Paper - Contribution to the structure and development of the vertebrate head 2
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Locy WA.Contribution to the structure and development of the vertebrate head. (1895) J. Morphol. 11(3): 497-595.
- Contents: General Introduction | Part I - Metamerism of the Head | Part II - The Sense-Organs | Figures
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Part II - The Sense-Organs (1895)
The Sense-Organs of Vertebrates embrace all those structures that are endowed with Special sensibility They differ from one another mainly in degree of differentiation, and form a series, at the lower end of which are simple sensory papillæ, and at the upper end are complex organs like the eye and ear. We recognize two orders of Sense-Organs — simple generalized ones, the ganglionic Sense-Organs, and more specialized ones» belonging to the so—ca1led five senses.
The question of the origin and relationship of these sense-— Organs is full of interest. From the combined results of investigations on both Invertebrates and Vertebrates it seemsaltogether probable that the higher· Sense-Organs have been derived from those of a lower order, and, indeed, that they have all been differentiated from a common sensory basis, and are therefore related in a direct way.
This view has been entertained by morpho1ogists for upwards of ten years, and has a firm foothold in philosophical discussions. The evidence favoring such an interpretation has been accumulating, and although still incomplete, the hypothesis was never so well supported as at present.
Professor Whitmarks researches were among the first to bring out this conception. Ten years ago he demonstrated in 1eeches the genetic relation between eyes and tactile papil1ae, and since that time it has no longer been a matter of pure assumption to say that certain end-organs are modifications of a common sensory basis. He shows that the sensory papillæ located on each bodycking in the leeches have a very interesting relation. In the hinder part of the body the papillæ are purely tactile, but passing headward they become gradually modified, and are at first mixed visual and tactile, and tinally the anterior ten pairs are purely visual. We have thus had for upwards of ten years a welbauthenticated case of the derivation of Sense-Organs of a higher grade from those of a lower order.
In a more recent publication, The Metamerism of Clepsine", 1892, Professor Whitman has added to the def1niteness of his earlier discoveries by observations on a new species, ask-r» Clepsine hol1ensis. He shows by a comprehensive study of the elements of the nervous System, the nerve distribution, and the sensillæ the complete homodynamy of all the segments.
Regarding the relationship of the eyes to other metameric sense-organs, he says: "One more peculiarity of this species may be noticed The median rows of sensillæ are quite well developed as eyes, at least on segments IV, V, and VI. The visual elements in Segment IV are quite numerous, and they are placed in a pigment cup quite like that of the principal eyes, only thinner and sma1ler, and directed backward instead of forward. The sensi1la of Segment V is a little sma1ler, but still presents the same general features. In Segment VI the organ is still eye-lilce, butiits visual elements are fewer andthe pigment cup imperfectly represented by loose pigment. In Segment VII we find one or two visual eells and a little scat . teredpigment In the following segments we usually find the sensillae in about the. same condition. In no other species hitherto described do we find the sensillæ passing by such gradations into the principal eyes. The serial homology of these organs with the eyes is, then, a fact demonstrated not only by the embryological development, but also by the structural gradations exhibited in the adult animal" (pp. 39I, 392).
The existence of a series of rudimentary sense—organs in Vertebratos was brought to 1ight in 1885 by Froriep and Beard. While it is by no means clear that they are the homo1ogues of the sensory papillæ of anne1ids, it is nevertheless probab1e. It is now generally admitted that they constitute the basis from which the higher Sense-Organs of Vertebrates are derived. These organs are arranged in 1ongitudinal series. The series has become rudimentary or lost in the adult forms of higher existing Vertebrates, but they are present in the embryos Kupffer and others have shown that there are at least two series of these Organs, the upper one corresponding to the lateral line of comparative anatomy, and the 1ower one embrac— ing the so-ca11ed branchia1 Sense-Organs of Beard. From the upper series the ears and nose are probab1y derived, and possibly the eyes.
Allis (’88), in his well-known memoir on the lateral line of Amia, gives Hgures that show a connection between the epithelium of the nasal pit and that of the1ateral1ine. His Fig. I, P1. XXX, representing a larval form one day old, shows the nasal epithelium forming part of the lateral line. In subsequent growth it follows the same course that the surface organs of the lateral System do when they become canal Organs. Allis says, p. 537: The nasal pits are inclosed in the same way that the lateral cana1s are, and the short canal so formed is at first continuous with the canal inclosing organ 5 infra-orbita1.
There is now genera1 agreement that the ears be1ong to the lateral line series, but there has been much dissent on the part of Some morphologists to admitting the eyes to the same category; but the grounds for the Opposition seem to me to be largely removed. It must be said, moreover, that the Organs of taste and touch have not as yet been shown to have any genetic relation to the ganglionic Sense-Organs.
I am g1ad to acknowledge my indebtedness to Minot’s introductory remarks to his chapter on the Sense-Organs. He says: "Very suggestive in this connection are the observations of H. V. Wilson («91), of a thickening of the nervous layer of the epiderrnis on either side of the head in the bass embryo (Serranus atrarius). This thickening forms a long, shallow furrow,
which subsequently divides into three parts, of which the first
becomes a Sense-Organ over the gil1-cleft, the second, the auditory invagination, and the third, the Anlage of the sense-organs of the lateral line. This peculiar development confirms the notion that all these organs belong in one series, but the appearance of a continuous thickening as the Anlage of them all has, as yet, been observed only in this fish, and may not indicate a corresponding ancestral condition Unfortunately, Wilson was unable to make out anything as to the connection of the sensory plate with the ganglia. The sense-organ above the gi1lcleft, although differentiated, is a larval structure only, and disappears in the adult."
A quite similar condition is now known to obtain in some elasmobranch forms. Mitrophanom in 1890, published a preliminary report of his observations on the lateral line of Acanthias and other Elasmobranchs In 1893 he published a full report of the same, illustrated by many iigures He describes a continw ous thickening of the epidermis along the sides of the head embracing the territory of the roots of the seventh to tenth nerves. From this thiclcening there is separated the material for the auditory saucer, the branchia1 sense-organs, and the beginning of the 1ateral line. My observations on this region in Squalus agree with those of Mitrophanow.
In looking over the literature on the sense-organs one cannot fai1 to be struck by the remarkable uniformity of the sensescells in the different kinds of sense-organs. , They are easily reducible to one type, and this, of course, favors the view that they have been derived from a common form.
I. The Lateral Eyes
Squalus acanthias is an especially favorab1e form for observing the beginnings of the optic vesic1es. They make their appearance in this animal at a very early stage, while the neura1 p1ate is broadly expanded, and before the neural folds have arched upwards in any part of the embryo. I should estimate their appearance in this animal to be as early as in any other anima1 in which they have been described. Previous to I889 it had been current opinion that the optic vesic1es appeared very early only in the class of Mammala Bischoff, Kö11iker, His, Van Beneden, Heape, and others had recorded their early appearance in that group ; but it was regarded as a precocious deve1opment for some reason confined to mamma1ian forms.
More careful observations on the earlier stages have shown that the optic vesic1es customarily arise in different anima1s much earlier than was supposed. In I889 Whitman called attention to the very early appearance of the optic vesicles in Necturus: « Its basis being discernib1e as a circular area—-— after treatment with osmic acid, followed by MerkePs fluid —- long before the closure of the neura1 folds of the brain." Dura1noted the early appearance in birds. In 1893 Eyc1eshymer gave notes upon their appearance and structure in Necturus and other Amphibia His most noteworthy observations are upon Rana pa1ustris, a form hitherto unstudied, in which he ftnds a remarkably early deve1opment of the primary optic vesicIes. They are very evident from surface study in this form, on account of the distribution of pigment granules in them, just after the neural ridges have begun to form. cross-sections- in these early stages show a considerable amount of histological differentiation from the surrounding ce11s.
The early differentiation of the optic vesicles had not been recorded in any e1asmobranch form unti1 the publication of my preliminary papers in 1893 and I894, where the optic vesic1es are described and their serial relation to other structures on the cepha1ic plate is show-vix. Since then I have had opportunity to coniirm my observations on Squa1us, and to extend them to some other forms ; for instance, in Diemycty1us and Amblystoma the optic vesicles are well deve1oped in very early stages In Torpedo oce1lata they show fairly well, and in the chick they may be readily made out while the groove is wide1y open.
In Squalus acanthias the optic vesicles are the first ruchments of Sense-Organs to appear. counting out the gastricular cavity (and the neural segments) they are the Hrst organs of any kind to be established. Strangely enough, their early history in these animals has been entirely overlooked. Balfour’s statement that « The eye does not present in its early development any very especial features of interest," seems to have withdrawn attention from that organ in the elasmobranch Hshes.
In order to iix clearly the time when the optic vesicles first appear in Squa1us, we must glance over the early steps in the formation of the head-plate. Pl. XXVL Figs. I, 2, Z, represent three stages that immediately precede the formation of the eyes. In Fig. I the embryo is well established; it is a stage slightly older than Balfoufs stage B. The front part of the embryo is not broader than the rest, and the curve of the anterior margin is uniform and unbrokerr In Fig. 2 we note two changes, vie» that the front end is broader than the part just behind it, which is apparently constricted, and the anterior margin is drawn out into a rounded median cusp. The headend continues to expand laterally (Figs. 3 andr4) until we may (with some appropriateness) speak of it distinctively as a headplate. In the meantime the trunk region has grown longer, and is relatively narrower, so that the entire axial embryo has a Characteristik: appearance of a narrow body terminated by a rounded plate-like head. When this stage has been reached the first satisfactory traces of the eyes become visible ; when first formed they present the appearance that might be produced by pressing down lightly upon a plastic surface with two rounded dies. They are circular areas slightly concave upwards, and occupy a position far forwards on the cephalic p1ate. The early stage of these vesicles may easily escape observation, as they become evident only when the light strikes them properly, and when they are in a favorable position for the observer.
Fig. 5 is engraved from a photograph in swhich the optic vesicles show very well. The specimen from which the photograph was made showed about six mesoblastic somites, and was ZHF mm. in length The circu1ar areas may be made out satisfactorily in slightly earlier stages, in which only three mesoblastic somites are differentiated, and in which the neural fo1ds of both head and trunk are not only broadly expanded, but are even ventrally —curved. The circular depressions are at first separated from one another by a raised welt. This has already been spoken of in Part I as atonguedike process extending from the mediananterior tip bacliwards to two—thirds the length of the cephalic plate. It continues to be a prominent feature of the cephalic plate for some time. I can offer no suggestion as to its signifis cance, outside of the obvious suspicion that it may represent a proboscis of Some lcind, or that it may be related to the 1arge notochord of this region. On this point 1 have no conjectures to make. The depression to form the infundibulum starts a little after the first appearance of the optic vesicles, and cuts the median process in two, so that the anterior tip is separated from the rest, and the depression for optic vesicles and infundibulum combined reaches across the median plane of the cephalic plate (Figs. 6, J, 8, 9, ers-J. The optic vesicles, however, do not reach to the 1ateral margins of the neural folds ; the latter are much expanded beyond them.
It is evident that we cannot, in a strict Sense, speak of such vesicles as "diverticula of the fore-brain." The "diverticula" are found before the fore-brain. The depressions once started grow deeper and press outward laterally ; when fully formed they are cupped, almost rounded in out1ine, concave from within, and they form rounded elevations where they come in contact with the outer layer of epiblast. Figs. H, J, 8, 9, Io, show very well the appearance presented by these primary optic vesicles when viewed from above. Fig. 7 is magnified higher than the others, and is, therefore, larger, but the others are all uniformly en1arged. These figures all show the interior of the optic pits. They represent a range of stages only slightly older than that shown in Fig. 5. In all of them the depression in which the optic vesicle lies extended across the median p1ane, and in its median portion, the depression develops into the infundibulum. The lateral depressions, which form the optic vesicle, sink downwards more rapidly than the median part of the depression, and as a resu1t there is left a ridge or keel separating the optic vesicle and connecting the anteriopmedian tip of the headp1ate with the Central tonguelike process. Miss P1att, in her studies on the head of this animaL says: "It may be here noticed that the downward growth in the Hoor of the brain,y which gives rise to the infundibulum, is prirnitively bilateraL and not median," and I think, therefore, that she saw these circular optic pits in an early stage, but interpreted them as 1atera1 pockets of the infundibu1um. There might be reason to doubt whether they are the optic vesicles, if they could not be traced with perfect continuity into the eyes, and if there were not evidence of histological differentiation in their areas.
Cut 8. - Eight transverse Sections of the embryo shown in Fig. s, Pl. XXVL x about zo diameters The nnmbers below the Sections refer to their Position in the series.
By the time the neural folds of the head begin to rise the optic vesicles are very prominent, and may be observed from the outside and from within. Fig. I 3 shows an embryo viewed obliquely from the 1eft side, and the optic vesicle of that side is seen from the external surface, where it presents the appearance of a rounded eminence 0n the opposite side the optic vesicle is seen from within. cut 9 shows transverse Sections of the head of the same embryo. Figs. 16 and 17 show embryos in which the optic vesicle Ho. A) shows from without, and also from within («:)P.i-). 1n Figs. I9 and 22 the optic vesicle shows clearly from the outside. Transverse Sections of the embryo photographed in Fig. 23, after partial closure of the neural groove, are illustrated in cut I0.
Cut 9.-— Eight transverse Sections of the embryo shown in Pl. XXVL Fig. is. I( about so characters. The accessory optic vesicles H. Ha) have been formed.
These optic vesicles are so well developed at an early period in Squa1us, that it seemed to me probab1e that similar early formed vesicles might exist in Torpedo oce11ata, and have been overlooked by the Zieglers, who have studied the early stages of that form in 1892; accordingly I procured embryos of Torpedo oce1lata from the Zoölogical Station at Naples, and studied the head region with some care. The first thing noted was that the embryos of Torpedo ocellata aire not nearly as favorable objects for observations as those of SquaIus. They are smaller and the beginnings of Sense-Organs, branchiæ, and other structures about the head are by no means so clear as they are in Squa1us. As far as I am ab1e to discern, there is no very distinct differentiation of the optic vesicles, in the nearly stages of this form; nevertheless, they are present at the stage designated C, by the Zieglers, and perhaps a little earlien In studying the actual specimens, I had occasion to note that the Zieglers’ models are admirable representations of the embryos of the form they are intended to represent The optic vesicles are much fainter than they are in Squa1us, but net-ertheless are simi1ar to them. I have studied them in Torpedo both in surface views and in Sections. One noteworthy fact is that they have been sectioned and actua11y Hgured by Ziegler in his Pl. IV, Fig. 19I. It will be noted in this iigure, that the brain-wa11s bulge outwards on either side and these depressions mark the interior of the optic vesicles P1. XXIX, Fig. 65, is a drawing of one of my Sections of Torpedo s1ight1y younger than the one of Zieg1er’s just referred to. The optic vesicles are indicated at pp.
Cut 10. — Twe1ve transverse Sections of the embryo shown in Pl. XXVL Fig. 23. x about zo diameters The number-s below the Sections reker to their positions in the series. In Section 14 the optic vesicles show below and the depressions for the mid-brain vesicles above.
The external appearance of the optic vesicles in later stages is shown in Pl. XXVII, Figs 34, 35, 36, 37. When the stage represented in Fig. 35 has been reached the lens shows externally ; it is forrned in the usual vertebrate way. The choroid fissure shows in Figs. 36 and 37. My studies have not been made to include the later stages of development of the optic vesic1e, but are coniined to its earliest differentiation.
Sections of the earliestckormed circular areas, show something in the direction of histological differentiation Mitoses are more frequent and more of the cells are e1ongated and pear-shaped than in the other parts of the cepha1ic plate. The differentiation is by no means as marked as in the Rana pa1ustris Hgured by Eycleshymer —- nevertheless indications of change are not wholly lacking My Sections are too thick (I0«- to I In) for satisfactory histological study, but one can see in them that the middle of the wa11s of the optic cups are areas of differentiation The differentiation does not progress far until later, but the frequency of dividing cells, and their change of form continue, in these early stages, to be marks for distinguishing visual epithelium from that of the surrounding brain—wal1s. The dividing cells are neurob1asts, and these are known to be points from which thenervedibres spring. Their presence in any considerable number would, therefore, indicate a differentiation in the direction of increase of sensibi1ity.
The eyes are the highest developed of the Sense-Organs, and in that particular stand at the head of the series. But, do the eyes belong to the same series with the other Sense-Organs, or do they occupy a position by themse1ves, can they be homo1ogized with the restk This is a puzzling question, over which there has been much controversy, and upon which there is still much difference of opinion among anatomists The stock objection to their being classed with the other Sense-Organs, is this:—— they have apparently been derived from a different basis, and their nerves are developed in a different way. The eyes are formed out of neural-plate material as diverticula of the fore-brain, while the other Sense-Organs arise outside the neural plate ; they have an independent epiblast origin.
It is questionable, whether any particular stress should be put upon that argument, as the neural plate is at best only a part of the modified epiblast ; moreover, the suggestion that the neural plate is, undoubtedly, very much widened and expanded from its ancestral condition, and that certain sensory area, originally lying outside, may have been included in it, does much to 0ffset that objection. The neural plate has been a prodigiously long time in forming, and the eyes have been brought into closer relationship with it. The plate is broadest in front, and the eyes are the most anterior sense-organs, and have become included in this expansion While there is not sufficient data to give a wholly satisfactory answer to the question propoundech the assumption that the eyes are closely related to all the others is not without foundation, and we are now in the attitude of awaiting further facts.
II. Accessory Optic Vesicles
The neura.1 plate, while still in very young stages, and while the neural grooves are widely open, becomes the seat of some accessory differentiations that resemble the optic vesicles. So far as I know no observations upon these structures have been recorded except those in my preliminary account in the JOURNAL OF« MORPHOLOGL I quote from that paper: But, more interesting than the fact of their (the optic vesicles’) very early appearance in Elasmobranchs, is their apparent relation— ship to other depressions that are formed upon the cephalic plate, behind the already established optic vesicles. The new involutions referred to, make their appearance upon the cephalic plate just baclc of the optic vesic1e. Two of them (Fig. 13, as. v) and Fig. U, as. Eil, ask. IN) take precedence of all others in deve1opment, and are so distinctly formed as to afford a good basis for cornparison with the optic vesicles. They are circular depressions formed in front of the latter, and they produce upon the exterior corresponding rounded e1evations.
The optic vesicles are formed first, and when, at a very litt1e later stage, the others arise behind them, it appears as if the process of eye-formation were repeating itself serial1y.
These circular pockets not only arise in a simi1ar way, but structura1ly resemb1e the optic vesic1es. In cross and 1ongitudina1 Sections the ce11s of the sunken patches are similar to those in the eye pockets. They may be designated accessory optic vesicles.
The assumption that they are primitively visual in character may be going too far, but the basis upon which it rests will be shown directly. The 1east that can be said of them (from their structure, the place and manner of their appearance), I think, is that they are segmental sensory patches.
These structures begin to appear soon after the eye vesic1es. There are several pairs of them stretching in rows baclc of the eyes, as stated above, the two anterior pair are the most prom— inent. They gradually grow fainterz the anterior one is the best differentiated; the second one is not so clear, and the succeeding ones grow fainter as we pass backwards I have counted as many as four pairs in surface studyz but Sections show that the series is more extensive and contains not less than eight pairs of these circular pits. The two anterior ones are well shown in Figs. 9 and ro. These are the earliest surface indications. In Fig. V, which is slightly more advanced, the accessory structures are clearly developedz the ftgure shows four pairs behind the eyes. In longitudinal Sections of the same specimen, Pl. XXIX, Figs. y6——87, eight pairs may be counted. The form of the depression, as seen at first from the surface view, is ovate; they soon become circular patches, and form themselves into concave, shallow cups. I have seen them in many specimens.
It will be noted that they make their Erst appearance while the neural plate is broadly expanded, and spring into prominence while the neural folds are growing upwards It is during the rise of the neural folds that similar, but fainteky vesicles can be made out, in surface study, behind the two anterior pairs.
Pl. XXX, Figs. 88 to 112 show twentyckive transverse sections of an embryo very slightly older than that represented in 5 58 LOCPI IJVo1.. XI,
Pl. XXVL Fig. 16. The Sections 88 to 100 lie in the region of the cepha1ic plate. The following Sections, 105 to 1I2, lie in the neck and trunk region. These Sections are remarkable in bringing to light serial depressions a1ong the walls of the neural folds. They show that the serial cup-like differentia— tions extend back of the cephalic plate. I have not been able to determine the number of serial differentiations of this character in the embryo, but it is clear that there are several pairs behind the cephalic plate upon which I have noted, in surface study, four pairs in addition to the primary optic vesicles.
My sections are not favorable for a critical study of the histological conditions, but it is clear, from them, that« a differentiation starts in the anterior patches simi1ar to that mentioned above, for the true eye vesicles. There is a greater frequency of dividing cells and many of the cells become elongated and pear—shaped.
comparisons of the two anterior pairs with the eye vesicles give the following points of resemblance They are formed in precisely the same way, and present the same general appearancez viewed from within they are all similar concave cups; they produce corresponding elevations on the outer surface, and, if observed from the« outside, they form a series of rounded knobs serially arranged, the eye being the anterior terminations of the row. The marked differentiation in the eye takes place in later stages. There are, however, the histological features spoken of above that serve to distinguish it in early stages, and the accessory vesicle shows the same characteristics
These resemblances, coupled with the existence of serial eyes in Some of the Invertebrates, has led to the assumption that in Vertebrates these rudirnents represent accessory optic vesicles. The fate of the anterior pair would also favor this view ; they pass into the pineal sense—organ, a structure whose visual character is acknowledged.
If the view expressed above is true, we have a multiple-eyed condition in the embryos of these animals. This is common enough in Invertebrates, but has not been previously noticed in Vertebrates.
These accessory structures are present for a brief time only. That region of the head, behind the optic vesicles and in front of the ears, becomes the seat of great modifications, and, as the neural groove closes, the structures described give way to later formed ones. They are, therefore, not only embryonic structures, but are also transitory, It is a truism in develop— mental history that the more primitive characteristics appear first, and the secondary modiiications come in later. The structures described are among the most primitive that have been preserved in this very ancient group of Vertebrates, and should be of signiiicance in indicating the ancestral relations of the eye. I have spoken of the disappearance of the Organs, but I have been able to trace the anterior pair further along, an account of which will be given in the next Section on The Pineal Sense-Organs.
I have also traced out simi1ar rudimentary organs in the embryo chick, at about the conventional 24-hour stage ; there is a series of seven vesicles behind the optic vesicles These structures are rudimentary, and very quickly fade away.
All this acquires new interest when taken in connectiion with the researches of Whitman on the eyes of leeches. As indicated above, he showed that the eyes of the leeches are derived from segmental sensory papillæ. It now appears that we have traces of a somewhat simi1ar line of segmental sensory organs in Vertebrates It is essential to say Drecke-F, for these structures are transitory, and do, not rise to a high grade of differentiation, except in the case of the pineal Sense-Organs. They may never, in the vertebrate group, have been raised to the rank of eyes, but it is certain that in Annelids simi1ar segmental sensory patches have been so raised. It is, however, reasonable to suppose that members of this series have been functionaL even in Vertebrates, when we recollect the highly deve1oped condition of the pineal eye in Some forms.
There is really very little direct evidence to support the proposition that the Vertebrates are derived from multiple—eyed ancestors, although it is aconception that has been in the minds of morphologists for Some time. Besides the indications of such a condition afforded by the facts of this paper there has recently come more evidence bearing in the same direction. This consists in the discovery of multip1e pineal eyes, in several distinct forms, and, in one case, the existence of three distinct nerves to the pineal organs (see Section on The Pineal Sense-Organs)
Whitman, in his paper just cited, says: ssAlthough the evidence appears to me conclusive that the eyes and the segmenta1 papillæ were, originally, morpho1ogical as well as physiological equivalents, it does not, of course, follow necessarily that both Organs now have the same fnnctional signifk cance. The original papillæ may have represented Sense-Organs of a more or less indifferent order, among which, inthe course of the historical development of the leech, a division of Iabor was introduced, a few at the anterior end becoming specialized as lighbperceiving Organs, the rest either remaining in their early indifferent condition or becoming specialized in Some other direction.
"The discovery that these papillæ are Sense-Organs might lead us to speculate on aHinities of a distant and somewhat uncertain nature, such as are supposed by the writerz in common with many others, to exist between annelid worms and Vertebrates At all events, the existence of such organs in the leech furnishes a broader basis for the discussion of the question whether the Vertebrates and Annelids have been derived from a common form possessing metameric Senseorgans, as was first argued by Dr. Eisig of the Naples station. Assuming that the Sense-Organs of the Vertebrate and the segmental papillæ of the leech may be traced to a common origin in some remote ancestral form, it does not follow that they should now present close structural resernblances It is far more important to show that they possess certain general features in common. The most important of their common features is undoubtedly their metameric origin. The nervesupply forms another feature of fundamental importance, in which, according to the interesting observations of Mr. Beard, on the segmental Sense-Organs of the lateral line (Z·:)x)Z. des» VIL Nos. 161 and I62) of the Vertebrate there is essential agreement. The developmenta1 history of these latera1 organs in the Hsh, where they make their first appearance as segmental papillae in the strictest sense of these words, cannot be explained on a more satisfactory hypothesis."
III. The Pineal Sense-Organs
1. Growth of Knowledge regarding the Pineal Sense-Organs
The pineal outgrowth has attracted much attention since the discovery, in I886, of its eye-like structure. Since then it has been accepted by morphologists as a rudimentary Sense-organ. The earlier 1iterature bearing on the subject has been theroughly reported on by Spencer («86) and Francotte («89). several recent1y published papers taken together put the subject in a new light, and, as Ritter («94) has said, «At no time since the epiphysis and pineal eye have been the topic of investigation has it been a more interesting or a more inviting one than it is just now." The growth of our knowledge regarding this remarkable sense-organ, scattered through the various publications of Spencer, Beraneclg Francotte, Strahl and Martin, Leydig, and others, may be reduced to the following summary : i First came the demonstration of its eye-lil(e structure in Amphibia, by De Graaf, and in Lacertilia by Spencer. De Graaf’s publication is much briefer, and was published only a few months before Spencer’s extensive study. Both authors investigated the structure in adult forms. Spencer studied its condition in twenty-eight different species of Lacertilia, and demonstrated very clearly that even in the adult forms of these animals, this organ has an eye-like Struc ture, with a 1ens, a pigmented retinal layer, and a nerve. It seems doubtful, in the light of recent work, whether the kibrous Strand connecting the eye-capsu1e with the epiphysial Stall-i, and designated the nerve by Spencer, is really nerve or not.
Following the study of the adult structure came investigations of the embryonic conditions The first studies dealingv with the embryonic development of this organ, and with its minute structure in the early stages, were those of Strahl and Martin. Along this same line were the studies of Båraneclg Francotte, Leydig, and others. By these investigators the organ was traced backwards to a certain point in its history, that is, to the time when it springs from the roof of. the thalamencephalon as a tubular outgrowth, like the tip of a finger of a glove. To this period also belongs the discovery of a nerve of transitory existence The investigations were coniined almost entire1y to two groups of animals, Amphibia and Reptilia. In the embryos of these animals many points of minute structure were worked out, such as the distribution of the nerve within the eye—like capsule, the division of the retinal part into distinct layers, the fact that the pineal eye is higher deve1oped in the embryonic periods than later, etc.
The next step was the discovery that there is more than one epiphysial outgrowth. Se1enka, Leydig, Eycleshymen Hill, successive1y ca11ed attention to the existence of two distinct outgrowths, or vesicles, from the roof of the thalamencephalon in Anguis, Amblystoma, and Teleosts.
Hi11’s studies on these two vesicles are the most complete He has shown the existence in Teleosts and Amia of two vesicles, anterior and posterior, and has traced them through the stages of deve1opment. The posterior develops into the epiphysis and the anterior degenerates. The dista1 portion of the former is histologically very complex, and receives a nerve from the posterior commissure. Hill made out the distribution of nervedibres in this part of the epiphysis. He traced quite clearly the history of the vesic1es.
Two vesic1es, an upper and a lower, have long been known to exist in Petromyzon, in both embryos and adu1ts. It is much to be regretted that we do not know their embryonic history, for much depends on this. The most recent paper on the epiphysial Organs in Petromyzon—that of studniöka—— does not clear up the question of the origin of the two vesic1es. Histological material was 1acking in just the stage required. The two outgrowths in embryos outside the Cyclostomes have been designated epiphysis, for the hinder, and pineal eye for the anterior. Studniöka designates them pineal and parapineal, respectively, in the Cyclostomes As I shall show latet, the so-ca11ed paraphysis is a different structure.
Beraneck showed that the pineal eye and epiphysis have been confused, and, on account of the existence of the nerve from outside sources, he argued for the complete independence of the pineal eye.
Klinckowstrom has p1aced himse1f in Opposition to this opinion, showing the eye vesicle is formed as an outgrowth from the epiphysis.
Next to the discovery of two or more epiphysial outgrowths may be mentioned the existence of accessory pineal eyes The ear1iest publication containing an account of such Structur-es is that of Duva1 and Kalt, in I889. In a brief note (Des Yeux Pinåaux Multiples chez 1’Orvet) these authors record the fjnding of two or three accessory pineal vesic1es in Anguis. They do not say whether adu1t or embryonic forms were studied.
Carriere ('89), in the same year, noted the occurrence in embryos of Anguis of a very rudimentary accessory pineal vesic1e.
Leydig ('90) observed these structures independently in embryos of the same anirnals He showed that there is much variabi1ity, both as to the presence and the histological Structure of these accessory capsu1es. In Some individuals they are lacking, and in others of the same age present and well deve1oped. He records the occurrence, in Some cases, of two accessory Organs, a. larger and a smaller one. The larger one resembles the pineal eye in structure and arrangement of pigment. The smaller one is very rudimentary Leydig com— pared them to the ocelli of insects. It is c1ear from his account that variabi1ity and inconstancy are characteristics of these accessory Organs.
Prenanh in the latter part of I893, again records the occurrence of one accessory pineal eye in the embry0s of Anguis He directs attention to the fact that up to that time they had been discovered only in a single species, VIII» Anguis fragilis, and further, that they were all in ernbryos, and not in adults. Ritter (-94) has recently recorded the, occurrence of such an» accessory org-an in the adult Phrynosoma.
The above observations, taken together, give suflicient positive data to establish the fact that there appears, from time to time, in some animals the vestiges of accessory pineal Organs, and that they are variab1e and inconstant in their occurrence.
The most recent advances consist in carrying the history of these organs baclcwards, and showing them to be connected with patches of sensory epithelium that arise on the cephalic plate in very young stages, and in the discovery, in Iguana, of three distinct nerves connected with the epiphysial outgrowths. The former work was done by the present writer, and the latter by Klinckowström.
As indicated in the preceding Section, there are several pairs of cups on the cephalic plate back of the optic vesicle, formed while the neural groove is widely open. I have designated them accessory optic vesicles In the process of closure of the neural groove the anterior pair are brought together, and form part of the thalamencephalon, from the roof of which the pinea1 outgrowth is derived. This process will be described more in detai1.
Klinckowström ('98) has recent1y shown in Iguana the presence of two nerves connected with the anterior outgrowth or pineal eye, and sometimes a third connected with the posterior soutgrowth or epiphysis.
These observations suggest new interpretationsz still it is not an auspicious moment to indulge in speculation. It is clearly evident that we need more data regarding these organs and their relationships But the trend of these discoveries is to strengthen the suggestion already made in this paper, that, primitively, the Vertebrates were multiple-eyed. The presence of accessory pineal eyes, the discovery of serial sensory areas on the cephalic plate from which epiphysial outgrowths arise, and the presence of two or three pinea1 nerves, are all consistent with this interpretation so far as the evidence goes, there is more than one epiphysial outgrowth, and therefore I have headed this Section, The Pineal Sense-Organs. In what way it will be necessary to qualify this proposition will depend on subsequent discoveries.
The results of all the embryo1ogica1 studies on the epiphysia1 outgrowth between 1887 and 1893 were to settle on one point of common agreement regarding its origin. Each investigator successiveiy found it to arise as a tubular outgrowth from the roof of the tha1amencepha1on comparative1y late in ontogeny, after the parts of the brain are estab1ished. No earlier trace of it had been found; but there was a previous undiscovered history, and it is now time1y to make the inquiry, What is the very beginning of the pinea1 Sense-Organs?
2. Remote Origins of the Pineal Outgrowth
As already indicated, there are upon the cephalic plate accessory eye vesicles which have made their appearance while the neural groove is widely open; and in tracing their fate I shall attempt to f111 up that gap in the history of the pineal outgrowth from the time these cup—1ike structures appear upon the cepha1ic p1ate to the time the tubu1ar outgrowth begins from the tha1amencephalon.
As the wa11s of the neural groove grow upwards these Cuplike structures are carried up vvith them; and there comes a time (Figs. I3, 3I) when they may be seen from the outside as rounded eminences, and from the inside as concave cups.
When the edges of the neural groove meet in the middle line the cups are approximatech and come to form part of the thalamencepha1on.
While these changes have been going on, further differentiations have been taking place in the wa11s of the brain. The bulging of the wa11s to form the mid-brain vesicle has come on insidious1y, and has taken a position behind the vesicles of the paired eyes, in apparent1y the same position previously ssoccupied by the accessory vesicles. These transformations are confusing, as the wa11s of the mid-brain resemble the accessory optic vesicles grown largerx 1n an ear1ier published paper I made the mistake of identifying the mid-brain with the accessory optic vesicles; but it was merely an error of identification, and did not affect the main contentionof that artie1e.
In working out the details of the formation of pineal outgrowth in Squa1us, it soon becomes apparent that the surface contours of the head are considerably altered by the distribution of mesoblastic Hcells lying between the external layer of epiblast and the brain-waIIs. The mesoblast forms a pad of varying thickness in close contact with the brain—wa1ls; the depressions are tilled, and in some places thick patches of mesoblast give rise to external rnarkings that render the surface appearances untrustworthy. There is, for instance, a deep pad of mesoblast tilling up a depression at the sides of the cerebellum, and also extending forwards on to the midbrain; at the external surface the pad assumes a circular form, and it is that pad of mesoblast which is seen in Fig. 32. The true mid-brain vesicle in that Hgure is the eminence marked wo. The accessory optic vesicles are present, but are not well marked externally. They are in front of the protuberance marked »O.
In order to get a view of the brain-walls I have found it necessary to remove the mesoblast and its coverings of epib1ast, and thus to complete1y expose the brain-wal1s. The brain thus laid bare, enables one to see with complete satisfac— tion its different parts and their relation to one another. A very interesting relation comes to light through the dissections. Somewhere between the stage with an open neural groove (Fig. 3I) and the completely closed groove (Fig. 33) the midbrain vesicle insidious1y takes the former position of the accessory optic vesicle while the latter is carried forward by the pr0cess of cranial flexure During the formation of the midbrain vesicle the two structures become incorporated into one faintly bi1obed protuberance (PI. XXVIIL Figs. 38 and 39). The anterior part is the accessory optic vesicle, and the posterior one the mid—brain. The separation soon becomes com— p1ete, and by the stage represented in Fig. 40 the two are completely separatedz but owing to the arrangernent of the mesoderm the accessory optic vesicle is rendered indistinguishable from the outside. When the mesoderm is entirely removed it may be seen. All this takes place in very brief intervals of time, and a complete series of embryos representing the different phases of the closure of the neural groove is required to see it all. I must say also that the changes are so perplexing that there exists in my mind Some doubt as to the adequacy of the above account. 1t may have to be modified, but I have given the facts as they now appear to me.
Figs. 38-54 show a series of embryos that have been partly dissected in the way just indicated They show characteristic changes in the brain-wa1ls as seen from the exterior.
In Fig. 38 the vvalls of the neural groove have not yet met in any part of their course, and the thalamencephalon cannot be distinguished from the rest of the brain. The first pair of accessory vesic1es are visible; they soon become inc0rporated in the walls of the tha1amencephalon. I have not been able to determine whether it is the epithe1ium of the first pair of accessory vesicles only, or whether epithe1ium from the second pair is also included. Either the epithe1ium of the two pairs is incorporated into the walls of the tha1amencepha1on by being carried together, or the epithe1ium of the second pair fades into the surrounding substance of the brain-wa11, which almost immediately grows into the vesicle of the mid-brain. Although I am doubtful as to whether the second pair of accessory vesicles pass into the inter——brain, I am sure that the epithe1ium of the anterior pair is so included.
Fig. 39 shows the first accessory optic vesicle and the midbrain vesicle forming a common protuberance, but they are very quiclcly separated, and the anterior vesicle goes into the tha1amencepha1on.
In Fig. 40, which is somewhat o1der, this has occurred The lateral wa11s of the thalamencephalon now consist of two shallow cups, approximated so that the structure looks lenticular when viewed from above. As a usual thing, the thalamencephalon is not evident at this stage before the removal of the overlying tissues. Fig. 4I shows, however, a specimen in which it could be seen before any dissection. Compare this with Fig. 32, which is the more usual appearance of an embryo of this age.
Fig. 42 is a dissection of the embryo shown in Fig. 41. The mid-brain, which, from external view, appears like a single rounded eminence, is shown after exposure to be bilobed.
Figs. 43, 44, and 45, which are successively o1der, show an increase in the size of the thalamencephalon, and marked changes in the mid-brain.
In Figs. 46 and 47 a faint furrow has appeared in front that serves to mark the boundary between the thalamencephalon proper and the cerebral lobes. This furrow is clearly deiined in Figs. 48 and 49, so that we have now the thalamencepha1on distinctly marked off from the rest of the fore-brain.
In Fig. 48 the optic vesicle and all the mesob1ast have been removed, and the view is talcen from the right side. The brain— walls show very clearly. The tha1amencephalon is now bounded anterior1y and posterior1y by furrows. The two furrows run nearly parallel to one another, and they serve to mark oft« the inter-brain very distinct1y.
The roof of the thalamencephalon is at this stage raised into two rounded eminences of nearly equal dimensions, an anterior and a posterior one. The posterior is, from the beginning, somewhat in advance of the other. It becomes the pineal out— growth, while the posterior eminence goes on deve1oping. The anterior one becomes greatly reduced by compression from the rapidly growing adjacent brain-walls. It very soon loses its rounded Character, and becomes pressed into a semicircular fo1d lying in front of the epiphysis. It is, I thinl(, equivalent to the Zirbelpolster of German author-s. If longitudinal sections be made at the stage represented in Fig. 48, the two e1evations show as in Minot’s Fig. 3I9, p. 5y2, of his Hasen« EmZ--;1-·c)!og·j-. In that Hgure they look like two equal vesicles springing from a single elevation of the brain roof, and«opening by a common wide passage into the brain vesicle No especial signif1cance, I think, is to be attached to the fact that there are at first two equal embossrnents from the roof of the tha1amencephalon. The anterior one is converted into a fold of the roof of the ’tween—brain that has long been recognized in different anima1s. It is designated Zirbelpolster by Burckhardt It does not in any sense correspond to the anterior epiphysial vesicle discovered by Hill in Teleosts, but rather to the fold of ’tween-brain that lies in front of both epiphyses. Studniölca has, however, marked the fold in diagramrnatic figures of Teleosts, the anterior vesicle of Hi1l, but this identification is not right.
We have in this specimen the first appearance of the tubular outgrowth, which, accorciing to previous authors, marks the beginning of the epiphysia It is somewhat important to fix the stage at which this outgrowth begins. Its very first appearance as an elevation of the roof of the thalamencephalon is in the stage represented in Fig. 36 after the saucersstage of the ear is past.
In Fig. 49 the posterior protuberance of the roof of the thalamencephalon is assuming a tubular form. It becomes the epiphysis The surface of the mid-b1«ain is now considerab1y changed. Two furrows have made their appearance, which divide the 1ateralwalls into three nearly equal 1obes. These do not show at all from the exterior before the removal of the epidermis and the mesoblast.
The gradual reduction of the thalamencephalon by compression between the Central hemispheres and the mid-brain is shown consecutively in Figs. 53, 54, II, If, and 6o. In Fig. 54, and 1ater, the epiphysis appears pear—shaped from the side, but from in front the enlargement on the distal end is seen to be broader than thick. It is borne upon a hol1ow sta1k.
Fig. 57 represents the brain of an embryo considerably older than in Fig. 56. The thalamencephalon is now very much compressed. The anterior half of it, which originally had a rounded protuberance growing from its roof, is now reduced to a small semicircular fold in front of the epiphysis.
Fig. 59 is a view upon the epiphysis of the same embryo from directly in front. It was obtained by removing the cerebral 1obes, which lie in front of, and partly hide the epiphysia The epiphysis is seen to be composed of a Stall( and an en1arged dista1 extremity ; leading from the Stall( on either side are the epiphysialpedunc1es. In this hgure we are looking through the ventricle of the thalamencephalon into that of the midbrain. On the inside of the lateral walls of the opening are two enlargements, the beginnings of the thala1ni optici. These contain the ganglia hebenulae.
In this specigmen the paraphysis had made its appearance as an outgrowth from the cerebrum. It is relatively late in arising, and is entirely distinct from any outgrowth from the thalamencephalon The lateral wal1s of the mid-brain are no 1onger10bed as in earlier stages. Fig. 58 is the same brain viewed from above ; we are looking directly into the cavity of the fourth ventricle.
Fig. 6o shows the brain of a still older embryo. It is the largest embryo of squalus in my collection, measuring 35 mm. in length. The cerebral lobes are divided into right and left halves by a sha11ow furrow. From the back of the cerebrum rises the paraphysis (P); just behind this is seen the semicircu1ar fo1d, belonging to the ’tween-brain, and behind that is the epiphysis, rising above and resting upon the anterior wal1 of the mid-brain. Fig. 6I shows a front view of the cerebral lobes after being removed from the rest of the brain ; the paraphysis is also brought into view. Fig. 62 is the same brain with the cerebral lobes removed, and p1aced so as to give a ful1-face view on the epiphysisz the stalk is considerably longer than in Fig. 59.
I have not had materia1 to work out the subsequent history of the epiphysis In specimens about six inches long it is situated in the wall of the cranium, directly over the cerebral lobes, and is connected to the roof of the ’tween-brain by a long threadJilce stallc The enlarged distal end is about the same size as in Fig. 62. cattie shows it in the adult with several capsules on the end of the stalk.
Sections of the stages just studied serve to coniirm the observations made from the outside. They do« not add very much to its features, speaking strictly morphologically The surface views have been prepared in such a way that they may stand for reconstruction, but they are more accurate than any actual reconstruction can be, as they show the relations entirely undisturbed.
Figs. 119—124 show a series of sagittal Sections covering the period of the first appearance of the epiphysis, the reduction of the anterior part of the thalamencepha1on, and the beginning of the paraphysis and the choroid plexus.
Fig. I19 is a specimen of the same age as that shown in Fig. 48. It shows that the roof of the thalarnencephalon is raised into two nearly equa1 protuberances.
Fig. I20 shows the beginning of the reduction of the anterior of these elevations, and the increased growth of the posterior one. In Fig. 121 the posterior e1evation, or epiphysis, has become a tubular outgrowth, while the anterior one is reduced and depressed.
In Fig. 122 the tubular Stall( of the epiphysis is considerably elongatech while that part of the roof of the thalamencephalon in front of it has become reduced to a narrowfo1d that has been carried ventracL The paraphysis has now arisen from the roof of the prosencepha1on. The beginning of the choroid plexus extends from the structure down into the ventric1e of the fort-brain, and its posterior part is connected with the fold in front of the epiphysis The anterior and posterior Commissures are now distinguishab1e.
Figs. 123 and 124 exhibit snbstantially the same relations ; the choroid plexus has considerably increased in extent, and is rapidly becoming much convolutedz the brain vesicles have become reduced. In Fig. 124 the dista1 end of the epiphysis is inserted into a cavity in the roof of the cranial Wall, that is, the mesenchyme forms a cup over its rounded end.
3. Comparisons between Epiphysial Outgrowths
The recent studies have done much towards establishing a basis for comparison between the epiphysial outgrowths in different groups of anima1s. Beraneclg Francotte, and others claim that the anterior epiphysial vesic1e—common1y designated the pineal eye — is formed independently, having no generic connection with the posterior one, or epiphysis Hill’s observations on the Teleosts coincide with this view. Klinclcowström, on the other hand, claims that the anterior one is formed at the expense of the posterior one, and Substantiates his conclusions with figures of Sections of 1guana, showing the two vesiclesin union with one anothetc It is not slifficult to conceive how the condition represented by Klinckowström might be brought about, and still not be inharmonious with the other observations. The two vesicles might, in ear1ier stages than he represents, have been formed indepencL ently, side by side, and thrown into communication by an upward growth of the brain roof involving them both. Or, indeed, there may be variations in the details of the formas tions of these two vesic1es.
However this may be, the resu1ts of the different observers all go to show conclusively that there are two (not counting the paraphysis) distinct outgrossvths from the roof of the thalas mencephalon of Petromyzon, Teleosts, and Lacerti1ia. The fate of the anterior vesicle varies in the different groups, while the posterior one persists in all as the epiphysis The anterior one may be formed as a rudimentary organ of transitory existence, as in Teleostsz or it may be developed into the pineal eye, in front of the epiphysis, as in Lacertilia.
The sources of nervesupply to the two vesic1es have been made known in Cyc1ostomes, Teleosts, and Lacerti1ia, and these facts are important in establishing homologies. It has been shown (studniåka, Gaskelh KlinckowströrrO that the upper capsule of Petromyzon receives its nerves from the posterior commissure, and that the 1ower vesicle is innervated from the left ganglion hebenu1a. Hi1l, in embryos of Te1eosts, has traced a nerve from the posterior commissure into the epipipzysis and has studied its distribution in detail in that structure The anterior vesicle of Te1eosts disappears before it has formed any nerve connection with the brain. Klinckowström («9s) has observed very interesting conditions in embryos of Iguana. In this anima1 he found in one case three nerves distributed to the parietwpineal Organs: one coming from behind and entering the epiphysis, and two coming from right and left ganglion hebenula, respective1y, and entering the pinea1 eye. He found the right and left nerves in three different embryos of Iguana eighteen days old. In one case the nerve from the left ganglion hebenula was nearly the same size as that from the right, but in the other two individuals the left nerves were much reduced l It should be added that Klinckowstrom has found the nerve to the pineal eye in this animal to come normally from the right ganglion hebenu1a, while in Petromyzon it comes from the ganglion of the left Ziele. But, his discovery of the occasiona1 presence of two nerves in Iguana, the left one being smallen affords an interesting transition between the two. It also enables us to account for the marked asymmetry in the ganglia hebenulæ in Petromyzon, on the hypothesis that the right ganglion, in Cyclostomes has, for some reason, ceased to have any nerve connection with the pineal Organ, while the left ganglion has retained its connection.
As Beraneck has shown, a nerve leading to the pineal eye arises in front of the epiphysis, and has but a transitory existence.
Putting the facts together and basing relationships on thenervesupply, and comparative study of the vesic1es, we may express the probable homologies in the following diagrams and tab1e :
I. Uppser vcsicle in Petkomyzon, epiphysis in Teleost and Lacerti1ia. II. Lower vesicle in Petrotny2on, HiIPs anterior vesicle in Teleosts, pineal eye in laccrtilia IV. Nerve to posted-im« seminis-sure. XI. Nerve from lower vesicle in Petromyzon to Ganglion hebenuliy embryonic nerve of transitory existence in Lacertiliix
+++Reformat table below+++
s I PETRoMYzoN. THE-USE. . LACERTILIA. I H . . . « Posterior epiphysial Is the superior Is the epiphysis Is the epiphysis vesicle of I-Ii1l. vesicle. Nerve- with nerve con- net-ve- s u p pl y
niislca. « posterior com— posterior com- commissure. Epiphysis of others. [ missure. missure.
Pinealorgan of stucl- I. 1 supply from necting it to from posterior
Anterior epiphysial f "Is the inferior Is formed as in Becomes pineal
vesicle of Hill. ] l l vesicle. the two other eye supplied
Parapineal Organ of Ell. groups, but with a nerve
studniislca. j l aborts. which early cle[ generates
If these comparisons are well taken, we have in the Fette— myzon, the epiphysis in its highest state of development Its distal end is enlarged into an eye—like capsu1e that is more differentiated than their inferior vesicle which represents the pinea1 eye. This is the reverse of what is true in most Lacertilia, and, on this account the upper capsule of Petrornyzon has usually been eompared with the pineal eye of Reptilia and Amphibia But the fact that the superior capsule in Petrornyzon receives its nervesupply from the posterior commissure, corresponding in this particular with the epiphysis of other animals, has much more weight in determining its homo1ogies than any purely structural resemblances We are on a better foundation, therefore, in comparing the upper capsule of Cyclostomes to the epiphysis of other animals, than in taking the other Position. It may be noted, in passing, that the superior capsu1e is in the same relative position as the epiphysis, and the inferior one corresponds in position with the pineal eye.
This view would make the epiphysis eye—like in Character, and there are many facts that weigh in that direction. Hil1’s studies speak strongly for the visual nature of the epiphysis He shows a high degree of differentiation in the nerve (:ells to which the nerve from the posterior commissure is distributed. There is also formed in Iguana, Plica, and Phrynosorna an eyelike enlargement at the distal extremity of the epiphysis.
Between the condition in Petrornyzon and that in the Lacertilia many gradations have already been brought to light.
In Teleosts, the anterior vesicle arises, as in the other groups, but it is transitory, ancl soon degenerates It would appear from a recent paper by Ritter, that the anterior vesicle in Phrynosoma is more persistent than in Teleosts, but does not reach the grade of differentiation exhibited in the Lacertilia. He gives a Hgure showing the rare occurrence of the anterior vesicle in an adult Phrynosoma, and although large this capsule is not so highly differentiated as the capsule behind it, but to compensate for this the distal end of the epiphysis is highly developed.t· ln other korms of Lacertilia, the anterior vesicle reaches a high grade of perfection, while the epiphysis is not well developed. In most cases, however, there is a deposit of pigment in the distal Position of the epiphysis. The nerve distribution in that organ is known only in the Teleosts.
We might carry our comparisons further and inquire whether the structure present in selachians is to be homo1ogized with the epiphysis or the pineal eye of other forms. The nerve relations are not known in the Se1achians and, theref0re, we have not, as yet, a satisfactory basis for comparison. In its structure and persistent attachment to the brain roof by a sta1k, the outgrowth in Se1achians resembles the epiphysis and one would be inc1ined to say that only the epiphysis is represented in the Elasmobranchs, and that the pineal eye is lacking. Klinckowström has offered the ingenious suggestion that in Se1achians and birds the second or anterior epiphysis is never ful1y separated from the posterior, and this is worth thinking about. There is no positive evidence to support it.
However the above question may be settled, it seems to me that the fundamental features of the morphology of the epiphysis are tolerably clear, and that future work on this subject will be main1y in the direction of ampliiication and discovery of details.
The invertebrate homologies of the pineal eye are not known. Spencer supposed it to be homologous with the median eye of Tunicates, but that relation is a strained one.
Various authors have compared the pineal outgrowths to the ocelli of Arthropods Leydig compares them to the stemmata of Insects, and shows that there are many resemblances to support the comparison.
Patten has recent1y studied the deve1opment of the eyes of Limulus, and argues for a homo1ogy between the median eyes of these Arthropods and the pineal eye of Vertebrates He shows the median eye of Limulus to. arise by the fusion of paired eye-stallcs, giving us a case of undoubted union of originally paired Sense-Organs. I have c1aimed a similar occurrence in the epiphysis of Squa1us. But it seems to me that the evidence is too circumstantial at present to bear the interpretation that we already know the invertebrate homologue of the vertebrate pinea1 Organ.
4. The Double Nature of the Epiphysis
Receives support from two sources. It is diflicult to interpret Klinckowströmk diScovery of a double nerve in Iguana on any other hypothesis That interpretation is also strengthened by the claim I have made of tracing the epiphysis in Se1achians back to a pair of epithelial cups on the cepha1ic plate.
Hill ('94) regards the two vesicles he has discovered in Teleosts as having been primitively side by side; and Ritter («94) has recently suggested that the epiphysis and the parapineal organ of Studniälca are right and left mates rather than independent eyes of double origin. It will be remembered, in this connectiorn that Klinckowström found three nerves in one individual of Iguana, two from the ganglia hebenulae, and one from the posterior commissure; that Leydig discovered two accessory pineal organs in Anguis ; Duval and Kalt found as many as three of these Organs, and I have found several pairs of epithelial patches back of the eyes on the cephalic plate, and have traced one pair into the epiphysis It seems to me that all this evidence is more favorable to the idea that the pineal Sense-Organs are multip1e, and individually of paired origin, than to the idea expressed by Ritter.
Considerab1e confusion has arisen in identifying the paraphysis in different forrns. As soon as it was made known that there are two outgrowths from the roof of the fore-brain, it was at once assumed that the most anterior one is the paraphysis, and the posterior one the epiphysis But there is now evidence that there are, at times, more than two outgrowths Hill has shown in Amia the presence at one and the same time of three tubular outgrowths from the roof of the fore-brain. Two of these come from the thalamencephalom andthe anterior one arises from the prosencephalon He points out that the 1atter is the paraphysis proper, and that in Amia we have to deal with two other outgrowths from the roof of the thalamencepha1on. These he makes homo1ogous with the two vesicles he has described in Teleosts. Hill concludes that in the 1atter group the paraphysis is lacking on account of the nomdevelopment of the choroid plexus This identification is in harmony with the original description of the paraphysis Francotte, who described it in 188 J, and Selenka (-90), to whom the credit of its discovery is usually accorded, both state that it arises from the prosencephal0n. Selenka says: « Wie das Zwischenhirn seine Epiphyse, so hat das Vorderhirn seine Paraphyse.« Francotte suggested that it represents the beginning of the choroid plexus.
Minot ('92), p. 69o, designates the anterior vesicle discovered by Hill (-91) in Coregonus the «paraphysis," but the Structure in question arises from the roof of the thalamencepha1on, and between it and the prosencephalon is a marked downward fold in the brain-wal1, which in many other forms is present behind the paraphysis. It is more probable, as Hill suggests in his1ater paper («94), that the paraphysis is lacking in the Te1eosts.
It is to be understood that in some forms there are two epiphysia1 outgrowths from the thalamencepha1on that are entirely independent of the paraphysis When the latter Structure is present in conjunction with the former two, as in Amia, there are then three separate outgrowths from the roof of the fore-brain.
As already indicated in this paper, the paraphysis is present in Squalus as an outgrowth from the prosencephalon, and is connected with the choroid plexus.
IV. The Beginning of the Auditory Organ
The formation of the auditory saucer is preceded by a thickening of the epidermis along the sides of the head in the auditory region. The thickening portion occupies a larger area than that used by the ear vesicle when it is Erst formed, and it is coniiuent with the epidermal thickening just above the branchiae As Mitrophanow has pointed out, this thickened patch of epithelium is composite in character; it embraces not only the epithelium for the formation of the auditory plate, but that to form the so—called branchial Sense-organs, and also the beginning of the lateral line. I had noted this relationship in Squa1us before seeing Mitrophanoufs paper, and 1 lind no com— parison that my resu1ts correspond —— so far as the earlier Stages are concerned—very closely with his. He has, however, carried his studies considerably farther, and has shown how the different branches of the lateral-line System arise.
The general relationship noted between the auditory plate, the branchia1 Sense-organs, and the lateral line is very similar to what Wilson found in sea bass. In that form the Connection between ear, branchial Organs, and lateral line is evidently more c1ear on account of the presence of a distinct sensory furrow and the way in which the separation into three parts takes place.
The general thickening in the sharks is not so well circumscribed. The fact that these different organs proceed from a common epithelial thickening indicates relationship of a fundamental kind. After separation the ear gives further evidence of its relationship with the lateral organs by preserving its canal (endolyrnphatic duct) connection with the exterior and developing in a manner that is characteristic of the canal organs of the lateral line.
Mitraphanow departs from the usual point of view that the organs of the lateral line are metarnericz and in that particular, I think, I should be inclined to follow him.
The auditory plate is at first differentiated from the general thickening, and its epithelium then becomes gradually rounded up into a circu1ar area, that is depressed in the centre—the auditory saucer. This structure sometimes covers the space of three neuromeres, and I have one surface preparation in which the outer epithelium shows three bars running vertically across it; these bars correspond with the underlying neuromeres.
We may interpret these superficial bars either as being moulded upon the underlying neural segments, or as being due to the same general cause as the neura1 segments I am of the opinion that the latter alternative is the correct one.
When iirst formed, the auditory saucer is opposite the ninth neuromerez but subsequent1y, while it is still in the saucer condition, it shifts backwards, and cornes to lie at the side of the tenth neural segrnenh and later still, as a capsule, it lies opposite the eleventh neuromere.
The history of the auditory vesicle in sharks has been worked out beyond this period by Ayers (-92), and it is very clear from its mode of growth that it is directly related to the canal organs of the lateral 1ine.
A consideration of the so—ca11ed branchia1 Sense-Organs and their ganglia is reserved to be published later with the part on the Nerves.
- Note. — I have been indebted to the persons mentioned below for material that has helped jill up the gaps in rny collection of embryos, and I desire here to express Iny appreciation of their Kindness.
- To Miss Julia B. platt, for the loan of Sections of Acanthias ; to Dr. E. L. Mark, for young stages of Squalus ; to Mr. F rank smith, for a considerable number of embryos ; to Professor J. E. Reighard for the loan and free use of his entire collection of elasmobranch embryos; to Professor A. D. Morrill, for early stages of Squalus.
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- Locy 1895 Contents: General Introduction | Part I - Metamerism of the Head | Part II - The Sense-Organs | Figures
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