Paper - The development of the paraphysis and pineal region in mammalia
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Warren J. The development of the paraphysis and pineal region in mammalia. (1917) J. Comp. Neural. 28(1): 75-134.
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- 1 The Development Of The Paraphysis And Pineal Region In Mammalia
- 1.1 Introduction
- 1.2 Part I. Sheep Embryos
- 1.3 Part II. Human Embryos
- 1.4 Part III. Iviammalian Embryos In General
- 1.5 Conclusions
- 1.6 Bibliography
- 1.7 Plates
The Development Of The Paraphysis And Pineal Region In Mammalia
From The Anatomical Department, Harvard Medical School
The object of this paper is to describe the appearance of the primary arches in the roof of the forebrain which have not as yet been reported in mammalia and also the presence of a mammalian paraphysis, as there has been much doubt as to its existence in any members of this class of Vertebrates. The other features of this region, the velum, choroid plexus, epiphysis and commissures will only be discussed in a general way, especially in the older stages, .as a considerable amount of research has already been devoted ‘to their study and description.
Part I. In Part I the development of the pineal region in sheep embryos will be considered, as these embryos in practically every case studied from 21 mm. up to 48 mm. possess a distinct paraphysis and differ in this respect from all other mammalian embryos that were available for study in the Harvard Collection.
Part II. In Part II the development of certain special features only of the pineal region in human embryos will be discussed. These are the formation of the primary arches, which can be readily demonstrated in early human embryos; the formation of the paraphysis, which can be seen in a few specimens only as a very inconstant and a relatively rudimentary structure as compared with that of vertebrates below the mammalia; finally attention will be called particularly to a peculiar set of tubular outgrowths springing from the velar end of the postvelar arch and overhanging the paraphysal arch and the paraphysis when present. These will be referred to as the postvelar tubules or diverticuli.
Part III. Part III will contain a brief review of the development of the forebrain roof in embryos of the opossum, rat, rabbit, cow, pig, deer, cat and dog. Wax models on a scale of 40 diameters have been made to demonstrate the whole of the forebrain in sheep embryos from 9 mm. up to 48 mm. and additional models on a scale of 100 diameters to show the details of the paraphysis only in embryos of 28 mm. and 48 mm. Two drawings on the same scale are added to point out the histological structure of the organ in those two specimens. To illustrate the development in human embryos models were reconstructed of the whole forebrain at a magnification of 40 diameters of embryos of 15 mm., 16 mm., and 23 mm. Other models have also been reconstructed on a ‘scale of 80 diameters of the paraphysal arch, velum and oral or velar end of the postvelar arch only and are intended to demonstrate details of the paraphysis and of the velar end of the diencephalic roof. The figures of the wax models are in all cases reduced to one—half the linear dimensions of the Wax models themselves.
Part I. Sheep Embryos
1. The primary arches and the subdivisions of the forcbrain
The primary arches were first described by Minot (16) in Acanthias and have since been demonstrated in practically all vertebrates below Mammalia. In a previous article, Warren (24), the primary arches and the three main subdivisions or segments of the forebrain in vertebrates were considered and the previous work on these subjects was reviewed at length. The primary subdivisions of the forebrain consist of a telencephalic segment whose roof is formed by the paraphysal arch. This is separated by the velum from the first diencephalic segment (Kupffer’s parencephalon) with the postvelar arch and later the epiphysal arch in its roof. This in turn is followed by the second diencephalic segment bounded above by the pars intercalaris (Kupffer’s synencephalon) which contains a portion of the posterior commissure and is limited caudally by the groove and ridge between the fore- and midbrain. These subdivisions were demonstratedin Reptilia and also in the pig and sheep, Warren (24), figures 28 to 37. For further details on this subject and on the question of neuromeres readers are referred to the review and discussion of the previous investigations on the subdivisions of the forebrain in the above article.
figure 1 is from a model of a sheep embryo of 9.9 mm. sagittal series which shows the subdivisions of the forebrain as outlined in the previous paragraph into the telencephalic segment anterior to the velum, V, with the paraphysal arch, P.A., forming its roof, the first diencephalic segment the roof of which is made up of the postvelar arch only, P.V.A. (the epiphysal arch is not as yet formed) and separated by an internal ridge from the second diencephalic segment. The roof of the latter, the pars intercalaris, P.I., is practically filled by the posterior commissure, R0. The midbrain is composed of two segments separated from the forebrain and from each other by distinct internal ridges. Compare this model with those for Reptilia, Warren (24), figures 3, 4, 17. All of the primary arches and the velum are shown in figure 2 taken from an embryo of 14. mm. sagittal series and they correspond to the conditions already described in other vertebrates, Minot (16), Terry (22), Neumayer (17), Warren (24). The extension of the posterior com— missure into the forebrain is very striking and it is evident that this belongs partly to the forebrain and partly to the midbrain. The primary arches, including the pars intercalaris, and the three primary subdivisions of the forebrain can therefore be said to be definitely established in sheep embryos.
2. Paraphysal arch and paraphysis
The paraphysal arch is clearly seen in the last two figures and passes over into a relatively low velum. The earliest signs of a paraphysis could be made out in an embryo of 21 mm. (fig. 3). It is a tiny conical structure, P., springing from the telencephalic roof plate immediately in front of the velum. figure 4, an embryo of 26 mm., shows a good sized paraphysis, P., with a relatively wide opening lying between the foramina of Monro and the anlagen of the lateral telencephalic plexuses and inclined backwards towards the velum.
The next stage appears in figure 5 and is from an embryo of 29 mm. In the last stages the wall of the paraphysis consisted of cells similar to those in the roof plate of the forebrain. Here the wall of the organ has become much thicker, the distal end more solid and the cavity much reduced. The outline is quite irregular and this feature is shown clearly in figure 6, which represents a model of the paraphysis of the same embryo at 100 diameters. The View in the picture is from the front and outside of the brain looking in between the walls of the hemispheres in the direction of the guide line from the letter P in figure 5. The base of the paraphysis where it is attached to the telencephalic roof plate, is noticeably wide and thick and is prolonged into a narrow tapering extremity, which contains a partial cavity and is apparently the main portion of the original outgrowth. On each side are two lateral prolongations from the base showing a striking uniformity which are practically solid. Directly behind the paraphysis in the mid-line the postvelar arch inclines forward and upward above the organ. A swelling on its right extends into the cavity of the diencephalon. This represents the right half of the diencephalic choroid plexus, D.C.P. The velum, V, is indicated by a slight fold which is much better marked towards the mid-line and is not shown well in this view. This stage was selected on account of the peculiar appearance of the paraphysis, which was the most irregular specimen seen in any of the sheep embryos, though there were several other cases, notably in embryos of 21 mm. frontal series, H. E. C. no. 1686; 25 mm. frontal series, H. E. C. no. 1690; and 40 mm. H. E. C. no. 1691, where the structure was almost as irregular but varied in size. Its markedly solid formation differentiates it sharply from the tubular and glandular types so characteristic of Reptilia and Amphibia. In the following two embryos no paraphysis could be found, no. 1111, 24.4 mm. transverse series, no. 1240, 26.6 mm. transverse series, so that the paraphysis is not absolutely constant in the sheep embryos. figure 7 gives the details of a section along the line A-B (fig. 6), taken just above the opening into the brain. The thickness of the main part of the organ is well shown and the cavity has gradually faded out into two narrow prolongations, the most peripheral being its main continuation. One of the solid projections on the right is cut separately and both of these on the left. The general detail of structure can be seen in the drawing and compared with that of the brain wall and the double commencement of the diencephalic plexus also can be made out, D.C.P.
In an embryo of 34 mm., H. E. C., no. 1692, there is only a rudimentary paraphysis not as large as that shown in figure 4, while in an embryo of 40 mm., H. E. C. no. 1691, the organ is relatively small as compared with that shown in figure 5 and has a central projection with two smaller ones in front and behind which are nearly solid and contain only a slight trace of a cavity. The last three figures are taken from the oldest specimen in the collection, an embryo of 48.4 mm., H. E. C. no. 1696. In figure 8, representing a model of the forebrain, there is a picture of the paraphysis in median section, and in figure 9 a View of the paraphysis is given practically similar to that seen in figure 6. One is looking from the front and a bit to the right of the midline at the outer surface of the lamina terminalis, L.T., and of the paraphysis, P. Behind the paraphysis the diencephalic choroid plexus, D.C.P., bulges backward into the diencephalon. The space between the paraphysis and the wall of the plexus is of course filled with vessels and connective tissue which could not be modeled. The base of the paraphysis is nearly as broad as that shown in figure 6 and is likewise prolonged into a narrow tube which is hollow up to the tip, but no lateral projections are seen. A distinct fold, V, marks the position of the velum. figure 10 is a section through the paraphysis and the diencephalic plexus at the line A-B (fig. 9). The section is through the broadest part of the organ, the wall of which is thick and well defined and stands out in contrast to the cells covering the folds of the plexus. The cells in the paraphysis form a double row while those covering the folds of the plexus are for the most part arranged in a single row. This specimen is hollow and differs in this respect from the more solid types seen in the embryos of 29 mm. and 40 mm. and a good view of the character of the tufts of the diencephalic plexus is given in the picture. The paraphysis is thus shown to be practically a constant structure in embryos of 21 mm. to 48.4 mm. and differs markedly from the elongated tubular organ of Reptilia and the complicated glandular structures of Amphibia, its chief characteristics being its short, broad and irregular form and its solid character.
3. Velum tromsversum
The velum appears in some of the earliest embryos and can always be clearly followed up to the oldest stages studied for this article. At first it forms a comparatively slight fold in the roof of the forebrain which, with the elongation of the postvelar arch, becomes more accentuated especially towards the median line, but as the choroid plexuses develop it tends to be more obscured. The observations made here agree with those previously made in other Mammalia, Johnston (12), and with Neumayer’s pictures of early sheep embryos (17). See also Ziehen (25).
4. Postvelar arch
At first the postvelar arch forms a short curve in the roof of the first diencephalic segment and later with the appearance of the epiphysal arch becomes more elongated. The diencephalic choroid plexus begins to form at the velar end of the arch and appears at first in embryos of about 25 to 26 mm. (fig. 4, D.C.P.). The plexus develops at first on either side of the median line forming two marked tufts of tissue bulging into thediencephalon and beginning just behind the diencephalic leaf of the velum. After a short distance the separate tufts become more or less condensed into a single median tuft which invaginates the diencephalic roof plate in the mid-line (D:C.P., figs. 6 and 7). In an embryo of 32 mm; the plexus has involved a large part of the postvelar arch and in the oldest specimens has extended dorsally nearly to the epiphysis. The size and character of this formation are well shown in figure 10. The lateral choroid plexus is first seen in embryos of about 20 mm. arising on either side of the telencephalic roof plate and of the opening of the paraphysis. Bailey in two excellent papers, the one on the “Morphogenesis of the Choroid Plexuses,” the other on the “Morphology of the Roof Plate of the Forebrain and Lateral Choroid Plexuses in the Human Embryo” has considered so carefully the origin of the plexuses, especially that of the lateral plexus, that it is unnecessary to add anything further here.
5. The epiphysvis
The epiphysal arch appears first in an embryo of about 14 mm. (fig. 2) and is relatively short but well marked. It continues in approximately this condition up to about 24 or 26 mm., when its walls thicken and it becomes more elevated above the surface of the brain. It now may truly be called the epiphysis and both commissures are placed close against its anterior and posterior Walls (fig. 4). In figure 5 it is more clearly defined and begins to incline slightly backward a position which is more strikingly shown in the oldest embryos (fig. 8). In the specimens studied there was no sign of the differentiation of a pineal eye and an epiphysis, a condition which seems to be peculiar to Reptilia, and the structure is relatively much suppressed as compared with its development in birds, Reptilia and Amphibia. As it is intended to indicate only the early appearance and topographical position of the epiphysis in this paper, no attempt will be made to consider its histological structure. For histological details see Jordan (13) on the epiphysis of the sheep and Jordan (14) on the structure of the same organ in the opposum. For a review of the appearance of the epiphysis in other vertebrates see Warren (24). ‘6. The commissures
The posterior commissure appears relatively early in sheep embryos and at first begins partly in the forebrain, in the hinder end of the pars intercalaris. In this respect it resembles the early form of the posterior commissure in Reptilia but is even more precocious in its development. It apparently must be regarded as belonging partly to the forebrain and is not Wholly confined to the midbrain as is usually stated. It is often difficult in the younger stages to determine accurately its limits, which tend to gradually fade out into the outer layer of the brain wall, but in all the younger stages it had approximately the extent as shoW.n in figures 1 and 2. In embryos of about 20 mm. the commissure has completely filled up the pars intercalaris and its cephalic end lies directly against the dorsal wall of the epiphysis. This same condition is also found in young human embryos of from 10 to 20 mm. The extent of the pars intercalaris in the roof of the early mammalian brain, Where it really should be considered as one of the primary arches or segments in the roof of the brain, is one of the most striking features in these embryos. It is constant in all Vertebrates and always precedes the superior commissure except in ammocoetes.
The superior commissure can first be observed in sheep embryos of 24 to 26 mm. and lies directly against the anterior Wall of the epiphysis. It occurs in practically all vertebrates including man and its development has been thoroughly described by Minot (16), Neumeyer (17), Cameron (3), Osborn (18), and Dexter (4).
No trace of the velar commissure described in Reptilia by Elliot Smith and others occurs in the Mammalia studied for this paper.
1. The primary arches consist of the paraphysal arch, the postvelar arch, the epiphysal arch and the pars intercalaris (synencephalic arch) and together with the velum are formed in.the roof of the forebrain of early sheep embryos.
2. The paraphysis can be followed in practically all sheep embryos from 20 mm. up to 48 mm. It is characterized by its short, broad and irregular outline and its solid structure, the cavity being in most cases reduced to a minimum.
3. The diencephalic choroid plexus and lateral telencephalic plexuses are well marked and develop essentially as described in other vertebrates. .There is no trace of the median telencephalic plexus so noticeable in Amphibia. V
4. The epiphysis forms a short hollow stalk with thick walls and inclined slightly backward over the posterior commissure.
5. The superior and posterior commissures are formed as in other vertebrates. The posterior commissure is characterized by its precocious development and by the extent that it invades the pars intercalaris of the forebrain in early embryos.
Part II. Human Embryos
1. Primary arches in the roof of the forebrain
These structures have not as yet been described in human embryos and their present demonstration fulfils the prophecy of the late Professor Minot made in 1901 that they would eventually be found in all vertebrates, Minot (16). In an embryo of 10 mm. (fig. 11), the primary arches in the roof of the forebrain are as clearly marked as in any of the lower vertebrates. The postvelar arch is relatively short and thick, while the epiphysal arch is, on the other hand, rather longer than in lower forms. The pars intercalaris is quite extensive and contains in its posterior end a portion of the posterior commissure as was the case in the corresponding stage in sheep embryos. In 84 JOHN WARREN
an embryo of 15 mm. (fig. 12),‘a slight hint of a paraphysis appears in the paraphysal arch. The postvelar arch has become greatly elongated and thinner, while the epiphysal arch has become much condensed with thicker walls. There is a long pars intercalaris and the posterior commissure has continued its encroachment therein. In figure 13, a picture of the median section of the forebrain in an embryo of 23 mm., the arches have passed out of their primitive condition and have assumed their more advanced characteristics. The long extent of the postvelar arch is noticeable and the peculiar modification of its velar end has commenced. The velum is small but distinct and a small paraphysal outgrowth is seen in the much reduced paraphysal arch. The epiphysis has been definitely formed and the posterior commissure has developed forward through the whole extent of the pars intercalaris up to the dorsal wall of the epiphysis. No sign of a superior commissure could be seen in the embryo, though Bailey (2) shows it in an embryo of 19 mm.
1 The pars intercalaris has now become much reduced in length due to the increased development of the midbrain and corresponds in this way to a similar process in lower vertebrates.
2. Pamphysal arch and pamphysis
Very few observations had been made on the pineal region in human embryos until the appearance of Bai1ey’s (2) paper in 1916. Selenka (19) gave the first description of the paraphysis in opossum embryos, but showed no figures of the structure. D’Erchia (5) has described the paraphysis as a simple fold in an embryo of about 30 mm., and Francotte (8) shows a picture of a section through a tubular paraphysis in a three weeks embryo. In a previous article, Warren (24) figure 39, there appeared to be a distinct paraphysal outgrowth in a section of an embryo of 28.8 mm., H. E. C. no. 1598, but as will appear later this observation was incorrect. Bailey (2) described the roof of the forebrain in three embryos of 19 mm., 28 mm., and 32 mm. respectively. His observations were based especially on the development of the choroid plexuses. As regards a paraphysis, he states that no glandular structure was present in his specimens but that the paraphysal arch was in all cases well defined. The velum was also clearly traceable in his embryos and my own observations as to the velum and the paraphysal arch coincide with his results. I have been able,. however, to demonstrate the presence of a paraphysis. It exists in a rudimentary form as compared with that of lower vertebrates and is very inconstant. Out of the embryos in the Harvard collection used in the preparation of this paper I have been able to find it in only eight cases. These specimens were embryos in good preservation and any that were at all damaged in the region of the forebrain were excluded, though two or three of‘ these showed signs of a possible paraphysal outgrowth. It is owing to the relatively large number of human embryos in the Harvard Collection that the writer was fortunate enough to find the few specimens containing a paraphysis, which will here be described.
In an embryo of 15 mm. (fig. 12), a slight median elevation is seen in the paraphysal arch in the true morphological position of a paraphysis. It is composed of‘ a solid clump‘ of cells with no cavity with the exception of a very tiny dimple on the under- side of the arch just beneath it A similar structure could be found in other embryos of about the same size, H. E. C. no. 2044, 16 mm. and H. E. C. no. 1707, 16.4 mm. From the above stages up to embryos of 23 mm. no signs of a paraphysis could be found with the exception of three doubtful cases in embryos of 19 to 20 mm. which were, however, excluded as the roof of the forebrain was somewhat damaged. In an embryo of 23 mm. (fig. 13), however, a tiny paraphysis with a slight cavity is clearly seen. This model was made from a sagittal series and the paraphysis only extended through four sections and was at first overlooked. Three other embryos of approximately this same size showed no signs of any paraphysal outgrowth and had a simple paraphysal arch as shown by Bailey (2), figure 18.
figures 14, 15 and 16 are taken from two models of the paraphysal and velar region‘ of the roof of the forebrain of an embryo of 25 mm. This embryo was’ in an excellent state of preservation and has the best specimen of a paraphysis of any in the Collection.
figure 14 shows a median section of this part of the roof of the forebrain. The velum, V, forms a deep fold and immediately in front of it can be seen a well marked paraphysis, P, with a wide opening and a large cavity. On the opposite side of the velum the diencephalic roof bulges forward and overhangs the paraphysis. In figure 15 there is a view of the external surface of the roof of the forebrain in this same region seen from in front and a little to the right as in figures 6 and 9. The paraphysis, P, overlies the telencephalic roof plate. Immediately behind the velum, V, arise several outgrowths from the diencephalon, the so-called postvelar tubules to be considered later. It is easy to see that in a series of sections at right angles to the telencephalic roof plate these prolongations might be mistaken for a paraphysis if that organ was actually absent, and it is not easy in such a series to determine at once on what side of the velum these outgrowths really belong until an actual model has been made. '
figure 16 represents the same model seen from the interior of the brain to show the relative positions of the opening of the paraphysis and the openings of the postvelar tubules with reference to the velum and the telencephalic and diencephalic roof plates. The extent of the remainder of the diencephalic roof and the position of the epiphysis and commissures are well shown in Bailey’s models of 28 and 32 mm. embryos ( (2) figs. 20 and 22). The next stage in which a paraphysis could be shown was an embryo of 36 mm., again a specimen in excellent condition. Between this specimen and the previous one the collection contained several embryos of 28.8, 29, 30 and 31 mm. In an embryo of 31 mm., sagittal series, there was a very slight elevation in the paraphysal arch close to the velum which might be regarded as a mere rudiment of a paraphysis (fig. 17). In the others there was no sign of a paraphysis. The telenceph alic roof plate formed a well defined paraphysal arch similar to the condition shown by Bailey (2), figures 20 and 22. For misinterpretation of the paraphysis and velum see Warren (24), figure 29, Where the velum is incorrectly placed and what is labelled paraphysis should really be a postvelar outgrowth from the diencephalon.
figure 18 is from a model of a median section of the forebrain of the 36 mm. embryo similar to figure 14. The velum corresponds to that in the 25 mm. embryo and in front of it a relatively small paraphysis, P, projects forward from the telen— cephalic roof plate. It is connected to the telencephalon by a solid stalk with a slight depression indicating the original opening and a small cavity persists in the distal end only. The paraphysis and telencephalic roof plate are completely buried under the mass of tubules which spring from the postvelar arch and extend forward. In three embryos of 37, 40, and 42 mm. respectively, all in good preservation, there was no sign of any sort of a paraphysis. An embryo of 44.3 mm., the oldest embryo in the collection, isrepresented in figure 21, which gives an external view of the roof of the telencephalon and dienceph— alon similar to figures 15 and 19. Here a tiny paraphysis, P, can be seen in the midline of the telencephalic roof buried under the mass of diencephalic tubules. It extended through a very few sections, only one of which had a cavity, and is attached by a thin solid stalk to the brain. The paraphysis in this specimen is about to degenerate and entirely disappear.
Of all the embryos studied, excluding doubtful cases and slightly damaged specimens, a more or less rudimentary paraphysis could be found in only eight instances. It therefore may be said to exist in human embryos as a very inconstant and variable structure.
3. Postvelar arch and postvelar tubules or diverticuli
The earliest sign of this peculiar modification of the velar end of the postvelar arch appears in embryoslof from 19 to 23 mm. In figure 13, an embryo of 23 mm., it forms a simple transverse fold somewhat tent-shaped in the mid-line. This was earlier mistaken by the author for a paraphysis, as the real paraphysis was here so small that it was at first overlooked.
In three or four other embryos of approximately this same size a similar formation could be seen which in some cases was carried still further forward into a finger—like projection in the mid-line, which gave in the section the impression of a small rounded tube whose walls were rather thicker than the diencephalic roof. The embryo of 25 mm., a median section of whose brain is seen in figure 14, shows this projection more distinctly. figure 15 gives an external view of the same specimen, where the details of the tubules can be clearly seen. There are two main pouchlike projections on either side of the mid-line, which show a tendency to give off smaller tubules and to cover up the paraphysis. They are limited below by the fold in the brain wall formed by the velum and have quite definite limits above, the extreme lower end of the postvelar arch being alone concerned in this formation. Their relations to the brain cavity are best seen in figure 16, which gives a View from the interior of the brain. In the lower part of the figure is the lamina terminalis, L.T., which passes over into the forebrain roof, T.R.P. The opening of the paraphysis, P, lies just cephalad to the velum, V, and there can be no doubt of its telencephalic origin. On the diencephalic side of the velum appear the two large pouches in the depths of which are the openings of the smaller tubules. This outgrowth as a whole begins at the diencephalic lip of the velum, involves practically the whole width of the postvelar arch at this point and ends fairly definitely above. Beyond this point the diencephalic roof arches upwards with a perfectly smooth and gently curved surface. This structure is clearly of diencephalic origin and should never be mistaken for the paraphysis.
Embryos of 28.8, 29, 30, and 31 mm. were next examined and in all of them some similar formation was to be seen, which varied from a relatively simple fold like that in figure 13 to a more complicated replica of what has just been described. The embryo of 31 mm., H. E. C. no. 1706, was selected in order to show the separation of some of the tubules from the brain and their formation into blind cysts or vesicles. figure 17 shows a model of this embryo seen from the front, as is the case with figures 15, 18 and 21. The paraphysal arch or telencephalic roof plate is here rather sharply elevated and then bends abruptly downward into the lamina terminalis, L.T., between the median walls of the hemispheres. In the depths of the model a very rudimentary paraphysis, P, can just be seen. The diencephalic roof is prolonged forward into a large projection a little to the left of the mid-line, below which several smaller tubules can be seen lying nearly side by side on either side of the mid-line. Tubule number 1 represents an absolutely blind vesicle, which is fused to a hollow stalk, number 2, that communicates by means of a narrow opening with the brain cavity. Number 3 also opens by a wide opening into the lower part of the large projection. Number 4 is a closed sac and is fused to the wall of number 5. It is almost exactly similar to number 1, while number 5, like number 2, communicates with the brain by a narrow opening. Histologically the walls of vesicles 1 and 4 are thinner than those of the others and show signs of degeneration. It is evident that they represent the distal ends of the original tubular outgrowths which are about to become detached entirely from the brain and probably will eventually disappear.
Figure 18 is from an embryo of 36 mm. which was in an excellent state of preservation and, together with the 25 mm. embryo shown in figures 14, 15 and 16, represent two of the best specimens in the Harvard Collection. It shows a median section of this part of the brain. The velum forms here a well developed fold and beginningiimmediately above it on the diencephalic side a large tubule partly subdivided is thrust forward, which extends over nearly the whole of the telencephalic roof plate. Above this tubule is seen the section of a larger diverticulum. In figure 19 an external View of these tubules is seen similar to that in figure 15. Two large diverticuli are thrown out above from either side of the mid-line enclosing a smaller one between them, a section through which appeared in the previous figure. Below appears a number of smaller and more irregular tubules lying on the telencephalic roof and completelv burying the paraphysis. At least two of these have become blind vesicles, though still fused to the wall of a neighboring tubule. Their ‘walls are thinner than those of the others and are evidently about to separate off and probably degenerate. figure 20 gives a view of the same model seen from the inside of the brain. The velum, V, forms a well marked ridge, in front of which a slight depression in the telencephalicroof plate marks the former opening of the paraphysis. On the diencephalic side of the velum the roof plate ‘shows two large irregular openings separated by an irregular ridge in the mid-line, the one on the right being much larger than the one on the left. In the depths of these larger diverticuli are seen the openings of the smaller tubules, the majority of which communicate with the right diverticulum. It can blearly be seen that all of these represent outgrowths or evaginations from the roof plate and differ in this respect from the usual invaginations or ingrowths of the brain wall found in plexus formations. Above the larger prolongations the diencephalic roof plate sweeps gradually upward with only a slight median depression. In the depths of these tubules were two or three closed vesicles either entirely cut off from the brain wall or still partly fused and showing the same signs of degeneration as was seen in the 31 mm. embryo (fig. 17). An embryo of 37 mm., H. E. C. no. 820,. has a much simpler arrangement in this region. The evagina—tions from the diencephalic wall formed three fairly large pouches, one in the mid-line with two smaller ones on either side, and there was no sign of any smaller. tubules or detached vesicles, While in an embryo of 40 mm., H. E. C. no. 1917, there is a. simple medial pouch pushed forward over the telencephalic roof plate with a very few tiny outgrowths from it. An embryo of 42 mm., H. E. C. no. 838, showed practically the same conditions as in the 36 mm. embryo (fig. 19), and in the oldest embryo in the collection which is shown in figure 21. There was in this embryo, however, some little shrinkage in the walls of the postvelar tubules, which must have caused some slight distortion, but the general plan however is essentially similar to that shown in figure 19. There is one especially prominent tubule appearing below and in the mid-line. The other specimens almost always had some similarly placed tubule which at first caused confusion in studying sections as it was so similar to sections through a long tubular paraphysis. However it, together with all the others, opens into the larger outgrowths from the brain wall on the diencephalic side of the Velum and is of diencephalic and not of telencephalic origin. The View from the inside of the brain is essentially similar to that shown in figure 20 and immediately above these diverticuli the roof plate runs smoothly up and back. Real plexus infoldings in the roof plate do not appear until a point is reached roughly at least half way to the supra—pineal recess.
In all the specimens this tubular formation begins just at the diencephalic lip of the Velum and involves in all cases relatively about the same extent of the velar end of the diencephalic roof with quite definite limits caudally. When one takes into consideration the distance from the Velum to the superior commissure the formation as a whole is quite compact and definitely limited.
The question at once arises as to the character of this formation and its homologies in lower forms. It seems first of all to be in the nature of a transitory affair as shown by the tendency to the formation of detached and degenerating tubules and vesicles. The only specimen of an older stage available for study is shown in figure 22. This is a model of the forebrain of a human embryo of 80 mm. at a magnification of 20 diameters, for which I am indebted to the kindness of Prof. G. L. Streeter who loaned me this specimen about four years ago when the model was made. The low magnification is unsuited to bring out details but gives a general topographical View of the roof of the forebrain. The Velum, V, is very thick and dense and the paraphysal arch almost entirely suppressed. On the dienceph— alic side of the Velum the diencephalic roof plate is prolonged forward in the form of a wide pouch overhanging the roof of the telencephalon to a marked extent. Irregularities are seen '
in the lateral wall, which are apparently choroidal in characteror may be due to shrinkage, though the brain roof as a whole was in good condition. Streeter in his account in the KeibelMall Embryology states that the oral end of the roof plate of the diencephalon forms a large choroidal pouch overhanging the telencephalon and suggests that this is the homolog of the paraphysis of other vertebrates. In figure 22 a picture corresponding to this description appears, but as the pouch is on the diencephalic side of the velum it cannot be compared with the true paraphysis which always is of telencephalic origin. His (11), figure 56, shows a frontal section through that part of the diencephalon prolonged forward over the velum, in which a few choroidal folds appear, but no trace of any tubules or diverticuli and the picture corresponds closely with that seen in the sections of the embryo modeled in figure 22.
As regards the appearance of this formation in lower verte— brates, there is but little evidence on which to base conclusions. I have examined carefully the Harvard Collection of models of the forebrain of Necturus, Lacerta (agilis and muralis), and Chrysemys marginata. In all of these specimens the diencephalic roof plate is invaginated by the plexus formation in the roof of the forebrain. Here one finds solid projections into the brain cavity instead of hollow tubules or diverticuli growing out of that cavity. This is especially the case in Amphibia, where practically all of the postvelar arch is absorbed into a huge mass of plexus, which extends back to the hindbrain. In Chrysemys there is a slight projection of the diencephalic roof plate forward on either side of the paraphysis, but this seems to be due rather to the impression made by the latter on the diencephalic roof and practically the whole roof plate is invaded by masses of plexus. In Lacerta the plexus formation affects the roof of the diencephalon in the neighborhood of the epiphysis only and the velar portion is absolutely smooth and ﬂat. In birds Dexter (4), figures 5 and 7, shows an oval or a triangular shaped Vesicle lying dorsal to the paraphysis and close against the wall of the diencephalon. He finds this as an inconstant structure in chick embryos from 60 mm. up to young birds after birth. He mentions also the presence of detached vesicles referred to by Dendy in Sphenodon and by other authors in some of the Lacertilia. These latter, however, all seem to belong to the epiphysal part of the forebrain roof. Whether the Vesicle seen in certain chick embryos could be homologized with any of the detached_ tubules or cysts shown in the human embryos will require further study and confirmation. As regards Mammalia, there are no signs of these postvelar tubules in any of the sheep embryos here described. Heuser (9) shows several excellent models of the ventricles of the pig from early stages up to 260 mm., but nothing comparable to what has been described in human embryos occurs there and no traces can be found either in rat or rabbit embryos.
In dog embryos of 14 mm. and 17 mm. a median pouch-like prolongation from the diencephalic roof is clearly seen, figures 31 and 32, which resembles the similar projection in the human embryo in figure 13. A somewhat similar arrangement appears in cat embryos of 10 to 12 mm. (fig. 33) and in older embryos there is a marked projection of the diencephalic roof forward over a greatly reduced velum and paraphysal arch. In the cat, however, folds of plexus invade the prolongation in a manner quite different from that seen in human embryos, and- no tubular formation can be seen. These are apparently the only instances occurring in other forms which are in any way comparable to the condition just described in the human embryos. It might be asked justly whether this formation was not wholly pathological. It, however, occurs so constantly, although in a variable degree of complexity, that it would seem to be a normal phase in embryos between the ages considered here. It is also suggested that the possible persistence of some of these detached vesicles might account for some of the pathological conditions found in this general region. Conditions prevailing at the time when this paper was Written prevented the author from following out the comparative morphology of these postvelar tubules as thoroughly as shouldhave been done and more extensive study of comparative forms and of older human embryos will be needed to reach a satisfactory conclusion. The terms ‘postvelar tubules’ or ‘diverticuli’ are used here for lack of more suitable expressions and are rather unsatisfactory. It is hoped, however, that with further investigation a better description can be obtained.
1. The primary arches can be demonstrated in early human embryos from 10 to 15 mm. in length.
2. Of the embryos of 15 mm. and over examined in preparing this paper there were about thirty in which the brain was in suitable condition to warrant making observations and in addition to these a number of others were studied but excluded on account of injury or distortion of the forebrain. In the thirty specimens only eight showed any possible signs of a paraphysis and most of these were mostly rudimentary in character. By counting every possible case we get a result of 27 per cent. The fact remains, however, that the structure can be found in human embryos, though in a rudimentary and inconstant condition.
3. The so—called postvelar tubules or diverticuli can be clearly followed in every degree of complexity in embryos of 19 mm. up to 44 mm. and appear in every specimen studied in those stages. They begin at the diencephalic lip of the velum, have definite limits and involve a relatively short extent of the oral end of the diencephalic roof plate. They always appear as outgrowths from the brain roof and are to be distinguished from ingrowths due to plexus formation.
Part III. Iviammalian Embryos In General
As was mentioned in the introduction, it is intended to review generally the main features of the development of the pineal region in the other mammalian embryos which could be examined in the Harvard Collection. Of these only a few specimens of opossum, cow, deer, and dog embryos were available and the statements on them must necessarily be incomplete. '
Opossum. In an opossum embryo of 11 mm. (fig. 23) the primary arches are all recognizable. The paraphysal arch is short and is succeeded by a very low velum. The postvelar arch is long and flat, the epiphysal arch just developed and the pars intercalaris of relatively large size with the posterior commissure partly in it and partly overlapping into the midbrain. The next stage to be described is an embryo of 26 mm. shown in figure 24. The paraphysal arch, P.A., is reduced to a deep narrow fold which passes over into the velum, V, which has been wholly involved in the plexus formation. The roof plate above the velum swells forward over the telencephalon but this seems to be chieﬂy choroidal in character and is similar to what is seen in rat embryos (figs. 25 and 26) and especially in cat embryos (figs. 34 and 35). As is the case in the cat, the plexus has involved all of the diencephalon up to the deep suprapineal recess. A peculiar feature of this stage is the absence of an epiphysis, only the low arch just behind the pineal recess representing it. This however develops later. See Jordan (13) who gives a good account of the histology of the organ. The commissure shown here is the posterior commissure which fills up all of the pars intercalaris.
This was the oldest stage in the Collection with the exception of a set of sections through the head of an opossum after birth which were in too poor condition to be of any value. The original description of the paraphysis was given by Selenka in the opossum, but he contented himself with making the statement that it existed and gave no pictures of it as far as I can discover. It is hoped that some investigations will be published on older embryos and on the adult opossum in order to confirm the presence of the structure in this species.
The Harvard Collection contains an excellent series of these embryos up to 25 mm. in which the earlier development of the pineal region can be‘ advantageously studied. figure 25 shows the primary arches in an embryo of 9.6 mm. with the posterior commissure partly in the pars intercalaris of the forebrain, as is always the case in Mammalia. An embryo of 14.4 mm. is shown in figure 26. In the paraphysal arch there is a fold which runs across the brain at this point, in the position of a paraphysis, but which cannot be regarded as the real, structure. The velum is very small and the postvelar arch is now filled by the diencephalic plexus. The epiphysis is of large size and forms a long sack with rather thick walls and a wide opening into the brain. Just in front of its opening appears a small superior commissure and the posterior commissure fills all of the pars intercalaris and extends back into the midbrain. figure 27 is a median section of an embryo of 25 mm. constructed from three different sections and is the largest one in the collection. Immediately in front of the small velum is seen the same transverse fold that appeared in the previous figure, which however is deeper and extends entirely across the brain cavity. The postvelar arch has become long and more dome—shaped and contains masses of plexus. Above the superior commissure is a very deep suprapineal recess folded backward against the anterior wall of the epiphysis. Both commissures are well developed but the posterior seems to be much crowded together by the encroachment of the midbrain which has shortened up the pars intercalaris. The epiphysis is of unusual size and has developed backward over the pars intercalaris and the midbrain. Its cavity is at first wide but narrows towards the distal end, which however is thick and expanded laterally. There is apparently no real paraphysis in rat embryos up to 25 mm., its place being occupied by a simple fold running across the full width of the brain cavity in the paraphysal arch. The size and shape of the epiphysis are also very striking.
Early rabbit embryos give a picture of the primary arches essentially the same as that shown above for rat embryos. figure 28 is a transverse section of an embryo of 6 mm. showing the primary subdivisions of the forebrain into a telencephalic segment, T, and two diencephalic segments, I .D., and II D., as described in the sheep and pig embryos earlier in this paper and in a previous article (Warren (24), figs. 34 to 37).
An embryo of 14.5 mm. (fig. 29) has a much reduced paraphysal arch. The velum is rounded and as yet the postvelar arch is without any plexus formation. The epiphysis is much enlarged, contains a well marked cavity, with its tip directed forward and the whole structure is of somewhat irregular outline. The posterior commissure has completely filled the pars intercalaris and has developed backward into the midbrain.
The oldest stage at my disposal was an embryo of 22 days about 36 mm. in length, Which was not very well cut or preserved, but an embryo of 19 days, 30 mm. (fig. 30) shows essentially the same conditions. This was from a special series not catalogued in the collection. In the median plane the velum forms a distinct angle with a low fold or groove in the paraphysal arch which passes immediately over into the lamina terminalis. On either side of the median line the velum is much obscured by the plexus which has become much increased, leaving clear however a small suprapineal recess. The superior commissure is of good size, while the posterior commissure and the pars intercalaris seem somewhat reduced in length by the pressure from the midbrain behind. The epiphysis consists of a long tubular body, which ends in an enlarged tip and the central cavity in the body is prolonged into numerous smaller tubules in the extremity of the organ. The whole resembles somewhat the epiphysis of birds. The striking feature in ratand rabbit embryos is the extreme development of the epiphysis which differentiates them from the other mammalian specimens in the Harvard Collection, Where the organ remains in a more rudimentary condition.
No additional description is needed here to any extent, as the development in the sheep has been already discussed. As regards pig embryos Heuser’s (9) work on the shape of the ventricals covers the main points in the development of this part of the brain. See also Johnston (12) on the morphology of the forebrain in vertebrates for additional details, especially the velum. The formation of the primary arches and the three main subdivisions of the forebrain correspond to the account already given for sheep embryos. Heuser found no sign of a paraphysis but there is a deep fold running the whole width of the paraphysal arch, somewhat similar to the condition in rodents, which lies in the morphological position of the true paraphysis. Of two specimens of cow embryos one of 17 mm., sagittal series, H. E. C. no. 1126, showed beautifully all the primary subdivisions in the roof of the forebrain, and the older, a transverse series, was not reconstructed. The same is true of several early deer embryos which at 7 to 9 mm. showed the arches and subdivisions of the forebrain as previously described. See H. E. C. no. 1514, 9.8 mm. sagittal series and H. E. C. no. 1516, 7.3 mm. transverse series. The oldest deer embryo of 18.6 mm., H. E. C. no. 1230, had a very rudimentary epiphysis, velum and paraphysal arch and no sign of any paraphysis.
Of the three sets of sagittal sections in the collection those of 14 mm. and 17 mm. are here shown, as the youngest, 12.5 mm., was somewhat damaged in the paraphysal region. figure 31 is a median section of the forebrain of the 14 mm. embryo and shows the primary arches, although the postvelar arch is already somewhat invaded by the plexus, which was not the case in the 12 mm. embryo. The velum forms a well developed fold with a low paraphysal arch in front. On the diencephalic side of the velum the velar end of the roof plate bulges somewhat forward over the velum. It should be noted that the posterior commissure has filled nearly the whole of the pars intercalaris and extends practically up to the low epiphysal arch. In figure 32, embryo of 17 mm., there is no sign of a paraphysis, the velum forms a sharp fold and the diencephalic roof protrudes above it in a pouch-like projection very similar to what is seen in a median section of some of the human em bryos. This pouch makes a fold extending across the whole width of the roof. Owing to lack of material it was impossible to follow the development further, but it can be stated that the primary arches are present and that there is a hint of the diencephalic outgrowth described in human embryos.
The primary arches in cat embryos are quite similar to the pictures already shown here in other forms and there is no need of adding a special picture. The same is true of the three main subdivisions of the brain as seen in transverse sections (fig. 28). In figure 33, a cat embryo of 10.7 mm., there is a very low paraphysal arch and only a slight fold for the velum, which is less well marked than in earlier stages. Above it appears a wide projection in the diencephalic roof somewhat similar to that seen in figure 32. The rest of the brain roof shows merely the other arches. In an embryo of 24 mm. (fig. 34) the paraphysal arch is reduced to a mere slit, the velum is also very insignificant and immediately above it there is a large forward projection of the diencephalon followed by a mass of plexus which involves the whole of the roof. This projection has the same relative position as the one shown in human embryos but seems to become more involved in the plexus formation and to lack the tubular outgrowths which are so characteristic of the former. A small epiphysis which can be seen in embryos of 15 mm. is shown here inclined backward between the superior and posterior commissure. The largest embryo cut sagitally is showrrin figure 35. It is an embryo of 39 mm. and is in the main an exaggerated picture of that seen in figure 34. The greatly reduced size of the paraphysal ‘arch and velum is to be noted and also the excessive plexus development which involves the whole of the diencephalic roof up to the suprapineal recess. The pars intercalaris occupies a very prominent space in the roof of the diencephalon in this stage and in that shown in figure 34 and is not as much suppressed by the pressure from the midbrain roof as is usually the case. No paraphysis could be found in any of the cat embryos in the collection. Owing to the small size of the paraphysal arch and of the velum and to the extreme development of the diencephalic plexus, it is often difficult to fix accurately the true position of the velum and misinterpretations are easily made. The great temptation is to place the velum rather further back along the brain roof and describe the fold or folds which then seem to belong to the telencephalon as the paraphysis. The author feels convinced, however, that the velum in the above figures is properly placed, thereby confirming all the above folds to the diencephalon and having only a rudiment of a paraphysal arch in the roof of the telencephalon, with absolutely no sign of a paraphysis, which must be morphologically of telencephalic origin. The important features in the development of the pineal region in these embryos are the almost total suppression of the paraphysal arch, the much reduced velum and the luxuriant development of the plexus. An excellent account of the development of the forebrain in the cat is given by Tilney (23) and figures of embryos up to 70 mm. are there shown.
1. The primary arches in the roof of the telencephalon and diencephalon are all seen in early stages of mammalian embryos, and are all present at the same time. The marked extent of the synencephalic arch or pars intercalaris and its early appearance is a very striking feature in Mammalia.
2. The three main subdivisions of the forebrain, the telencephalic and the two diencephalic segments or subdivisions, can be followed in all the Mammalia referred to here.
3. The paraphysis is found definitely developed in sheep embryos only. In all other Mammalia except man it is represented merely by the paraphysal arch either with or without a transverse fold which passes across the whole width of the telencephalic roof. In human embryos it can be found in occasional cases as a rudimentary and very inconstant structure.
4. In all the earlier stages of mammalian embryos the Velum is fairly well developed. In later stages it becomes reduced, especially towards the mid—line, to a mere angle and is more or less absorbed in the folds of the diencephalic plexus.
5. The postvelar arch is always well developed. Later the diencephalic roof plate becomes incorporated with the tufts of the diencephalic plexus which occupies the greater part of its extent. There is in most cases a well marked suprapineal recess extending upward and backward over the supracommissure.
6. The epiphysis of ungulates and carnivora even in the oldest embryos studied is very small possessing thick walls and a small lumen. It is almost buried between the two commissures and covered anteriorly by the arched suprapineal recess. In rodents it undergoes a much more extensive development and resembles somewhat the large epiphysis of reptiles. In the cat and rabbit it is characterized by a long hollow stalk with fairly thick walls and an extended distal end, which shows distinct tubule formation and inclines backward over the posterior commissure.
7. The superior commissure is well developed and present in all forms studied. It is deeply placed between the epiphysis and the suprapineal recess. The posterior commissure makes a very early appearance, being present in almost all forms in the stages where the primary arches are present in the roof of the forebrain. Its first traces can be seen in both the forebrain and the mid brain and it soon occupies all of the pars intercalaris, extending close upto the dorsal wall of the epiphysis.
8. The diencephalic and lateral telencephalic plexuses are well developed in mammalian embryos but there is no trace of the median telencephalic plexus which is so distinctive in Amphibia.
(1) BAILEY, P. 1916 The morphology and morphogenesis of the choroid plexuses with especial reference to the development of the lateral telencephalie plexus in Chrysemys marginata. Jour. Comp. Neur., VOl. 26, no. 5.
(2) BAILEY, P. 1916 Morpliology of the roof plate of the forebrain and the lateral choroid plexuses in the human embryo. Jour. Comp. Neur., vol. 26, No. 1.
(3) CAMERON, J. 1904 On the presence and significance of the superior commissurc throughout. the Vertebrate. Jour. Anat. and Physiol., vol. 38.
(4) DEXTER, F. 1902 The development of the paraphysis in the common fowl. Am. Jour. Ans-Lt., vol. 2.
(5) D’ERCHIA, F. 1896 Contribute allo studio della volt-a del cervello intermedio e della rcgione parafisaria in embrioni di pesei e di mammiferi. Monit. Z001. Ita1., Ann. 7.
(6) FRANCOTTE, P. 1892 Contribution Q. l’é.tude de l’épiphyse et de la paraphyse chez les lacertilliens. Bruxclles, 1896.
(7) FRAN(.‘()T’I‘]-S, P. 1894 Note sue l’oeil pariétal, Pépiphyse, la parapliyse et les plexus choroides du troisieme ventricule. Bull. (lo l’Acad. roy. de Belg., Ser. 3, tom. 27, no. 1.
(8) FRAl\'C()'I‘TE, P. 1896 Contribution a l’étude de l’oei1 parietal de l’épiphyse ct de la paraphyse chez les Iacertilliens. Mém. couronnés do l’Acad. roy. de Belgique, tom. 55.
(9) Hnusnn, C. 1913 The development of the cerebral ventricles in the pig. Am. Jour. Anat., vol. 15, no. 2.
(10) HIS, W. 1889 Die Formentwicklung des menschlichen Vorderhirns vom Jnde des erstcn bis zum Beginn des dritten Monats. Abt. Mat. phys. C1. kon-Séichs. Ges. Wiss., Bd. 15, Leipzig.
(11) Hrs, W. 1904 Die Entwicklung des mcnschlichen Gehirns., Leipzig.
(12) JOHNST()'.\'_, J. B. 1909 The morphology of the forebrain vesicle in vertebrates. Jour. Comp. Neur., Vol. 19.
(13) JORDAN, H. E. 1911 The microscopical anatomy of the epiphysis in the opossum. Anat. Rec., vol. 5.
(14) JORDAN, H. E. 1911 The histogenesis of the pineal body of the sheep. Am. Jour. Anat., vol. 12.
(15) KUPFFER, C. VON 1903 Die Morphogenie des Nervensystems. In Oskar Hertwig’s Handbuch der vergleichenden u. experimentellen Entwieklungslehre der Wirbelthiere. Jena.
(16) MINOT, C. S. 1901 On the morphology of the pineal region based upon its development in Acanthias. Am. Jour. Anat., vol. 1.
(17) NEUMAYER, L. 1899 Studien zur Entwicklungsgesehichte des Gehirns der Siiugetiere. Festschr. z. 70, Geburtstag von. 0. von Kupﬂer. Jena, fischer.
(18) OSBORN, H. F. 1887 The relation of the dorsal eommissures of the brain to the formation of the encephalic vesicles. Amer. Nat., vol. 21.
(19) SELENKA, E. 1890 Das Stirnorgan der Wirbeltiere. Biol. Centra1bl., Bd. 10.
(20) SMITH, G. E. 1903 On the morphology of cerebral commissures in the vertebrate. with special reference to an aberrant commissure formed in the forebrain of certain reptiles. Trans. Linnean Soc. London, 8 (2nd Ser. Zool.).
(21) STREETER, G. L. 1911 Development of central nervous system. Keibel— Mall Text-Book of Embryology, vol. 2.
(22) TERRY, R. S. 1910 The morphology of the pineal region in teleosts. Jour. Morph., vol. 21.
(23) TILNEY, F. 1915 The morphology of the dieneephalic ﬂoor. Jour. Comp. Neur., vol. 25, no. 3.
(24) WARREN, J. 1911 The development of the paraphysis and pineal region in Reptilia. Am. Jour. Ana.t., vol. 11.
(25) ZIEHEN, TH. 1906 Morphogenie des Centralnervensystems der Saugetiere. In 0. Hertwig’s Handbuch der vergleichenden u. experimentellen Entwieklungslehre der Wirbeltiere. Jena, fischer.
I.D., first diencephalic segment II.D., second diencephalic segment D.C'.P., diencephalic choroid plexus D.R.P., diencephalic roof plate
E’.A., epiphysal arch
F.M., forarnen of Munro
L.C.P., lateral choroid plexus (telen cephalic) .L.T., lamina terminalis L.V., lateral ventricle
M., mesencephalon M.B., midbrain 0.0., optic commissure P., pamphysis P.I., pars intercalaris P.V.A., postvelar arch P.V.T., postvelar tubules or diverticuli ‘ P.C., posterior commissure S.C'., superior comrnissure T., telencephalon T.R.P., telencephalic roof plate Th., thalamus V., velum transversum
1 Sheep, 9.9 mm. H. E. C., sagittal series, no. 1339.
2 Sheep, 14 mm. H. E. C., sagittal series, no. 1330.
3 Sheep, 21 mm. H. E. C., sagittal series, no. 1687.
X 20. X 20. X 20.
4 Sheep, 26 mm. H. E. C., sagittal series, no. 1112. X 20. 6 Sheep, 29 mm. H‘ E. C., transverse series, no. 1689‘ X 50.
5 Sheep, 29 mm. H. E. C., transverse series, no. 1689. X 20. 7 Sheep, 29 mm. H. E. C., transverse series, no. 1689, section 236. X 50.
8 Sheep, 48.4 mm. H. E. C., transverse series, no. 1696. X 20.
9 Sheep, 48.4 mm. H. E. (7., t1*:u1sVe1*:sL» series, no. 1696. X 50. 10 Sheep, 48.4 mm. H. 1*}. C., transverse series, 110. 1696, section 414. X 50.
11 Man, 10 mm., H. E. C., transverse series, no. 1000. X 25.
12 Nam, 15 mm. H. E. C., trzmsvcrsc series, no. 2051. X 20.
13 Man, 23 mm. H. E. (7., sagittal series, no. 181. X 20.
14 Man, 25 mm. H. E. C., transverse series, no. 15 Man, 25 mm. H. E. C., transverse series, no.
16 Man, 25 mm. H. E. C., transverse series, no. 2043. X 40. 17 Man, 31 mm. H. E. C., sagittal series, no. 1706. X 40.
Man, 36 mm. H. E. C., transverse series, no. 2050. >< ~10. Man, 36 mm. H. E. C., trzmsvorse series, no. 2050. X 40. Man, 36 mm. H. 1*). 0., transverse series, 110. 2050. X 40.
Man, 44.3 mm. H. E. 0., transverse series, no. 1611.
r -‘ in 1'! PLATE 12 EXPLANATION or“ fiGURES 22 Man, 80 mm. Michigan Collection. X 10.
Opossum, 11 mm. H. E. C., sngitt:-11 series, no. 925.
Opossum, 26 mm. H. E. 3., sngittal series, no. 2077.
Rat, 9.6 mm. H. E. C., sagittal series, no. 1824. Rat, 14.4 mm. H. E. 0., sngittnl series, no. 1925.
X 20. X 20.
X 20. . Rabbit, 14.5 mm. H. E. C., sagittal series, no. 162.
Rat, 25 mm. H. E. C.,'-sagittal series, no. 1796. X 20. Rabbit, 6 mm. H. E. C., transverse series, no. 554.
Rabbit, 30 mm. H. E. C., sagittal series, special series.
X 20. X 20.
31 Dog, 14 mm. H. E. 0., sagittal series, no. 2052. X 20. 32 Dog, 17 mm. H. E. C., sagittal series, no. 2053. X 20. 33 Cat, 10.7 mm. H. E. C., sagittal series, no. 475. X 20.
34 Cat, 24 mm. H. E. C., sagittal series, no. 467. X 20. 35 Cat, 39 mm. H. E. C., sagittal series, no. 368. X 20.
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