Paper - The development of the paraphysis in the common fowl (1903)

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Dexter F. The development of the paraphysis in the common fowl. (1903) Amer. J Anat. 1: 13-24.

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This historic 1903 paper by Dexter described development of the paraphysis in the chicken.

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The Development of the Paraphysis in the Common Fowl


Franklin Dexter, M. D.

Associate Professor of Anatomy, The Anatomical Laboratory, Harvard Medical School.

With 9 Text Figures.

The moment that one begins to examine the literature of this subject he cannot fail to be impressed with the difficulties which present themselves. The forebrain has been a favorite subject for study, consequently a great deal has been written on it, and so it is impossible to feel certain that all pertaining to it has been read. It has been considered from many points of view, by means of different methods, and has received many names, either when considered as a whole, or in its subdivisions. It does not seem to me to be necessary to mention each individual paper which I have read, so propose to include only those in my list of literature which have an actual bearing upon this special subject.

There is a difference of opinion in regard to the best subdivision of the forebrain, but all who have described it, as far as I know, take the velum transversum as its primary subdivider into an anterior division or prosencephalon, and a posterior division or diencephalon. The nomenclature lately proposed by Minot (9) although a trifle longer than that adopted by some writers, has the advantage of being more specific, and consequently I shall follow it, with the exception of the first subdivision. He subdivides the median line of the diencephalic roof into six divisions. First, the region of the post commissure. As he later points out in his paper, and I thoroughly agree with him, that this commissure is probably developed from the midbrain, and therefore should properly be considered as belonging to that region. Since this is probably the case, I see no reason for describing it as a portion of the diencephalic roof.

We will therefore omit this subdivision, and will consider the posterior commissure as a part of the midbrain, and will subdivide the median line of the dienccpbalic roof into five regions:

First. The epiphysis. Second. The supra-commissure.

Third. The post velar arch. This extends from tlie supra-commissure to the vekmi.

Fourth. Tlie velum transversum. Fifth. The paraphysal arch. This extends from the lamina terminalis in front, to the velum transversum behind. It is in this subdivision, close to the velum, that the paraphysis is found.

The history of the paraphysis in the lower vertebrates has been repeatedly studied and described. It is curious that so little attention has been paid to it in birds, especially since its presence has even been demonstrated in certain mammalian embryos. Selenka (12) was the first to identify the paraphysis in chicks. Burckhardt (1) mentions it in a 2.5 mm. embryo crow, and states (2), " In birds the paraphysis remains rudimentary and later cannot be identified." Minot (9) also refers to it in an embryo chick of about seven days. D'Erchia (5) identified the paraphysis in fish, and in mammalian embryos, and believes it to be a constant structure in all vertebrates, but reports no observations on birds. Francotte (7) identified it in a human embryo of twelve weeks, and believes it to exist in all vertebrate embryos. This is all the literature I have been fortunate enough to find relating to the paraphysis of birds.

Many of the preparations employed in this piece of research work belong to the Flarvard Embryological Collection. Besides the sections here represented, intermediate stages of embryos were studied, as well as chickens varying from a few days to full-grown hens. The sections were invariably serial, and double stained with cochineal and orange Gr. Many specimens were hardened in Tellyesnicker's fluid, which gave on the whole better results than Zenker's, and of course has the great advantage of being without corrosive sublimate. Thirty-six hours was perhaps the usual time the adult brains were allowed to remain in this fluid, and an equal amount of time in running water, and then they were treated by the progressive alcohol method. Great difficulty was experienced in making true longitudinal median sections of the adult brains. This was largely due to the depth of the longitudinal fissure and to the very thin inner wall of the lateral ventricle, which often in the process of hardening becomes more or less twisted. Much less difficulty was usually encountered in the earlier stages where the entire head was cut without removal of the brain. Nos. 1, 2. 3, 4, 5 and 7 of the following figm-es were drawn on the same scale, and all of the sections in this paper were drawn with the aid of a camera lucida.

Figure 1 is a sagittal section of the forebrain of a 6.7 mm. embryo.

Fig. 1. Embryo of 6.7 mm. Harvard Embryological Collection. Sagittal series 477. Section 110. X 45.a

The midbrain is somewhat obliquely cut, but the section in the region of the paraphysis is nearly median. One recognizes the large cavity of the embryonic forebrain, with its correspondingly thin walls.

At this stage the epiphysis (Ep) is simply an evagination of the roof, in front of which the posterior velar arch forms a gentle curve. Neither the posterior nor the superior commissure has made its. appearance, nor as yet is there any indication of a choroid plexus. The velum (v) is plainly seen between the ventral portion of the posterior velar arch and the paraphysis. It appears as a somewhat triangular mass of mesenchymal tissue protruding into the cavity of the forebrain, but is actually separated from that cavity by the thin ectodermic wall. It extends transversely across the forebrain, and so divides it, as was previously mentioned, into the prosencephalon and diencephalon.

This is the earliest stage in which I have been able to identify the paraphysis. Embryos a trifle younger, present in sagittal section an appearance which closely resembles Fig. 1, with the paraphysis wanting. The paraphysis lies in the median line, immediately dorsad to the foramen of Munro, and anterior to the velum transversum. At this stage it i* a simple evagination of the brain wall, and is identical with it in structure. It contains a large cavity which communicates with that of the forebrain.

Fig. 2 is a most fortunate median sagittal section of a 19.5 mm. embryo in which the above-mentioned subdivisions of the forebrain may be readily identified. The posterior commissure is at this stage plainly visible. A well-developed epiphysis is present. This is the earliest stage at which I have been able to identify the superior commissure. It lies in its characteristic position, within the ectodermic brain wall, anterior to the opening of the cavity of the epiphysis. We will return again to this region, and will study it more closely with a higher power.

It is evident on comparing Figs. 1 and 2 that the posterior velar arch has now totally changed its shape. In Fig. 1, it forms a curve.

In Fig. 2, it consists of a liorizontal and a perpendicuhir arm, which form together what is not far from a right angle.

Moreover, it is perfectlj apparent from this drawing, that the choroid plexus of the forebrain is developed only from the perpendicular arm, or anterior portion of the arch.

The triangular form presented by the velum in Fig. 1, has now disappeared, and it is replaced by a fairly thick quadrilateral fold of mesenchyma which is distinctly broader and more conspicuous than any

Fig 3 Embryo of 19.5 mm. Harvard Embryological of the other folds. CoUection. Sagittal series 473. Section 334. X 45.2 diams. The paraphysis at this

stage is much more developed and contains a large cavity communicating with that of the forebrain. Its wall is distinctly thicker than in the younger embryo.

Fig. 3 is not as fortunate a median sagittal section as the last. The paraphysis is shown exceedingly well, but the communication of the cavity of the epiphysis with the forebrain does not appear in this section. It is from an embryo of 43 mm., and is naturally much more developed than the previous one. The posterior commissure is very large. Portions of the epiphyis are seen in the mesenchymal tissue above the roof of the third ventricle. The superior commissure has materially increased in size, and is found in its usual position. Anterior to this commissure a fold in the roof of the ventricle might easily be taken for the epiphysial opening, but such is not the case. The posterior velar arch has again changed its shape. What was formerly described as the horizontal arm is now distinctly ascending and forms with the perpendicular arm a fairly acute angle, with a direction of upwards and forwards. The perpendicular arm has n'ot altered its position, but the choroid plexus springing from it is thoroughly well developed, lying in many folds, some of which have been cut transversely and therefore appear separated from the roof of the ventricle. The velum transversum is very much changed in appearance. The mesenchymal tissue has thinned, its choroid fold is very prominent, and but for its specific position it would Ije impossible to differentiate it from any other fold of the choroid plexus. (The breadth of this fold, as well as its position and relation to the paraphysis, are well shown in Fig. 4.)

FiG.3. Embryo of 43 mm. Harvard Embryological collection, sagittal series 50it. Section 363. X 45.3

The paraphysis presents a wonderfully regular outline, as well as cavity. It seems to be distinctly smaller than in the previous stage, which is particularly true of its cavity, but on the other hand, its mesenchymal tissue ventrad to the epiphysis. Its position is very characteristic. It gives off lu'anclies which supply the choroid plexus of the third ventricle, and the vessel then divides dorsad to the paraphysis, and each terminal branch supplies the choroid plexuses of the lateral ventricles posterior to the foramina of Munro.

Fig. 4. Embryo of 45 mm. Harvard Emliryolcgi Wails are much thicker. A cal Collection. Frontal series 514. Section 7T;i.

large vessel is seen in the x-s- 5 diams.

Fig. 4 is a frontal section of an embryo's l)raiu 45 mm. It is cut obliquely to the cavity of the paraphysis, as seen in Fig. 3. The two lateral ventricles, witli i)ortions of their choroid plexuses, the cavity of the forebrain with its clioroid folds and optic thalami on each side, are all easily identified. The velum stretches transversely across the roof of the forebrain, between the paraphysis and tlie choroid plexus, and is continuous with the mesenchyma surrounding the optic thalamus. It is situated dorsad to the paraphysis. The paraphysis is seen lying in the mesenchymal tissue ventrad to the forebrain, and appears as an irregular ring of ectodermic tissue. Two of the above-mentioned vessels, which supply the choroid plexus, are met in this section.

Fig. 5 represents a sagittal section of a ten-days' chicken's brain. The section is not exactly in the median plane, consequently the choroid plexus has been separated from the roof' of the third ventricle. It seemed to me to be unnecessary to draw in all the structures that have been previously represented. The epiphysis and both commissures are omitted. The picture, when taken as a whole, resembles very closely Fig. 3, an embryo of 43 mm. The dorsal wall of the posterior velar arch is even more perpendicular than in the above-mentioned figure. This tends to make the angle formed by the two limbs much more acute, so that now both are quite perpendicular. The position of the anterior arm has changed very little, but the choroid plexus springing from it, is fully developed. Unfortunately, owing to the obliquity of the section, the velum has been separated from the roof of the ventricle, and consequently it does not appear in this section. Such a fold, were it present, would present no essentially different picture from that represented in Fig. 3. As we have already seen the velum is situated immediately behind the paraphysis. I believe that the first prominent fold of choroid plexus in an adult chicken, behind the paraphysis, represents morphologically the large, broad, welldeveloped velum transversum of the 6.7 mm. embryo, as is represented in Fig. 1. There is almost no change in the appearance of the paraphysis. Its cavity is more constricted, but its walls are of about the same thickness, and of course its position in the two plates is identical. Above and to the left of the paraphysis there is a curious vesicle (Ve) which I am at a loss to explain, but will refer to again farther on.

Figs. 6 and 7 represent the same sections under different powers of magnification. It was desirable to draw this section with the same power as was employed for the others, but the field was not large enough to include the entire section and so it was tliouglit advisable to first study its topography with a lower power, without which it does not seem to me that it would be intelligible. The figures represent frontal sections which pass through the paraphysis of a three-days' chicken's brain. In Fig. G one is able to distinguish the narrow lateral ventricles, the slit-like cavity of the forebrain lying between the two optic thalami, a very minute paraphysis, and just above it a triangular cavity or vesicle. This seems to be the same structure which we saw in sagittal section (Fig. 5).

Fig. 5. Sagitta] section of a 10 days' chicken's brain. X 45.2 diams.

On examination of Fig. 7 the same regions are much more easily distinguished. Here the paraphysis is seen clearly to be a portion of the epithelial roof of the forebrain. Its cavity presents a very uniform appearance, but wdth a higher power, small fissures are observed running off from it, and piercing its wall for variable distances. A large , transverse vessel separates it from the vesicle. The space between the paraphysis and the mesenchymal wall of the vessel, I imagine is due to shrinkage, but it must be said that it is peculiarly constant, and has been frequently observed as is here represented.

Figs. 6 and 7. Frontal sections of a 10 days' chicken's brain. Fig. 6 x 9.2 diams. Fig. 7 X 45.2 diams.

Fig. 8A represents a portion of the paraphysis as seen in Fig. 5 under a much higher power. Even with a comparatively low power, one can easily subdivide its wall into two regions. An inner, thinner layer, next to its cavity, and an outer, much thicker layer. The inner stratum stains more deeply than the outer, and is of about the same thickness as the ependymal layer of the brain. With a high power, the nuclei of this layer are seen to be somewhat oval in shape, and are crowded together in contradistinction to the outer layer wliere they are much more clearly defined, round in appearance, and more widely separated from each other. In a word, the outer layer seems to be a sort of modified ectodermic tissue. The above-mentioned clefts issuins: from the central cavity of the gland into its walls are in this section plainly visible.

It seems to me that Minot's (9) supposition regarding the paraphysis in amphibia and birds is a perfectly correct one. It is inconceivable that the paraphysis can for a moment be thought of as an organ of sense. It is much more probable that it is an appendix of the paraphysal arch, developed from the brain wall, and, as we have seen, its outermost layer in the adnlt is composed of a modified ectodermic tissue. In the younger stages its walls are thin and its cavity is large, but in the adult chicken or hen the reverse is true. A narrow, cleft-like cavity persists, surrounded by moderately thick walls. The gland is oval in shape and is not far from 150 // in its greatest diameter, which lies nearly parallel with the longitudinal axis of the cavity of the forebrain. It is an absolutely constant structure, and I have been able to identify it time and time again in the embryo, in the chicken, and finally in the full-grown fowl. Its position is very characteristic. The paraphysis is situated Sagittal immediately dorsad to the Frontal sec- forauieu of Munro, and anterior to a prominent fold of the choroid plexus which must morphologically correspond to the velum transversum.

Fig. 8B is a section of a portion of the peculiar vesicle above referred to shown under a high power. The preparation is from the same series as Fig. 7 but a different section, a little farther dorsad to it was chosen, as the cells seemed to show more distinctly than those in the previous section. The wall of the vesicle is granular in structure, with large round nuclei, which in each case contain a small, irregularly-shaped nucleolus. At times the nucleolus appears as a round dot, or presents a linear appearance. It may represent quite a regular cross, or even be star-like in shape. Externally there is a layer of mesenchymal tissue, and internally what presents the appearance of a distinct membrane. Within the cavity of the vesicle, there is almost invariably an appreciable amount of coagulum. Frequently close to its wall, some spherical bodies are seen. These vary much in size, as well as in number. I have no notion whatever as to what they are. The triangular vesicle has been a very interesting puzzle to me, nor is it as yet solved. I call it a vesicle simply for the want of a better name. For a long time it did not seem possible that it was not a blood-vessel, or perhaps a lymph space, and to-day I am unable to explain it, so must leave ^ the subject for future investigation. There are, however, a few curious facts in regard to it. It is an inconstant structure. I have seen it in embryos from 60 mm. in length, up to young chickens after birth, but I have never been fortunate enough to meet it in an earlier or in a later stage. At times it is present, but I am inclined to believe that it is more frequently absent. When present it seems to be situated in about the same spot, and may be identified in either a sagittal or a frontal section. In the former it is apt to be somewhat oval (Fig. 5); and in the latter triangular in section (Fig. 7).

Fig. 8A. Paraphysis of a 10 days' chicken gection. X 540 diams.

Fig. 8B. Vesicle of a 3 days' chicken tion. X 540 diams.

In one series of the adult chicken, where the vesicle was absent, the blood-vessel (which appeared in Fig. 7 as a compressed vessel beneath it), was triangular in form, and presented precisely the same shape as this vesicle. As the vesicle begins to appear in serial sections the wall is first met, then one meets the cavity, and lastly a Avail. Surely when one considers the structure of this object under the high power, it is difficult to conceive of its being either a lymph space or a blood-vessel, and after diligent search through many sections in several embryos, I have never been able to find a single blood corpuscle within its cavity, and as far as I know, its cavity does not communicate with that of the forebrain.

Dendy (4), in a most admirable and interesting paper on Sphenodon, refers to what he calls an accessory vesicle situated between the tubules of the paraphysis and the parietal stalk. He describes it as sacculated, irregular in shape, containing no blood corpuscles, unconnected with the forebrain, and as disappearing at a moderately late stage of development. He does not believe it to be either a vessel or a lymph space. Moreover, as pictured in his article, the lining epithelium seems to be of quite a different character from the vesicle in question Burckhardt (1) mentions a vesicle in lacerta vivipara, which is close to the epiphysis, and could not, I should fancy, be confounded with this one. Under the head of " Nebenorgan " and "Nebenscheitcl organ," difl^erent kinds of vesicles have been described in different animals by various authors, but I fancy that they should all be associated with the epiphysis rather than with this vesicle, which certainly can have no connection with it.

Eitter (11) also speaks of a parapineal organ wliich does not seem to me to have any resemblance to the one in question. It is certainly a good subject for future investigation, and with my present amount of material one ought soon to be able to arrive at some definite conclusion in regard to it.

The presence of the snpra-commissure in birds is denied by certain authors. Gronberg (8) states that it is not present in this class of animals, but admits its presence in all the mammalia. Dejerine (3) claims that it exists in all vertebrates. It has been described under various names, but as far as I know, nothing has been said in regard to its development in birds, although Edinger (6) describes it in pigeons under the name of tractus habenulo-peduncularis. To Osborn (10) the simple name of supra-commissure is due.

Fig. 9. Embryo of 25 mm. Harvard Embryological Collection. 516. Section 38G. x 230 diams. Sagittal series

In Fig. 2 an embryo of 19 mm. the supra-commissure is met for the first time, and after this date it must of course persist throughout life. The various descriptions of its fibers seem to vary in the number of words employed, rather than in any real difference of opinion in regard to the anatomy of the commissure itself. As far as T know all writers agree that its fibers arise from the ganglion of the habenula. and terminate in tlie ganglion interpedunculare of the midbrain.

Fig. 9 is a sagittal section of a 25 mm. embryo, and shows this region very clearly. The opening of the cavity of the epiphysis into the forebrain is very apparent (Fp), and immediately anterior to it, within the ectodermic wall, the supra-commissure is first seen in a chick of abont 19 mm. in length. Its position is very characteristic, for it is invariably described as being sitnated in this particular spot throughout all classes of vertebrates. The posterior commissure makes its appearance at an earlier date than the superior, and is consequently at 'this period very well developed. As was previously mentioned, it is properly considered to be a portion of the midbrain. This being the case I would call attention to the fact that its fibers terminate a very short distance from the epiphysial opening (Ep), or in other words, immediately posterior to it, consequently, it seems to me, that the opening of the epiphysis into the fore-brain is situated much nearer the line of division between the mid- and forebrains than it is commonly supposed to be, and that the portion of the roof of the midbrain which is formed by this commissure, extends a greater distance ventrad than we are apt to picture in our minds.

Some very curious, large nerve cells (N) may be observed situated in the wall of the midbrain. They contain a round nucleus, and an irregularly-shaped nucleolus. I call attention to them, since they are quite new to me.

To recapitulate:

  1. The paraphysis appears as an evagination of the roof of the forebrain, and first makes its appearance in a chick of about 6.7 mm. and is present throughout life. It is situated immediately dorsad to the foramen of Munro, is oval in shape, and contains a slit-like cavity which communicates with the cavity of the third ventricle.
  2. The choroid plexus of the third ventricle arises only from the anterior half of the embryonic post-velar arch,
  3. In the chicken, the first prominent fold of choroid plexus posterior to the paraphysis corresponds morphologically to the velum transversum of the embryo,
  4. The supra-commissure may be first identified in an embryo chick of about 19.5 mm.

Finally, I should like to acknowledge my indebtedness to Prof. C. S, Minot, not only for his valuable suggestions, but also for the interest he has taken in this piece of work.


Vli. P/., Choroid Plexus.

Ep., Epiphysis.

F. B., Forebrain.

Lat. Vent., Lateral ventricle.

M. B., Midbrain.

P., Paraphjsis.

Pc, Posterior commissure.

N., Nerve cells.

8c. , Superior commissure.

Ye., Vesicle.

Yes., Vessel.


1. BuRCKHARDT, E. — Die Ilomologien des Zwischenhirndaches bei Eeptilien und Vogeln. Anat. Anzeiger. IX Band. 1894. S. 320

2. BuRC'KHARDT, E. — Der Bauplan des Wirbelthiergehirns. Morph. Arbei ten. IV Band. S. 131.

3. Dejerine, J. — Anatoniie des centres nerveux. Tome 2.

4. Dendy, a. — On the Development of the Pineal Eye and Adjacent Organs in Sphenodon. (Hatteria.) Quart. Jour. Micros. Science. Vol. 42, p. 111.

5. D'Erchia, F. — Contributo alio studio della volta del cervello intermedio e della regione parafisaria in embrioni di pesci e di mammifera. Monit. Zool. Italy. VII. 75-80. 118-122.

6. Edinger, L., und Wallenberg, A. — Untersuchungen liber das Gehirn der Tauben. Anat. Aijzeiger. 1899. Band 15. p. 245.

7. Francotte, P. — Sur L'Oeil Parietal, L'Epiphyse, La Paraphyse et les Plexus Choroides du Troisieme Ventricule. Extrait des Bulletins de I'Academie royale de Belgique. 3d serie. Tome XXVII. No. 1. 1894.

8. Gronberg, G. — Die Otogenese eines niedern Saugergehirns nach Unter suchungen an Erinaceus Europaeus. Zoolog. Jahrbucher. Band XV. S. 261, u. 354.

9. MiNOT, C. S. — On the Morphology of the pineal region, based tipon its Development in Acanthias. American Jour, of Anat. Vol. I, p. 81.

10. OsBORN, H. F. — Preliminary Observations upon the Brain of Manopoma. Proc. Acad. Nat. Sci., Phila., 1884. p. 262.

11. Eitter, W. E. — On the Presence of Parapineal Organ in Phrynosoma Coronata. Anat. Anzeiger. Band IX. 1894. p. 766.

12. Selenka, E. — Das Stirnorgan des Wirbelthiere. Biol. bbl. X. S. 323.

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