griglio—Tos (’02) describes three dorso-lateral and three epibranchial thickenings related to the trigeminus ganglion in a 15-somite human embryo, all of which proliferate ganglionic material. He recognizes nine subdivisions of the trigeminus and finds a complete correspondence between the development of the trigeminus ganglion of man and the lamprey. And all this from the study of a single embryo! Giglio-Tos has obviously confused mesenchymal condensations with ganglionic material. He also describes (’02) dorsolateral and epibranchial placodes of the VII-VIII ganglion.

Celestine da Costa (’20, ’21, ’23) recognizes dorso-lateral and epibranchial placodes in connection with the V and VII nerves. The epibranchial placode of the V nerve is present at 6 somites in the guinea-pig, but the dorso-lateral placode does not appear until 16 to 18 somites, and is visible only during this time, after which it disappears suddenly. The most intimate relations exist between this placode and the trigeminal ganglion, making a placodal contribution to it possible. “En tout cas cette collaboration doit étre peu importante et bien plus réduite que celle qu’a décrite Brachet chez les Reptiles” (’23, p. 518).

Volker (’22) states that in the ground-squirrel the ganglia of the V, VII, IX, and X nerves fuse with the ectoderm and “erhalten von ihm Zuschuss zu ihren Zellen.”

Davis (’23) finds no true placode related to the trigeminus in a 20-somite human embryo.

The Writer has found that, in the case of each of the early ganglionic anlagen in the rat, the ganglion proliferates at first lateral to the paraxial mesoderm in close contact with the overlying ectoderm. This contact of the early ganglionic proliferations with the ectoderm is constant and characteristic. It is due, I believe, to two factors: First, it appears, as I have already pointed out (p. 64), that the ectoderm exercises a directive influence upon the rapidly proliferating ganglion cells, which migrate ventrally along its inner surface, a parallel case being the directive influence of solid objects upon the growth of tissues in vitro. Secondly, the rapid lateral expansion of the paraxial mesoderm tends to keep the ganglion in contact with the ectoderm. This expan~ sion is most marked and continues longest in the region of the mandibular arch. Consequently, the contact of the V ganglion with the ectoderm is most prolonged. The VII-VIII anlage soon becomes separated from the ectoderm by the invasion of mesenchyme lateral to it, its ventral extremity keeping contact with the epibranchial placode. The entire early IX—X anlage becomes separated from the ectoderm in a similar fashion. Wheii the vagus proliferation from the postotic crest occurs at 24 to 25 somites, it hugs the lateral surface of the vena capitis lateralis, which seems to exercise a directive or tactic influence.

The question as to what constitutes a true placode is a puzzling one, considerable confusion having arisen due to the fact that placodal thickenings have not always been differentiated from the diffuse and general thickening of the ectoderm over the entire branchial regionf‘ The thickening of

3 In an abstract appearing after the completion of the above text and too late for proper notice in the body of this paper, Bartelmez (’24) has described the extent of the thickened branchial ectoderm in young human embryos. He finds “clear evidences of cell migration from the area. V as well as the more caudal visceral arch areas.

In his complete monograph Bartelmez (’25) believes he has evidence that the placodes contribute largely to the ganglia. of the V and VII nerves and also

the branchial ectoderm begins first in the region of the mandibular arch, extending forward over the mouth clef t, thinning somewhat as it is traced forward. The thickening of the branchial integument proceeds caudally with the differentiation of the branchial pouches. At 14 somites the ectoderm over the branchial region caudal to the second branchial pouch is thickened as far caudally as the anterior border of the first somite, and the thickening extends ventrally over the pericardial region for a short distance (cf. figs. 72 to 79). After the formation of the last branchial pouch (the third in the rat), one finds a marked thickening of the ectoderm caudal to it, which does not, howe\>*er, extend as far caudally as the cranial border of the first somite as it did earlier.

As I have described, thetrigeminal ganglion has for as considerable time a broad area of contact with the ectoderm above the first branchial arch, but the ectoderm related to the ganglion is so slightly thickened and the slight thickening so diffuse that it is extremely improbable that it constitutes a true placode. The ectoderm over the ganglion is transitional in thickness between that covering the mandibular arch and that over the dorsum of the embryo, forming a natural transition between the two. The ectodermal thickening figured by Celestine da Costa (’ 3, fig. 3), and interpreted by him as an epibranchial placode related to the trigeminal ganglion, is regarded by the writer as merely thickened branchial ectoderm.

The epibranchial placode related to the VII nerve is the only one which, in the rat, is distinctly separable from the difl’use thickening of the ectoderm over the branchial region. The epibranchial placode related to the ninth nerve was found to be less clearly delimited from the branchial ectoderm than the epibranchial placode related to the VII ganglion. The epibranchial placode, with which the superior laryngeal nerve and ganglion nodosum fuse, is not separable from the thickstates that “ . . . . we are convi1'1ce(l that in Homo, as in many Icht.hyopsid:-'1,

the rostral neural crest furnishes a large contribution to the supporting elenients of the visceral region of the head

ened ectoderm caudal to the third cleft which becomes involved in the formation of the cervical vesicle. The fusion of the ninth ganglion with its placode was found to be less intimate than that of the VII or X ganglia—-—a condition somewhat different than has been described in other mammalian forms. In the case of the VII ganglion, a cord of cells connects the ganglion with its epibranchial placode; no such placodal branches of the petrosal or nodosal ganglia were noted. The only structure which may possibly be interpreted as a dorso-lateral placode in the mammal is the otic placode. In early embryos the VII ganglion makes intimate contact with the otic placode, but it was impossible to detect the migration of any cells from it into the ganglionic mass.

3. Do the placodes contribute to the formation of ganglionic material?

4. If the placodes do proliferate material incorporated in the ganglia, what part does this material play?

Chiarugi (’94), Griglio-Tos (’02), and Volker (’22) definitely commit themselves to the occurrence of placodal additions to the cranial ganglia in mammals.

Froriep (’85) concludes that the participation of the ectoderm in ganglion formation, if it occurs at all (‘Wenn sie fiberhaupt besteht’), must be confined to early stages and that such participationdoes not constitute the ultimate significance of the fusion of ganglion and placode.

Celestino da Costa (’23, p. 518) thinks that for the trigeminus a placodal contribution is possible, but in any case unimportant; in the case of the acoustic ganglion, ‘plus que probable,’ and ‘n’est guere niable’ for the geniculate ganglion.

Weigne1' (’01) was unable to be certain of ectodermal contacts of the V and VII ganglia and is inclined to believe that appearances which have been described as placodal proliferations are in most, if not all, cases due to oblique sectioning.

Streeter (’04) could find “no indication of the interchange of cells between ganglion and epidermis” in the epibranchial placodes of the ganglion petrosum and nodosum of the human embryo.


Turning to lower forms, Landacre (’10) found that in Ameiurus the visceral ganglion of the IX nerve is derived entirely from cells split off en masse from the epibranchial placode of IX. Since Herrick (’01, ’07) had found only special visceral fibers arising from the visceral ganglion of the IX nerve in that form, Landacre concluded that the “epibranchial placodes give rise to those ganglionic cells from wliich gustatory fibers arise.” According to Landacre, only general visceral and general cutaneous components are de— rived from the neural crest. Stone (’22) found that the neural crest yields only the general visceral component of the cranial ganglia in Amblystoma.

Tello ( ’23), studying the development of the cranial ganglia of the chick by use of the Cajal—pyridine-silver technique, came to tl1e conclusion that the real ganglionic elements are derived from the neural crest alone.

There is a complete lack of evidence concerning the fate of any cells which may be added to the ganglionic anlagen by placodal proliferation in the mammal. Viilker (’22) assumed the transformation of such cells into ganglion cells in the spermophile “auf grund von Beobachtungen von andern Forschern. ’ ’ I

In the rat I have found surprisingly few mitoses in the pla~ codes at any time-—~—far too few to account for any extensive addition of cells to the ganglia. Furthermore, if such proliferation were occurring, it would seem that in the examination of a large number of embryos one would be certain to find at least some numbers of mitosesiwith spindles arranged so that division would result in the addition of cells to the underlying ganglion and in addition some cells being pinched off from the ectoderm. The few mitoses found in the placodes were superficial in position (see, for instance, fig. 57) and only one or two were found with spindle axes perpendicular to the surface. Deceptive appearances were often caused by oblique sections. It was found that whenever the section was obviously true one had no hesitancy in deciding against proliferation and that when the section was obviously oblique then it became diflicult to be certain. It seems justifiable to conclude that it is improbable that ganglionic elements are contributed by the epibranchial placodes in the mammal. The absence of placodal contributions to the cranial ganglia in the mammalia would not exclude the possibility of such occurring in lower forms, and, conversely, it is dangerous to argue that, since the placodes appear to yield ganglionic elements in the Amphibia, they must do likewise in the bird, reptile, or mammal. Embryological work on the higher forms has been too often distorted by phylogenetic influences. Beard (’85), for instance, although he had not investigated the Mammalia, nevertheless expressed “a firm conviction that such rudiments (i.e., sense organs in connection with the ciliary and Gasserian ganglia) exist at some stage or other in mammalian development. This conviction rests upon a two-fold basis, an a, priori one (the italics are mine) that in Elasmobranchii the sense organs of the ciliary and Gasserian ganglia are Well developed,” etc. Giglio-Tos’ papers are another case in point. And What shall we say of Vi)'lker’s assumptions based upon others’ observations?

It may not be inappropriate to remark that the actual transformation of placodal cells into ganglion cells has not been observed in any form. Furthermore, it would seem that the study of Tello (’23), employing the Cajal-pyridine-silver technique, definitely rules out the transformation of placodal elements into ganglion cells in at least one form, the chick.

Landacre (’12) found in Ameiurus “a close correspondence between the size of the epibranchial placode and the number of gustatory fibers to which the ganglion gives rise in the adult,” and further states that, “In view of the reduced character of the gustatory system in many forms as compared with the fishes, it is not surprising that the actual contribution of cells by the epibrancliial placode should not be large and might take place during the period of contact and still be diificult to demonstrate.” If one grants the contribution of ganglionic elements by the placodes in the mammal, still no such correspondence between the size of the contact and the number of gustatory fibers in the adult nerve exists, so far as our present knowledge of the distribution of gustatory nerves in the mammal allows us to judge. The epibranchial contact of the vagus is most extensive in the mammal (cf. Froriep, ’85; Kingsbury, ’15), and yet of the three nerves (VII, IX, X) distributed to the taste buds, the vagus has the most restricted territory (of. Parker, ’22, and Zander, "97).

5. What is the fate of the placodes themselves?

6. Does the fusion of ganglion and placode exercise any mechanical influence in development?

In the rat epibranchial placode I becomes much reduced in thickness as development proceeds and finally disappears. Its contact with the VII ganglion is lost shortly after thirteen days of age. Epibranchial placode II becomes involved in the formation of ductus branchialis II and epibranchial pla code III becomes included in the cervical sinus, sharing the

fate of those structures, but it is hardly likely that the fusion of ganglion and placode is responsible in any mechanical sense for the formation of either ductus branchialis II or the cervical sinus, botl1 of which structures can be more adequately accounted for by the expansion of the surrounding arcl1 material.

7. What is the significance of the placodes?

This last question is a puzzling one. They have been regarded as rudimentary sense organs (Froriep, ’85; Johnson and Sheldon, ’86; Beard, ’86), as “différentiations provoquée par le voisinage du -tube neural, ou de ses émanations, vésicule optique ou crétes ganglionnaires” (Celestine da Costa, ’23, p. 519), or as “eines Uebergreifens des VVucherungsreizes von den ihnen engst benachbarten Ganglienanlagen auf ihren Zellen” (Viilker, ’22, p. 197). Johnston (’05) believes that the dorso-lateral placodes “represent material comparable to the neural crest which has separated more completely from the brain than has the remainder of the crest.”

To the writer it seems possible that the placodes arise dur ing development as a result of a tissue interaction (possibly

reciprocal) between the epidermis and the ganglion, or that they represent rudimentary sense organs developed under the stimulus of nervous contact, the placode undergoing retrogression when the contact is broken as a result of growth shiftings. In this connection one is reminded of the degeneration of certain end organs upon the severance of the nerves supplying them (viz., the taste buds). The pl.acodes need never have functioned as specific sense organs at any time during ontogeny or phylogeny. Any attempt to homologize them would be futile.

We come finally to a consideration of mesectoderm formation in the mammal. Veit (’18) found it impossible to delimit the diffuse neural crest in an 8-somite human embryo, and concluded that neural-crest cells were transformed into mesenchymal cells. Appearances similar to those described by Veit are observable in the rat. In most cases where the neural crest is diffuse one can draw no definite boundary between neural crest and mesenchyme. Now it is possible that cells may become detached from the neural crest, become a part of the mesenchyme, lose their ectodermal potentialities, and differentiate into some mesenchymal derivative, as Stone’s (’22) experiments seem to indicate for the Amphibia. Mangold (’24) has demonstrated that ectodermal cells transplanted into the mesoderm may differentiate into mesodermal structures, and it is quite possible that some such process may occur in the case of the neural crest. However, as Brachet (’07, ’21, p. 386), Celestino da Costa (’23, p. 519), and Keibel (’24, p. 38) point out, there is nothing to prove that cells detached from the crest do not differentiate into nervous structures.


There is, furthermore, no evidence, in the rat, of an organized contribution of the neural crest to the mesoderm, such as has been described in Amphibia by Platt, Landacre, and Stone. In the case of the rostral neural crest, which gives rise to the trigeminal ganglion, one could distinguish it fairly definitely from the surrounding mesenchyme as a lozenge-shaped mass of deeply staining cells even at 5 somites, and while cells might occasionally be found which seemed to be in process of being added to the mesenchyme, there was no evidence of a migration of the crest into the branchial arch. The acoustico—facial ganglionic mass is always compact, and it was possible to observe the condensation of the mesenchyme of the second arch independent of any neural—crest contribution in a close series of embryos from 8 to 12 somites. The postotic neural crest. is very diffuse in embryos of 9, 16, and 17 somites and no definite boundary between crest and mesenchyme can be set. The mesenchyme of the third and fourth arches becomes very compact some time before the neural crest has migrated ventrally to their upper levels, the condensation occurring independent of any neural-crest contribution. When the neural crest has finally reached its ventral limit at the dorsal levelsof the arches in question, the definite ganglionic masses appear for the IX and X nerves, the arch mesoderm is too compact to permit of any further ventral migration of the neural crest, and no such migration could be observed. Celestine da Costa (’23, p. 519) comes to a similar conclusion from his study of the guinea-pig.


Veit (’f2.2) has also expressed the belief that the mesectoderm is proliferated from the epidermis over the entire body. Appearances such as are pictured in his figures (7, 7 a, 8, 8 a) were observed in the rat, but are, I believe, to be differently interpreted. The first indication of the formation of a gill arch is a thickening of the overlying ectoderm. The condensation of the mesenchyme proceeds rapidly and the compacted mesenchyme comes to lie closely against the thickened branchial ectoderm——-so closely, in fact, that the line of separation is often obliterated, as one might expect when a closely compacted mass of cells comes in contact with an epithelium with no well-defined basement membrane. The difficulties of interpretation, as \Veigner (’O1) has observed, are increased by the curvature of an arch, so that sections are more than likely to be oblique. One gains the impression that possibly the mesenchyme is condensing under the influence of the ectoderm, or, conversely, that the condensation and expansion of the arch mesenchyme are responsible for the thickening of the overlying epithelium, as Harrison (’18) found to be the case in the limb bud. A striking illustration of the above was seen in rat embryos of 11 to 14 somites. Here the territory of the prospective third and fourth arches is indicated by an extensive area of thickened ectoderm which extends caudal to the second gill cleft almost to the border of the first somite. It extends ventrally a short distance over the pericardial region. Immediately beneath this area of thickened brancl1iogenetic ectoderm one finds the mesenchyme condensing and in close contact with the overlying epidermis, the condensation extending over the pericardium. Now the question arises, are these mesenchymal cells being proliferated from the ectoderm? I do not believe so. Very few mitoses could be found —-—not nearly enough to account for so extensive and profuse a proliferation. Those mitoses Whose spindle plane could be determined were parallel with the surface and no cells could be observed being pinched off from the ectoderm. The thickening of the ectoderm is accounted for by its rapid expansion, and it is to be expected that the rapid proliferation and condensation of the mesenchyme should bring it into contact with the ectoderm. Later, in embryos of 16 to 18 somites, the boundary between ectoderm and underlying mesenchyme becomes absolutely definite, except in the case of oblique sections, when it becomes difficult to delimit the ectoderm of the gill arch from the mesenchyme. Appearances figured by Veit and Esch (’22, fig. 7 as) are believed to be due simply to the adhesion of mesenchyme to the epidermis, although it is conceivable and not unlikely that occasionally cells may be detached from the ectoderm to be added to the mesenchyme.

Summary And Conclusions

1. The development of the neural folds of the rat is de~ scribed with the following results:

a. The rat ultimately develops seven typical rhombomeres. Rhombomere 1 is the cerebellar rhombomere. Rhombomere 2 is related to the trigeminal nerve, but rhombomere 3 is free of nervous connections. The facial nerve is attached to rhombomere 4 and the otic placode corresponds to rhombo— mere 5. Rhombomeres 6 and 7 are related to the IX-X anlage.

b. The expansion for the otic rhombomere appears earliest, i.e., at 3 somites.

c. A prominent sulcus (preotic sulcus)‘ lying in front of the otic rhombomeric expansion marks the site of rhombomere 3.

d. Rhombomere A1 appears anterior to rhombomere 3 at about 9 somites. Its anterior edge marks the cephalic limit of the hindbrain. It early gives evidence of its double value and soon divides into rhombomeres 1 and 2.

e. Rhombomeres 5, 6, and 7 differentiate in order caudal to rhombomere 4. It is doubtful if a primary rhombomere C can be recognized.

f. The recognition of primary rhombomeres is of doubtful value morphologically, but convenient for descriptive purposes.

g. The midbrain territory lies just caudal to the pronounced rostral flexure of the neural plate. Its floor is more restricted in extent than the more lateral expanded portions.

h. While it is impossible to carry the delimitation of regions into earlier stages than 4 or 5 somites, the conclusion that forebrain and midbrain are differentiated from a relatively small anterior portion of the neural plate seems justified.

i. Two more or less variable mesencephalic ‘segments’ are recognized.

j. Rhombomeres are regarded as expressions of a combination of growth factors——~rapid forward growth in a confined space and localized expansion looking forward to the development of the sensory mechanisms of the hindbrain. They probably have no phylogenetic import.


2. The neural crest is associated primarily with the neural plate, but whether it proliferates from the edges of the neural folds or from the roof of the neural tube seems to depend entirely upon the time of its appearance with respect to the differentiation and closure of the neural folds. Both conditions are found in the same embryo.

3. In the head region there are three neural—crest proliferations which give rise to the ganglia of the V, VILVIII, IX—X nerves. The proliferation for the fifth ganglion (rostral neural crest) extends along the territory of the prospective midbrain and rhombomere A1 of the 4- to 5~somi’te embryo. The proliferation for the VILVIII ganglion appears at 7 to 8 somites and is separated from the rostral neural crest in the territory of the preotic sulcus, and there is also a break in the neural crest between it and the IX-X anlage. The IX—X anlage can first be recognized at 8 somites, but it is not then in a state of active proliferation. It is from the first continuous with the spinal neural crest, which is actively proliferating at 8 somites.

4. The following conclusions are reached in regard to the development of the trigeminus ganglion:

a. The rostral neural crest (V anlage) soon loses its attachment to the midbrain and at 8 somites forms a mass attached to the neural plate for a short distance ahead of the preotic sulcus. The anterior portion of the rostral neural crest does not degenerate. There are three remaining possibilities as to its fate, viz.: 1) that it very quickly becomes diffuse, mingles with the mesenchyme, loses its specificity and individuality, and shares the fate of the mesenchymal cells in that region; 2) that it loses its attachment to the midbrain, becomes diffuse, and later recondenses to form the ramus ophthalmicus, or, 3) that the neural folds expand away from it, leaving it behind as the V ganglionic mass. The last is deemed most probable.

b. The ophthalmic ramus is not placodal in origin, since no placodal contacts can be found. It grows forward from the main ganglionic mass of trigeminus, keeping pace with the growth shiftings coincident with the establishment of the maxillary region, following closely the consequent shiftings of the optic vesicle.

0. The main ganglionic mass of the trigeminus has a broad area of contact with the epidermis, but apparently receives no contributions from it. No definite placodal structure is found at the place of contact and no cells could be observed actually proliferating from the ectoderm in a large number of embryos.

5. There is a well-developed epibranchial placode related to the VII ganglion.

6. The VIII ganglion is a derivative of the common acoustico-facial ganglionic mass, owing its origin entirely to neuralcrest proliferation.

7. The IX-X ganglia are derived from a continuous postotic neural-crest proliferation continuous with the spinal neural crest.

8. The IX ganglion establishes light contact with the ectoderm, but its epibranchial placode is less clearly delimited than that of VII. The ductus branchialis II is for a long time in contact with its lower pole.

9. The ganglion nodosum makes contact with the ectoderm on the dorsal edge of the cervical sinus and the cervical vesicle remains in contact with its lower pole for some time. The growth of the ganglion nodosum caudally over the territory caudal to the third branchial pouch is thought to be influenced by the expansion of the caudal pharyngeal complex.

10. Since no placodal contributions could be made out, the conclusion is reach ed that the ganglia of the V, VII-VIII, and IX-X nerves are entirely of neural-crest origin.

11. No organized contribution of ectodermal cells to the mesoderm occurs in the rat. .

12. The writer is inclined to regard the placodes as rudimentary sense organs differentiated under the influence of the ganglionic contact with the ectoderm. This does not imply that they have ever been functional in the course of phylogeny.

13. Ganglionic ‘spurs’ of the V and VII ganglia are described. It is suggested that these may furnish the neurilemma cells for the motor roots of these nerves, since the motor fibers grow out into them.

14. The morphology of the ganglia of the embryo seems to be determined by the growth of parts to which they are related. '


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Explanation of Figures

7 Model of the neural folds of a 1-somite rat embryo (Qer. 64) X 100. The egg chamber has been cut away from above and in front, exposing the dorsum of the embryo.

8 Model of the neural folds of a 2-to—3-somite rat embryo (Ser. 68 b) X 100. The egg chamber has been cut away from above and in front exposing the dorsum of the embryo.

9 Camera-lucida drawing of a section through the edge of the cranial neural folds in the territory of the prospective rostral neural crest of a 4—somite rat embryo (Ser. 287: 3: 3: 4 X 270), to illustrate the rearrangement of cells which occurs at the boundary between the edge of the neural folds and the ectoderm immediately preceding the active proliferation of neural crest. A slightly later stage of the rostral neural crest is illustrated in figure 25.

10 Ant.ero—lateral aspect of a model of the head of a 5-somite rat embryo (Ser. 110) X 100, showing the cranial neural folds. A section through the otic rhombomere is illustrated in figure 23.

11 Dorso—lateral aspect of the model shown in figure 10. X 100. Compare figure 1, which is a plotting of the same.

Abbreviations

Ec., ect-oderm P:r.ot.s., preotic sulcus Mcs., mesoderm Pr.st., primitive streak N.f., neural fold Rh., rhombornere 0p.f., optic fovea Som., somatopleure Pr.tn,f., primitive infundibulum Spl., splanchnopleure

9 Figures 7, 8, 10, 11, and 12 were drawn from models made by Miss Janet A. Williamson, who generously allowed me to use them here.


PLATE 1

PLATE 2

12 A11te1'o-la.tera.l aspect of a model of the head of a 6-somite rat 0lI11)1‘_')'0

(Ser. 80) )< 100. 13 Anterior aspect of a. model of the head of an 8-somite rat embryo (Ser.

8521.) X 100. Lateral and dorsal views of the same model are given in figures 14 and

ABBREVIATIONS A.br._, branchial arch Pm3?Lf., primitive infu11d'ibulum Op.f., optic fovea P-r.ot.s., preotic sulcus

126 NEURAL FOLDS AND CRANIAL GANGLIA OF RAT PLATE. 2 HOWARD B. ADELMANN

127 PLATE 3 EXPLANATION or FIGURES

14 Lateral View of :1. mode]. of the head of an 8-somite rat embryo (Ser. 85 a) X 100. The ectoderm and mcsoderm of the left side have been dissected away to reve:-.11 the anlagen of. the cranial gan.g1ia.. The extent of the otic placode is iiidicated by a broken line. Compare with sections of the same embryo (figs. 26 to 31).

15 Dorsal View of the same mode]. >( 100.

ABBREVIATIONS

A.b’}‘., branchial arch P-r.ot.s., preotic sulcus

1?,‘w,t., entoderm Rh.-., rhombomere

G., ganglion 8.1, first somite

Op.f., optic fovea. (p0st.ero-lateral View S.br., branchial pouch

in fig. 1-1) S.w,.e., neuro-ectoderma1_ sulcus

0t.pl., otic placode Sp.'n.c., spinal neural crest NEURAL FOLDS AND'CRANIAL GANGLIA OF RAT PLATE 3 HOWARD B. ADELMANN

1:4‘

PI’. 0L 8. Rm "

% Rh. 4

PLATE 4


16 Lateral aspect of a model of a 10—somite rat embryo, dissected to reveal the neural folds. The V and VII-VIII ganglia are outlined by a broken line. The IX—X. anlage which begins just caudal to the otic pla.code is not shown, because the plane (sagittal) was not favorable for its exac.t delimitation. The neural folds are still open everywhere cephalad of the a.nterior boundary of the fifth rhombomere, but it was not possible to determine the exact degree of fusion cauclal to that point. The point marking the ventral lip of the neuropore is indicated by an asterisk. A sagittal section of this embryo is illustrated in figure 21.

17 Lateral aspect of a model of the head of a 14-somite rat embryo (Ser. 228) X 100, dissected to reveal the neural tube and the anlagen of the cranial ganglia. The ot.ic pit and the thickened epithelium surrounding it are left in place. The neural tube is completely closed wit.hin the. limits of the model. The relation of the ganglia to external features of this embryo is shown in figure 4. Figure 33 is a photograph of a section through the trigeminal ganglion ofthe same.

ABBREVIATIONS

A.A., aortic arch Me.-Ii’.h., boundary between mesenA.LIr., branchial arch cephalon and rhombencephalon l")i.—M€., di—n1esencephalic boundary 0p.'v., optic vesicle

(4., ganglion 0t.pl., otic placode

}.{ep.(li1J., hepatic diverticulum Pr.inf., primitive infundibulum

.11., mesencephalic ‘segment’ .Rh., rhombomere

316., mesencephalon 8.1, first somit.e

8.?) r., branchial pouch

130 NEURAL FOLDS AND ORANIAL GANG-LIA_-OF RAT PLATE 4 HOW’.-&RD B. ADELMANN

% Rh; A.-1. Rh;3 _R h_. 4

‘T5’ Pr. ma” - Rh. 2 R113‘ tin.-1 G. IX-X G. V- .5, 'vn_vm Rh. 6% s. 1

Rh. 1 ma. 5 0:; pl. Rh. 7

‘ OP» '4 A. br.l


PLATE 5


18 Lateral View of a model of the head of an 18—somite rat embryo (Ser. 233) X 50. The model has been dissected to reveal the neural tube and the anlagen of the cranial ganglia. Sections of this embryo are illustrated in figures 34 and 35, 51 and 52.

19 Lateral View of a. dissected model of the hea.d of a _-26—somit-e rat embryo (Ser. 250) X 50. Sections of this embryo a.re illustrated i11 figures '39, 84 to 87.

ABBREVIATIONS

A.A., aortic arch 0t.*v., otic vesicle

A..bcr'., branchial arch Rh., rhombomere

Di-., diencepha.lon R.0p.V., ophthalmic ramus of the triI)'£—.—Mc., di-mesencephalic boundary geminus

G., ganglion 8.1, the first somite

Hcp.d'irv., hepatic diverticulum S.br‘., _branchia.1 pouch

L.b., lung bud ' - Ten, telencephalon

2%., meseliceplialic ‘segment’ Ult., ultimobranchial body

0pxv., optic vesicle



20 Rat, 9 somites, Ser. 223: 3: 3: 5 X 111.

Frontal section, showing rhombomeres A1 (1 and 2), 3, 4, and 5, and their relations to the V and VII-VIII ganglia and the otio placode. Note the double nature of rhombomere A, on the left, only faintly indicated on the right.

21 Rat, 10 somites, Ser. 91: 2: 3: 2 )( 111.

Sagittal section, showing the two mesenoephalic segments and the two internal sulci corresponding to rhombomere A1.

22 Rat, 3 somites, Ser. 315: 2: 2: 4 X .111.

Sagittal section, showing the expansion for the otic rhombomere and the preotie and postotic sulci.

23 Rat, 5 somites, Ser. 110: 4: 5: 6 )< 111.

Trztnseotion through the otie rhombomere, showing the difI"use ect-oderma] th'1e.kening which constitutes the lotic placode.

24 Rat, 8 somites, Ser. 221: 3: 2: 4 X 111.

A sa.gitt.a.1 section, showing the extent of the trigeminal anlage. The rostra] neural crest (V anlage) has lost its attachment to the midbrain and is now attnelred only to rhombomere A1.

ABBREVIATIONS G-., ganglion Pr.ot.s., preotie sulcus M., inesencephalic ‘segment’ Rh.,1'}1om'l)ome1'e Me-s.br., 1'>ra.nchia.1 mesoderm 8.1, the first somite 0t.pl., otic plaeode V4).-r., vns primitivurn 1'11on1be11cepl1a1i


PLATE 6

135 PLATE 7

25 Rat, 5 somites, Ser. 284: 2: 3: 7 X 123.

"I.‘ransection through the region of rostral neural crest. Note the neuroectodermal sulcus. Note a.lso that the neural crest is not continuous with the branchial mesoderm. An arrow marks its ventral limit.

26 Rat, 8 somites, Ser. 85a: 3:3: 6 X 123.

The section passes through the trigeminal anlage and the first branchial arch on the left side. Note the neuro~ect-odermal sulcus. The ganglionic anlage and the mesodermal condelisatipn of the first arch are distinct. An arrow marks the ventral limit of the ganglion.

27 Rat, 8 somites, Ser. 8:’) a: 3: 1: 6 X 123.

The section passes t.hrough the acoustico-facial ganglion on both sides. The right side of the section lies more anterior than the left side.

28 Rat, 8 somites, Ser. 85 a.: 2: 3: 10 X 123. Section through the IX-X anlage.

29 Rat, 8 somites, Ser. 85 a: 2: 3: 2 X 123. Section through the first somite. The spinal neural crest is being proliferated from the edges of the neural folds a.t this level.

30 Rat, 8 somit.es, Ser. 85 a: 2: 1: 10 X 123. The section lies caudal to figure 29. The neural crest lies dorsal to the closed neural tube at this level.

ABBREVI ATI ON S

Ec.l)'r., thickened ectoderm over bran- R.n..c., rostral. neural crest

chial region S.n..e., neuro-ectodermal sulcus (3., ganglion Sp.a.c., spinal neural crest El-[cs.b-r., branchial mesoder1n T/.p.'r., vas primitivum rhombencephali

0t.pl., otie placocle


PLATE 8

31 Rat, 8 somites, Ser. 85 a: 4: 1: 7 X 123.

This section passing through the optic pits illustrates t11e absence of any perioptic neural crest. There is no proliferation of neural crest from the lips of the anterior neuropore or from the margins or walls of the optic pits.

32 Rat, 11 somites, Ser. 218: 6: 1: 4 )< 123.

33 Rat, 14 somit-es, Ser. 228: 1: 5: 1 X 123.

Sections through the trigeminal anlage. Note the prominent notch at t.he boundary between ga.nglion and arch mesoderm. An arrow marks the ventral limit of the ganglion.

34 Rat, 18 somites, Ser. 233: 3: 1: 7 X 123.

Transection through the body of the trigeminal ganglion, to show its contact with the ectoderm. The ganglion and the mesoderm of the first arch meet at an obtuse angle. A ‘spur’ of ganglionic material extends from the base of the

ganglion ventrally along the sides of the neural tube. An arrow marks the lower limit of the ganglion.

35 Bat, 18 somites, Ser. 233: 3: 3: 3 X 123. Transection through the ophthalmic ramus of the trigeminus, to show its

independence of the ectoderm. Note the absence of any special ectodermal thickening related to it.

ABBREVIATIONS G., ganglion Sp.V., ganglionic ‘spur’ of the triMcs.br., branchial mesoderm ' ‘ geminal ganglion R.op.V., ophthalmic rarnus of the tri geminus


PLATE 9

36 Bat, 18 somites, Ser. 234: 4: 2: 8 X 123.

Oblique frontal section to show the eftect of oblique sectioning. The contactof the V ganglion with the ectoderm is shown. The oblique sectioning produces an appearance suggestive of proliferation which is 11ot reproduced on th.e other side of the same embryo (cf. text).

37 Rat, 21 somites, Ser. 246: 3: 2: 5 X 123.

Transeetion, to show the eetodermal contact of the trigeminal ga.nglion. Cells containing degeneration granules may be detected at the boundary between ganglion and mesoderm. Note the ganglionic ‘spur’ to which attention was called in the description of figure 34.

38 Bat, 26 to 27 somites, Ser. 258: 4: 2: 2 X 123.

Transection through the main ganglionic. mass of the trigerninus. The ganglion is no longer in intimate contact with the ectoderm. The ganglionic ‘spur’ is still well marked.

39 Rat, 26 to 27 somites, Ser. 250: 7: 2: 8 X 64.

Tra.nsect.ion through the ophthalmic ramus of the trigerninus to show its independence of the ectoderm still (see also fig. 40).

ABBREVIATIONS 133)., epibranchial placode Rh, rhombomere (L, ganglion R.op.'V., ophthalmic ramus of the triMes.br., hranehial mesoderm geminus 0t.'v., otie vesicle Sp, ganglionic ‘spur’



PLATE 10 EXPLANATION or FIGURES

40 Rat, 2 somites, Ser. 251: 2: 4: 8 X 64.

The section passes t.hrough a.lmost the entire extent of the ophthalmic ra.mus of the trigeminus and illustrates well its independence of the ectoderm. Tl1e slightly thickened ectoderm along its course is only a single layer in thickness and represents the upper margin of the diffusely thickened ectoderm over the branchial region (maxillary territory).

41 Rat, 12 sornites, Ser. 200: 1: 4: 3 X 123.

Transection, showing a postotic ectoderrnal thicke11ing which is not placodal in nature, but simply a thickening above the mouth which is continuous with the diffuse and extensive thickening of the bra.nchial ectoderm. The lnesodermal condensation above (really caudal to) the eye is continuous with the condensed Inesoderm of the mandibular arch.

42 Bat, 31 somites, Ser. 266: 5: 3: 3 X 123.

43 Bat, 34 somites, Ser. 265: 7: 2: 3 )( 123.

Transections, showing the invasion of the ganglionic ‘spur’ of the trigeminus by the motor root fibers. At 31 somites (fig. 42) the ‘spur’ is a loose mass of cells lying between the main ganglionic mass and the brain. At 34 somites it is much more diffuse and the delicate motor fibrils have grown out into its substance. Figure 43 has been retouched to bring out the motor fibers which photographed too lightly for reproduction.

ABBREVIATIONS

Eel)-r., thickened ectoderm over the R.op.V., ophtha.ln1'ic ramus of the tri branchial region geminus G., ganglion Sp, ganglionic ‘spur’ Mcs., condensed rnesoderm Vm., motor root of the trigeminus


PLATE 11

EXPLANATION or FIGURES

44 Rat, 9 somites, Ser. 220: 4: 3: 8 X 123.

Oblique transeetion through the acoustieo-faeia.l ganglion. The lower part of the figure lies caudal to the upper parts.

45 Bat, 11 somites, Ser. 213: 4: 2: 7 X 123.

Section through the VII-VIII ganglion.

46 Bat, 11 somites, Ser. 213: 4: 3: 1 X 123.

The section passes just anterior to the aeoustieo-facial ganglion on the left side and shows the dorsa.l extent of the mesoderm of the second branehial arch. On the right side a small part of the aeoustieo-facial ganglion is cut as well as the anterior edge of the otie plaeode.

47 Rat, 12 somites, Ser. 200: 2: 2: 13 X 123.

48 Rat, 13 somites, Ser. 204: 2: 5: 1 X 123.

49 Bat, 13 somites, Ser. 204: 2: 4: 7 X 123.

Figures 47 to 49 illustrate the relation of the aeoustico-facial ganglion to the eetoderm. Between 12 and 13 somit-es the aeoustieo-facial anlage begins to be separated from the eetoderin by the invasion of mesenchyme lateral to it. The boundary between ganglion and arch mesoderrn is marked by a pronounced median notch. The dorsal extent of the branchial a.r(:h mesoderm is well shown on the left side of figure 48.

ABBREVIATIONS E-e.?)'r., loranehial ectoderm Mes.br., branchial mesoderm G., ganglion 0t.pl., otie placode


PLATE 12

EXPLANATION or FIGURES

50 Bat, 14 sornites, Ser. 227: 1: 6: 17 X 123.

The VII-VIII ganglion is now separated from the ectoderm by considerable inesenchyme. The dorsal extent of tl1e a.rch mesoderin is shown on the right side of the figure. The section passes through the second branchial arch of both sides. Note that although the ectoderm is thickened for some distance above the dorsal level of the branchial arch mesoclerm, it does not come in contact with the gaiiglion; it shows no evidence of proliferation and a distinct placode is not distinguishable from the rest of the branchial ectoderm. The placode soon becomes well defined (16 to 18 somites. See figs. 51 and 52).

51 Rat, 18 somites, Ser. 233: 2: 4: 11 X 123.

Section through the Ventral tip of the VII-VIII anlage, the third section caudal to the first branchial pouch. The tip of the ganglionic mass just touches the epibraiichial placode. The arch mesoderm extends dorsally to the level of the ventral surface of the neural tube.

52 Bat, 18 somites, Ser. 233: 2: 4: 3 X 123.

Section passing throughthe attachment of the VII-VIII ganglion to the brain, eight sections caudal to figure 51. It illustrates the dorsal extent of the branc.hia.l arch mesoderm.

53 Rat, 14 somites, Ser. 209: 1: 2: 8 X 123.

Frontal section, showing the relation of the acoustico-facial ganglion to the ectoderm and to the otic placode. Note the prominent anterior lip of the otic pit. Rhomboineres 2 to 6 are well shown. In the lower left—hand side of the figure, the upper part of the rnesodermal condensation for the posterior branchial arches is cut.

ABBREVIATION S A.A., aortic arch Mes.br., branchial mesoderm Ec.br., thickened ectoderm over bran- 0t_pz_, otic pmcode chial region Rh., rhombomere Ep., epibranchial placode 77.01., V8113. capitis lateralis


PLATE 13

EXPLANATION 0-1+.‘ FIGURES

54 Rat,21somites, SeI'.246§w%3:4:5 x 123.

55 Rat, 21 somites, Ser. 246:3: 4.: 8 X 123.

56 Rat, 21 somites, Ser. 246£'3: 4: 13 X 123.

A series of sections through the acoustico-facial ganglion. Figure 54 is most ante:-i.or. Figure 56 most can! l. Note that the ventral tip of the ganglion extends into the angle between the dorsal end of the first branehial pouch and the epiI:)ra..nehia.l plaeode. '. '

57 Rat, 26 somites, Ser. 2543 4: 1: 9 x 128.

58 Rat, 26 somites, Ser. 254‘: 4: 2: 1 X 128.

A series of sections through the aeoust.ieo~faeial ganglion. Figure 58 is more caudal. Note how (fig. 57) 1.11% Ventral tip of the ganglion extends into the angle between the first. pouch. 2-11. 1. the epibranehial plaeode. In figure 57 special a.ttent.ion is called to the two’ n2lL;()ses Visi?~g= in the epibra.nchia.l plaeode. The spindle axes are parallel to the surface.

59 Bat, 26 somites, Ser. 254: 5: 1: 8 X 123.

Section through the aeo'ustic0-facial ganglion, showing how the ventral tip of the ganglion rests against. the epibranchial placode.

ABBRE-VIATIONS Ep., epib1'a11cl1ia.l plaeode V S.'b~r., bra.nchia.l pouch G., g::111glion Sp,, ganglionic ‘spur’ M es.br., br:1nel1ia.l Inesoderm V.c.l., vena. eapitis lateralis

()txv., otie vesicle

PLATE 14

EXPLANATION or FIGURES

60 Rat, 34 somites, Ser. 265: 6: 1: 9 X 139.

Tr:1.nsect.ion, showing the pla.eod:1,l rzlmus of. the genieulate ganglion.

61 Rat, 34 somites, Ser. 267: 13: 2: 3 X 139.

Front:1l section through the ventral end of the genieulate ganglion, showing the plaeotlal ramus and the great superficial petrosal nerve.

62 Bat, 27 somites, Ser. 258: 4: 4: 4 )( 185.

63 Rat, 34 somites, Ser. 265: 6: 2: 6 X 185.

Two tra.nsect.ions showing the ganglionic ‘spur’ of the aeoustieo-facial. It is especially well marked at 34 somites (fig. 63). In figure 63 a few mot-or fibers are found at the base of the spur. The photogra.ph h:’1.s been retouehed to bring out the motor fibers which photographed too lightly for reproduction.

ABBREVIATIONS

Ep., epibranchia.1 placode I VII, pl:1.e.o(la.l ramus from the tip of F.3)r., branehial celft genieulate ganglion to the ep'1br:1n— G., ganglion ehia]. plaeode A-".3.p., greater superficial petrosal Sp., ganglionic ‘spur’

nerve VII in-., motor root of the facial nerve


PLATE 15

EXPLANATION or FIGURES

6-1 Bat, 13 days, Ser. 294: 7: 1: 5 >_( 139.

The motor root fibers of V11 are now growing into the base of the ga.nglionic ‘spur.’ What remains of it now lies between the motor root and the base of the VII-VIII ganglion. The photograph has been retouched to bring out the motor fibers. '

65 Rat, 13 days four hours, Ser. 137: 6: 2: 4 X 139.

The ganglionic ‘spur’ is now completely obliterated, the motor root having taken its place. Unretouched photograph.

66 Rat, 31 somites, Ser. 266: 9: 1: 1 )< 139.

Figure showing the relation of the ga.11glionic crest to the motor root in the trunk region for comparison with figures 62 to 65. At 31 somites the ganglionic. crest is a lens-shaped mass closely applied to the neural tube and extending ventrally to the level at which motor fibers later emerge. At 34 somites (not. illustrated) the motor fibers emerge and the tip of the ganglion rests against the motor root for which it furnishes the neurolemnia cells. A similar relationship between the ganglionic ‘spur’ of the VILVIII and V ganglia and the motor roots of those nerves has suggested that the ‘spur’ may be the source of neurolcmma cells. I

ABBREVIATIONS (2., ganglion S-p.n.c., spinal neural crest Sp., ganglionic ‘spur’ VII m., motor root of the facial nerve

PLATE 16

EXPLANATION or FIGURES

67 Rat, 19 somites, Ser. 236: 2: 3: 2 X 111.

Sagittal section, showing t.he acoustico-facial anlage. It does 11ot rest against the otic vesicle at this time and no acoustic ganglion can be recognized.

68 Bat, 24 somites, Ser. 259: 5: 2: 2 )< 111.

The acoustic ga.nglionic mass is now growing away from the caudal aspect of the ac-oustico—fa.cia1 ganglion and rests against the otic vesicle.

69 Bat, 27 somites, Ser. 248: 4: 1: 7 X 111.

70 Bat, 30 somites, Ser. 111: 4: 3: :3 X 111.

Two later stages in the development of the acoustic ganglion. At 30 somites (fig. 70) the acoustic ganglion has acquired a separate attachment to the brain and is rather distinctly demarcated from the geniculate ganglion.

ABBREVIATIONS F.br., branchial cleft 0t:v., otic Vesicle G., ganglion S.b?'., branchial pouch


PLATE 17

EXPLANATION or FIGURES

71 Rat, 9 somites, Ser. 220: 4: 1: 7 ‘X 139.

An oblique section through the IX—X anlage. The lower part of the figure lies cephalad of the upper portion. T.he IX-X ganglion lies just beneath the ectodcrrn lateral to the vas primitivum rliornbencepliali. The section passes through the caudal edge of tlie otic placode.

72 Rat, 11-12 somites, Ser. 219: 7: 2: 5 X 1.39.

73 Rat, 11——12 somites, Ser. 219: 7: 1: 5 X 139.

74 Rat, 11——12 somites. Ser. 219: 6: 5: 8 X 139.

A series of sections, showing the relation of the IX-X crest to the vas_ primitivum rhombencephali. Figure 72 is most cephalad. The anlage of the IX-X lies just beneath the eetoderm. In figure 73 the vas primitivum rhombencephali ha.s just started its ventral bend toward the heart, the lower portion of its course (anterior ca.rdinal vein?) being cut in figure 74. In figure 74 both sides of the embryo are shown. The section is quite oblique. The right side lies anterior to the left, passing on the right through the curved surface of the caudal wall of the second branchial arch, whence the exaggerated thickness of the ectoderm on that side. The section is also oblique dorso—ventrally, passing more anteriorly ventrally than dorsally. On the left of this figure the neural crest has been cut at the place of transition between the IX—X crest and the spinal neural crest, i.e., just after it has curved around the caudal wall of the upper portion of the anterior cardinal vein and just ahead of the first somite.

75 Bat, 13 somites, Ser. 204: 2: 2: 3 X 139.

76 Rat, 1.3 somites, Ser. 204: 2: 1: 4 X 139.

The IX-X a.nlage is still superficial, but a new vascular channel (/v.c.1.) is just beginning to be established lateral to it (fig. . On the left side of figure 76 the vas primitivum rhombencephali seems to be surrounded by neural-crest cells. In figure 76 the anterior cardinal vein is cut longitudinally on the right. The neural crest lies lateral to it, but more caudally passes around itszposterior surface and becomes continuous with the spinal neural crest.

ABBREVIATIONS

Ec.br., thickened ectoderni over bran- 0t.pl., otic placode chial region ‘ S.br., branchial pouch G., ganglion 'V.c.a., anterior cardinal vein Mes.b-r., branc. mesoderm ' V.c.Z., vena capitis lateralis N.c., neural crest V.p.r., vas primitivum rhombencephali

PLATE 18

EXPLANATION OF FIGURES

77 Bat, 14 somites, Ser. 228: 2: 1: 11 >( 148. 78 Bat, 14 somites, Ser. 228: 2: 1: 17 X 148. 79 Bat, 14 somites, Ser. 228: 2: 2: 11 )< 148.

80 Bat, 14 somites, Ser. 228: 2: 3: 12 )( 148.

A series of sections through the IX-X anlage. 011 the right of figure 77 the new Venn eupitis lateralis is seen to be well est:'1b1ished 1atem.l to the IX-X uiilage, wl'1iel1 is now sepzwzzted from the eetoderm by considerable mesenchyme. The old VHS primitivum rl1om1'Je11eepl1:1li lies embedded in the ganglionic mass (right side of fig. 77), or it may be oblitemted (fig. 78, left). As one proceeds caudally (fig. 79), the crest becomes exeeediiigly diffuse and difiicult to delimit and fi11:1ll.}' becomes continuous with the spinal neural crest (fig. 80).

ABBREVIATION S

Er*.b2'., thiekeiied eetoderm over bra.n- 8.1, first somite

chin] region 8.?)-r., -hr:,u1ehial pouch G., grmglion Sp.n.c., spinal neural crest Me.9.(n"., b1-:111e.l1ial mesoderm V.('.l., vena eapitis lateralis

012121., otie plaeode


PLATE 19

EXPLANATION OF FIGURES

81 Bat, 21 somites, Ser. 246: 4: 2: 9 X 148.

82 Rat, 21 somites, Ser. 246: 4: 3: 3 X 148.

Two sections through the IX-X anlage. Figure 81 passes through the slender IX anlage just caudal to the second branchial pouch. The IX anlage extends ventrally to slightly above the dorsal level of the dorsal aorta, to which point the condensed arch mesoderm extends dorsally. The downgrowth for the IX ganglion measures 30 ,u. in.a.ntero-posterior extent at this time, but subsequently increases greatly in size through further proliferation from the common IX-X ganglionic mass. Figure 82 passes through the caudal portion of the common IX-X anlage, which further caudally becomes continuous with the spinal neural crest. No definite vagus downgrowth can be recognized as yet. This section passes through the caudal portion of the ‘third pharyngeal complex’ and shows well the level to which the condensed arch mesoderm extends dorsally.

83 Bat, 24 somites, Ser. 251: 5: 3: 5 X 148.

A section through the vagus anlage which is first recognizable at this time. Note that it lies lateral to the vena vapitis lateralis.

84 Bat, 26 somites, Ser. 250: 4: 5: 8 x 148.

A section through the IX anlage. It extends ventrally, at this time, to a point (marked by arrow) a little below the upper border of the dorsal aorta. In two more caudal sections of the IX anlage of the same embryo (fig. 85, 86) its ventral limit lies approximately at the dorsal border of the dorsal aorta.

ABBREVIATIONS A.br., branchial arch Mes.br., branchial mesoderm Ep., epibranchial placode V.c.l., vena capitis lateralis

G ., ganglion

PLATE 20

EXPLANATION on FIGURES

85 Bat, 26 somites, Ser. 250: 4: 5: 4 )< 123.

86 Rat, 26 sornites, Ser. 250: 4: 4: 5 X 123.

87 Rat, 26 somites, Ser. 250: 4: 3: 8 X123.

Three sections through the IX aulage of the same embryo as figure 84. Figure 86 passes t.hrough the caudal edge of the ganglion. It is just about to join the two communicating cords shown in the next figure, which is a section just caudal to the IX anlage, showing the two eorn1nunie:1t.ing cords, of neural-crest origin, between the IX and X ganglionie anlagen.

88 Rat, 26 somites, Ser. 247: 5: 1: 9 X 123.

Transeetion through the X ganglion, which lies la.t.e1'a1 to the vena eapitis 1o,teralis. The anlage passes around the ventral surface of the vein to join the ventral eomrnunicating cord of the IX and X zinlogen.

ABBREVIATIONS A.A., aortic arch Mes.br., l'>r:111e.l1io.l mesoderm I).0.c., dorsal communicating cord be- S.br., braneliial pouch tween IX and X anlagen 'V.c.c., ventral communicating cord, beEc.br., branehial ectoderm tween IX and X anlagen (4., ganglion V.c.l_., vena eapitis lateralis


PLATE 21

EXPLANATION or FIGURES

89 Bat, 29 somites, Ser. 124: 11: 2: 3 X 111.

Transection through the vagus anlage, showing how it curves around the ventral surface of the vena capitis lateralis to join the ventral communicating cord between the IX and X anlagen. A 11ew venous channel is now present lateral to the X anlage, so that the latter is included within a venous ring. Compare figures 89 and 88, noting the absence of any mesenchymal condensation in the former above the level of the dorsal aorta.

90 Bat, 31 somites, Ser. 266: 8: 1: 8 X 111.

91 Rat, 31 somites, Ser. 266: 8: 2: 1 X 111.

Two sections passing through the IX anlage. Figure 90 passes through the caudal surface of the second branchial cleft and figure 91 lies a little more caudal. The figures show how the ventral extremity of the ganglion rests against the epibranchial placode in the angle between the pouch and the placode.

92 Bat, 31 somites, Ser. 266: 8: 3: 5 X 111.

Section through the dorsal and ventral communicating cords of the IX and X ganglia, showing the ventral cord passing ventral to the vena capitis lateralis to join the ganglion nodosum farther caudally.

ABBREVIATIONS

A.A., aortic arch M’es.br., branchial mesoderm I).c.c., dorsal communicating cord be- V.c.c., ventral communicating cord be tween the IX and X anlagen tween the IX and X anlagen Ec.br., thickened ectoderm over bran— V.c.l., vena capitis lateralis

chial region V.c.l.2, branch of the vena capitis latEp., epibranchial placode eralis around the lateral surface of 0., ganglion the vagus anlage


PLATE 22

EXPLANATION OF FIGURES

93 Rat, 31 somites, Ser. 266: 8: 4: 5 X 111.

Transaction through the X ganglion.

94 Bat, 31 somites, Ser. 266: 9:2: 1 X 90.

Transection through the caudal extension of the ganglion nodosum. No ectodermal contact ha.s been established a.s yet. Note the caudal extension of the vagal crest which lies against the neural tube and which becomes continuous with the spinal neural crest farther caudally. The section passes through the level of the ultimobranchial body. Note the thickened branchial ectoderm which later becomes included in the cervical. vesicle.

95 Bat, 34 somites, Ser. 265: 4: 3: 7 X 60.

Transection through the IX anlage, showing the first indication of a division into ganglion of the root and ganglion petrosum.

96 Rat, 34 somites, Ser. 265: 4;: 3: 1 X 60.

Transection through the dorso-caudal wall of the ultimobranchial body, show» ing the extensive ganglion nodosum which extends farther ventrally and ca.uda.llv in sections just caudal to the ultimobranchial body.

ABBREV1 ATI ON S

Ec.br., thickened ectoderm over bran— Ult., ultimobranchial body

chial region V.c.l., vena capitis lateralis 0., ganglion V .c.Z.2, branch of the vena. capitis "latMes.br., branc.hia.l mesoderm eralis lateral to the X ganglionic N.c.X, caudal extension of the dorsal anlage portion of the ganglionic anlage


PLATE 23

EXPLANATION on FIGURES

97 Rat, 34 somites, Ser. 267: 12: 3: 3 X 154.

Frontal section, showing the contact of the IX ganglion with the ectoderm of the ductus branchialis II.

98 Bat, 34 somites, Ser. 267: 12: 1: 6 )< 154.

Front:-1.1 section, showing the ectodermal contact of the X ganglion and the superior laryngeal nerve with the thickened ectoclerm caudal to the third cleft. The vague placode never becomes separable from the diffusely thickened branchial

ectoderm caudal to the third cleft.

ABBREVIATION S

.D.b'r._, ductus branchialis G., ganglion Ep., epibranchial placode N.l.s., s11perior_-laryng-eal nerve F.br., branchial cleft "


PLATE 24

EXPLANATION on FIGURES

99 Rat, 12 days 18 hours, Ser. 142: 9: 3: 1 X 139.

Transection, showing contact of vagus with the ectoderm. The cervical sinus is beginning to deepen as a result of postbranchial expansion, the ectoderm related to the vagus ultimately becoming included in the cervical sinus.

100 Bat, 13 days, Ser. 135: 8:41:41 X 75.

The cervical vesicle is being cut off from the exterior due to the expa.nsion of the postbranchial region, and the vesicle so cut off lies in conta.ct with the lower pole of the ganglion nodosum. Note the small size of the third arch.

101 Rat, 13 days 4 hours, Ser. 168: 11: 2: 10 )< 139.

Section, showing the relation of the ganglion nodosum to the cervical vesicle. The vesicle lies in contact with the lower pole of the ganglion at the point of origin of the superior" laryngeal nerve.

ABBREVIATION S

A.br., branchial arch S.br., branchial pouch

Ep., epibranchial placode 8.0., cervical sinus

G., ganglion Ult., ultiinobrancliial body N.Z.s., superior laryngeal nerve V.C., cervical vesicle I N.XII, hypoglossal nerve II

Footnotes

  1. Since the completion of this work, I have had the opportunity of examining the manuscript of Bartelmez and Evans’ (’25) significant monograph on “The development of the human embryo during the period of somite formation, including embryos with 2 to 16 pairs of somites,” in which they deal exhaustively with the development of the neural folds and cranial ganglia. The exchange of manuscripts was effected through the kindness of Prof. C. J. Herrick. Footnotes will call attention to comparisons made in the course of the paper.
  2. Bartelmez (’25) now believes that rhombomeres 3 and 4 appear at first as a single ‘segment’ (rhombomere B ).



Cite this page: Hill, M.A. (2019, January 24) Embryology Paper - The development of the neural folds and cranial ganglia of the rat. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_development_of_the_neural_folds_and_cranial_ganglia_of_the_rat

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