The Works of Francis Balfour 1-20
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol I. Separate Memoirs (1885)
XX. On the Development of the Skeleton of the Paired Fins of Elasmobranchii, considered in Relation to its Bearings on the Nature of the Limbs of the Vertebrata
(With Plate 33.)
SOME years ago the study of the development of the soft parts of the fins in several Elasmobranch types, more especially in Torpedo, led me to the conclusion that the vertebrate limbs were remnants of two continuous lateral fins 2 . More or less similar views (which I was not at that time acquainted with) had been previously held by Maclise, Humphrey, and other anatomists ; these views had not, however, met with much acceptance, and diverge in very important points from those put forward by me. Shortly after the appearance of my paper, J. Thacker published two interesting memoirs comparing the skeletal parts of the paired and unpaired fins 3 .
In these memoirs Thacker arrives at conclusions as to the nature of the fins in the main similar to mine, but on entirely independent grounds. He attempts to shew that the structure of the skeleton of the paired fins is essentially the same as that of the unpaired fins, and in this comparison lays special stress on the very simple skeleton of the pelvic fin in the cartilaginous Ganoids, more especially in Acipenscr and Polyodon. He points out that the skeleton of the pelvic fin of Polyodon consists essentially of a series of nearly isolated rays, which have a strikingly similar arrangement to that of the rays of the skeleton in
1 From the Proceedings of the Zoological Society of London, 1881.
2 "Monograph on the Development of Elasmobranch Fishes," pp. 319, 320.
3 J. K. Thacker, " Median and Paired Fins ; a Contribution to the History of the Vertebrate Limbs," Trans, of the Connecticut Acad. Vol. in. 1877. "Ventral Fins of Ganoids," Trans, of the Connecticut Acad. Vol. iv. 1877.
SKELETON OF THE PAIRED FINS OF ELASMOBRANCHS. 715
many unpaired fins. He sums up his views in the following way 1 :
"As the dorsal and anal fins were specializations of the median folds of Amphioxus, so the paired fins were specializations of the two lateral folds which are supplementary to the median in completing the circuit of the body. These lateral folds, then, are the homologues of Wolffian ridges, in embryos of higher forms. Here, as in the median fins, there were formed chondroid and finally cartilaginous rods. These became at least twice segmented. The orad ones, with more or less concrescence proximally, were prolonged inwards. The cartilages spreading met in the middle line ; and a later extension of the cartilages dorsad completed the limb-girdle.
" The limbs of the Protognathostomi consisted of a series of parallel articulated cartilaginous rays. They may have coalesced somewhat proximally and orad. In the ventral pair they had extended themselves mesiad until they had nearly or quite met and formed the hip-girdle ; they had not here extended themselves dorsad. In the pectoral limb the same state of things prevailed, but was carried a step further, namely, by the dorsal extension of the cartilage constituting the scapular portion, thus more nearly forming a ring or girdle."
The most important point in Thacker's theories which I cannot accept is the derivation of the folds, of which the paired fins of the Vertebrata are supposed to be specializations, from the lateral folds of Amphioxus ; and Thacker himself recognizes that this part of his theory stands on quite a different footing to the remainder.
Not long after the publication of Thacker's paper, an important memoir was published by Mivart in the Transactions of this Society 2 . The object of the researches recorded in this paper was, as Mivart explains, to test how far the hard parts of the limbs and of the azygos fins may have arisen through centripetal chondrifications or calcifications, and so be genetically exoskeletal 3 .
1 Loc. cit. p. 298.
2 St George Mivart, "On the Fins of Elasmobranchii," Zoological Trans. Vol. X.
3 Mivart used the term exoskeletal in an unusual and (as it appears to me) inconvenient manner. The term is usually applied to dermal skeletal structures ; but the
46 2
716 DEVELOPMENT OF THE SKELETON
Mivart's investigations and the majority of his views were independent of Thacker's memoir ; but he acknowledges that he has derived from Thacker the view that pelvic and pectoral girdles, as well as the skeleton of the limbs, may have arisen independently of the axial skeleton.
The descriptive part of Mivart's paper contains an account of the structure of a great variety of interesting and undescribed types of paired and unpaired fins, mainly of Elasmobranchii. The following is the summary given by Mivart of the conclusions at which he has arrived ' :
" i. Two continuous lateral longitudinal folds were developed, similar to dorsal and ventral median longitudinal folds.
" 2. Separate narrow solid supports (radials), in longitudinal series, and with their long axes directed more or less outwards at right angles with the long axis of the body, were developed in varying extents in all these four longitudinal folds.
" 3. The longitudinal folds became interrupted varidusly, but so as to form two prominences on each side, i.e. the primitive paired limbs.
" 4. Each anterior paired limb increased in size more rapidly than the posterior limb.
" 5. The bases of the cartilaginous supports coalesced as was needed, according to the respective practical needs of the different separate portions of the longitudinal folds, i.e. the respective needs of the several fins.
"6. Occasionally the dorsal radials coalesced (as in Notidanus, &c.) and sought centripetally (Pristis, &c.) adherence to the skeletal axis.
" 7. The radials of the hinder paired limb did so more constantly, and ultimately prolonged themselves inwards by mesiad growth from their coalesced base, till the piscine pelvic structure arose, as, e.g., in Squatina.
" 8. The pectoral radials with increasing development also coalesced proximally, and thence prolonging themselves inwards to seek a point cTappui, shot dorsad and ventrad to obtain a firm support, and at the same time to avoid the visceral cavity.
skeleton of the limbs, with which we are here concerned, is undoubtedly not of this nature.
1 Loc. cit. p. 480.
OF THE PAIRED FINS OF ELASMOBRANCHS. 717
Thus they came to abut dorsally against the axial skeleton, and to meet ventrally together in the middle line below.
" 9. The lateral fins, as they were applied to support the body on the ground, became elongated, segmented, and narrowed, so that probably the line of the propterygium, or possibly that of the mesopterygium, became the cheiropterygial axis.
" 10. The distal end of the incipient cheiropterygium either preserved and enlarged preexisting cartilages or developed fresh ones to serve fresh needs, and so grew into the developed cheiropterygium ; but there is not yet enough evidence to determine what was the precise course of this transformation.
" II. The pelvic limb acquired a solid connection with the axial skeleton (a pelvic girdle) through its need of a point cVappui as a locomotive organ on land..
" 12. The pelvic limb became also elongated ; and when its function was quite similar to that of the pectoral limb, its structure became also quite similar (e.g. Ichthyosaurus, Plesiosaurus, CJielydra, &c.) ; but for the ordinary quadrupedal mode of progression it became segmented and inflected in a way generally parallel with, but (from its mode of use) in part inversely to, the inflections of the pectoral limb."
Giinther 1 has propounded a theory on the primitive character of the fins, which, on the whole, fits in with the view that the paired fins are structures of the same nature as the unpaired fins. The interest of Giinther's views on the nature of the skeleton of the fins more especially depends upon the fact that he attempts to evolve the fin of Ceratodus from the typical Selachian type of pectoral fin. His own statement on this subject is as follows z :
" On further inquiry into the more distant relations of the Ceratodus-\\mb t we may perhaps be justified in recognizing in it a modification of the typical form of the Selachian pectoral fin. Leaving aside the usual treble division of the carpal cartilage (which, indeed, is sometimes simple), we find that this shovellike carpal forms the base for a great number of phalanges, which are arranged in more or less regular transverse rows (zones) and in longitudinal rows (series). The number of phalanges of
1 " Description of Ceratodus,'" Phil. Trans. 1871. ' 2 Loc. cit. p. 534.
7l8 DEVELOPMENT OF THE SKELETON '
the zones and series varies according to the species and the form of the fin ; in Cestracion philippi the greater number of phalanges is found in the proximal zones and middle series, all the phalanges decreasing in size from the base of the fin towards the margins. In a Selachian with a long, pointed, scythe-shaped pectoral fin, like that of Ceratodus, we may, from analogy, presume that the arrangement of the cartilages might be somewhat like that shewn in the accompanying diagram, which I have divided into nine zones and fifteen series.
" When we now detach the outermost phalanx from each side of the first horizontal zone, and with it the other phalanges of the same series, when we allow the remaining phalanges of this zone to coalesce into one piece (as, in nature, we find coalesced the carpals of Ceratodus and many phalanges in Selachian fins), and when we repeat this same process with the following zones and outer series, we arrive at an arrangement identical with what we actually find in Ceratodus"
While the researches of Thacker and Mivart are strongly confirmatory of the view at which I had arrived with reference to the nature of the paired fins, other hypotheses as to the nature of the skeleton of the fins have been enunciated, both before and after the publication of my memoir, which are either directly or indirectly opposed to my view.
Huxley in his memoir on Ceratodus, which throws light on so many important morphological problems, has dealt with the nature of paired fins 1 .
He holds, in accordance with a view previously adopted by Gegenbaur, that the limb of Ceratodus "presents us with the nearest known approximation to the fundamental form of vertebrate limb or archipterygium," and is of opinion that in a still more archaic fish than Ceratodtis the skeleton of the fin " would be made up of homologous segments, which might be termed pteromeres, each of which would consist of a mesomere with a preaxial and a postaxial paramere." He considers that the pectoral fins of Elasmobranchii, more especially the fin of Notidamts, which he holds to be the most primitive form of Elasmobranch fin, " results in the simplest possible manner from the
1 T. H. Huxley, " On Ceratodus Fosteri, with some Observations on the Classification of Fishes," Proc. Zool. Soc. 1876.
OF THE PAIRED FINS OF ELASMOBRANCHS. 719
shortening of the axis of such a fin-skeleton as that of Ceratodus, and the coalescence of some of its elements." Huxley does not enter into the question of the origin of the skeleton of the pelvic fin of Elasmobranchii.
It will be seen that Huxley's idea of the primitive structure of the archipterygium is not easily reconcilable with the view that the paired fins are parts of a once continuous lateral fin, in that the skeleton of such a lateral fin, if it has existed, must necessarily have consisted of a series of parallel rays.
Gegenbaur 1 has done more than any other living anatomist to elucidate the nature of the fins ; and his views on this subject have undergone considerable changes in the course of his investigations. After Gunther had worked out the structure of the fin of Ceratodus, Gegenbaur suggested that it constituted the most primitive persisting type of fin, and has moreover formed a theory as to the origin of the fins founded on this view, to the effect that the fins, together with their respective girdles, are to be derived from visceral arches with their rays.
His views on this subject are clearly explained in the subjoined passages quoted from the English translation of his Elements of Comparative Anatomy, pp. 473 and 477.
"The skeleton of the free appendage is attached to the extremity of the girdle. When simplest, this is made up of cartilaginous rods (rays), which differ in their size, segmentation, and relation to one another. One of these rays is larger than the rest, and has a number of other rays attached to its sides. I have given the name of archipterygium to the ground-form of the skeleton which extends from the limb-bearing girdle into the free appendage. The primary ray is the stem of this archipterygium, the characters of which enable us to follow out the lines of development of the skeleton of the appendage. Cartilaginous arches beset with the rays form the branchial skeleton. The form of skeleton of the appendages may be compared with
1 C. Gegenbaur, Untersuchungen z. vergleich. Anat. d. Wirbelthiere (Leipzig 1864-5): erstes Heft, "Carpus u. Tarsus;" zweites Heft, " Brustflosse d. Fische." "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere," Jenaische Zeitschrift, Vol. v. 1870. " Ueb. d. Archipterygium," Jenaische Zeitschrift, Vol. vn. 1873. " Zur Morphologic d. Gliedmaassen d. Wirbelthiere, " Morphologisches Jahrbuch, Vol. II. 1876.
72O DEVELOPMENT OF THE SKELETON
them ; and we are led to the conclusion that it is possible that they may have been derived from such forms. In the branchial skeleton of the Selachii the cartilaginous bars are beset with simple rays. In many a median one is developed to a greater size. As the surrounding rays become smaller, and approach the larger one, we get an intermediate step towards that arrangement in which the larger median ray carries a few smaller ones. This differentiation of one ray, which is thereby raised to a higher grade, may be connected with the primitive form of the appendicular skeleton ; and as we compare the girdle with a branchial arch, so we may compare the median ray and its secondary investment of rays with the skeleton of the free appendage.
"All the varied forms which the skeleton of the free appendages exhibits may be derived from a ground-form which persists in a few cases only, and which represents the first, and consequently the lowest, stage of the skeleton in the fin the archipterygium. This is made up of a stem which consists of jointed pieces of cartilage, which is articulated to the shouldergirdle and is beset on either side with rays which are likewise jointed. In addition to the rays of the stem there are others which are directly attached to the limb-girdle.
" Ceratodus has a fin-skeleton of this form ; in it there is a stem beset with two rows of rays. But there are no rays in the shoulder-girdle. This biserial investment of rays on the stem of the fin may also undergo various kinds of modifications. Among the Dipnoi, Protopterus retains the medial row of rays only, which have the form of fine rods of cartilage; in the Selachii, on the other hand, the lateral rays are considerably developed. The remains of the medial row are ordinarily quite small, but they are always sufficiently distinct to justify us in supposing that in higher forms the two sets of rays might be better developed. Rays are still attached to the stem and are connected with the shoulder-girdle by means of larger plates. The joints of the rays are sometimes broken up into polygonal plates which may further fuse with one another ; concrescence of this kind may also affect the pieces which form the base of the fin. By regarding the free rays, which are attached to these basal pieces, as belonging to these basal portions, we are able to
OF THE PAIRED FINS OF F.LASMOBRANCHS. 721
divide the entire skeleton of the fin into three segments pro-, meso-, and metapterygium.
"The metapterygium represents the stem of the archipterygium and the rays on it. The propterygium and the mesopterygium are evidently derived from the rays which still remain attached to the shoulder-girdle."
Since the publication of the memoirs of Thacker, Mivart, and myself, a pupil of Gegenbaur's, M. v. Davidoff 1 , has made a series of very valuable observations, in part directed towards' demonstrating the incorrectness of our theoretical views, more especially Thacker's and Mivart's view of the genesis of the skeleton of the limbs. Gegenbaur 2 has also written a short paper in connection with Davidoff's memoir, in support of his own as against our views.
It would not be possible here to give an adequate account of Davidoff's observations on the skeleton, muscular system, and nerves of the pelvic fins. His main argument against the view that the paired fins are the remains of a continuous lateral fin is based on the fact that a variable but often considerable number of the spinal nerves in front of the pelvic fin are united by a longitudinal commissure with the true plexus of the nerves supplying the fin. From this he concludes that the pelvic fin has shifted its position, and that it may once therefore have been situated close behind the visceral arches. Granting, however, that Davidoff's deduction from the character of the pelvic plexus is correct, there is, so far as I see, no reason in the nature of the lateral-fin theory why the pelvic fins should not have shifted ; and, on the other hand, the longitudinal cord connecting some of the ventral roots in front of the pelvic fin may have another explanation. It may, for instance, be a remnant of the time when the pelvic fin had a more elongated form than at present, and accordingly extended further forwards.
In any case our knowledge of the nature and origin of nervous plexuses is far too imperfect to found upon their characters such conclusions as those of Davidoff.
1 M. v. Davidoff, " Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. Fische, I.," Morphol. Jahrbuch, Vol. V. 1879.
2 "Zur Gliedmaassenfrage. An die Untersuchungen von Davidoff's angekniipfte Bemerkungen," Morphol. Jahrbuch, Vol. v. 1879.
722 DEVELOPMENT OF THE SKELETON
Gegenbaur, in his paper above quoted, further urges against Thacker and Mivart's views the fact that there is no proof that the fin of Polyodon is a primitive type ; and also suggests that the epithelial line which I have found connecting the embryonic pelvic and pectoral fins in Torpedo may be a rudiment indicating a migration backwards of the pelvic fin.
With reference to the development of the pectoral fin in the Teleostei there are some observations of 'Swirski 1 , which unfortunately do not throw very much light upon the nature of the limb.
'Swirski finds that in the Pike the skeleton of the limb is formed of a plate of cartilage continuous with the pectoral girdle, which soon becomes divided into a proximal and a distal portion. The former is subsequently segmented into five basal rays, and the latter into twelve parts, the number of which subsequently becomes reduced.
The observations which I have to lay before the Society were made with the object of determining how far the development of the skeleton of the limbs throws light on the points on which the anatomists whose opinions have just been quoted are at variance.
They were made, in the first instance, to complete a chapter in my work on comparative embryology ; and, partly owing to the press of other engagements, but still more to the difficulty of procuring material, my observations are confined to the two British species of the genus Scy Ilium, viz. Sc. stellare and Sc. canicula; yet I venture to believe that the results at which I have arrived are not wholly without interest.
Before dealing with the development of the skeleton of the fin, it will be convenient to describe with great brevity the structure of the pectoral and pelvic fins of the adult. The pectoral fins consist of broad plates inserted horizontally on the sides of the body ; so that in each there may be distinguished a dorsal and a ventral surface, and an anterior and a posterior border. Their shape may best be gathered from the woodcut (fig. i) ; and it is to be especially noted that the narrowest part
1 G. 'Swirski, Untersuch. ilb. d. Entivick. d. Schtdtergiirtds u. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1 880.
OF THE PAIRED FINS OF ELASMOBRANCHS.
723
of the fin is the base, where is it attached to the side of the body. The cartilaginous skeleton only occupies a small zone at the base of the fin, the remainder being formed of a fringe supported by radiately arranged horny fibres 1 .
FIG. i.
Pectoral fins and girdle of an adult of Scyllium canicula (natural size,
seen from behind and above).
co. Coracoid. sc. scapula, pp. propterygium. me p. mesopterygium. mp. metapterygium. fn. part of fin supported by horny fibre.
FIG. 2.
JJTL
Right pelvic fin and part of pelvic girdle of an adult female of Scyllium
canmila (natural size).
il. iliac process, pn. pubic process, cut across below, bp. basipterygium. a/, anterior cartilaginous fin-ray articulated to pelvic girdle, fn. part of fin supported by horny fibres.
1 The horny fibres are mesoblastic products; they are formed, in the first instance, as extremely delicate fibrils on the inner side of the membrane separating the epiblast from the mesoblast.
724 DEVELOPMENT OF THE SKELETON
The true skeleton consists of three basal pieces articulating with the pectoral girdle ; on the outer side of which there is a series of more or less segmented cartilaginous fin-rays. Of the basal cartilages one (J>p) is anterior, a second (mep] is placed in the middle, and a third is posterior (mp}. They have been named by Gegenbaur the propterygium, the mesopterygium, and the metapteryginm ; and these names are now generally adopted.
The metapterygium is by far the most important of the three, and in Scyllium canicula supports 12 or 13 rays 1 . It forms a large part of the posterior boundary of the fin, and bears rays only on its anterior border.
The mesopterygium supports 2 or 3 rays, in the basal parts of which the segmentation into distinct rays is imperfect ; and the propterygium supports only a single ray.
The pelvic fins are horizontally placed, like the pectoral fins, but differ from the latter in nearly meeting each other along the median ventral line of the body. They also differ from the pectoral fins in having a relatively much broader base of attachment to the sides of the body. Their cartilaginous skeleton (woodcut, fig. 2) consists of a basal bar, placed parallel to the base of the fin, and articulated in front with the pelvic girdle.
On its outer border it articulates with a series of cartilaginous fin-rays. I shall call the basal bar the basipterygium. The rays which it bears are most of them less segmented than those of the pectoral fin, being only divided into two ; and the posterior ray, which is placed in the free posterior border of the fin, continues the axis of the basipterygium. In the male it is modified in connection with the so-called clasper.
The anterior fin-ray of the pelvic fin, which is broader than the other rays, articulates directly with the pelvic girdle, instead of with the basipterygium. This ray, in the female of Scyllium canicula and in the male of Scyllinm catulus (Gegenbaur), is peculiar in the fact that its distal segment is longitudinally divided into two or more pieces, instead of being single as is the case with the remaining rays. It is probably equivalent to two of the posterior rays.
1 In one example where the metapterygium had 13 rays the mesopterygium had only 2 rays.
OF THE PAIRED FINS OF ELASMOBRANCHS. 725
Development of the paired Fins. The first rudiments of the limbs appear in Scy Ilium, as in other fishes, as slight longitudinal ridge-like thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins.
These ridges are two in number on each side an anterior immediately behind the last visceral fold, and a posterior on the level of the cloaca. In most Fishes they are in no way connected ; but in some Elasmobranch embryos, more especially in that of Torpedo, they are connected together at their first development by a line of columnar-epiblast cells. This connecting line of columnar epiblast, however, is a very transitory structure. The rudimentary fins soon become more prominent, consisting of a projecting ridge both of epiblast and mesoblast, at the outer edge of which is a fold of epiblast only, which soon reaches considerable dimensions. At a later stage the mesoblast penetrates into this fold, and the fin becomes a simple ridge of mesoblast covered by epiblast. The pectoral fins are at first considerably ahead of the pelvic fins in development.
The direction of the original epithelial line which connected the two fins of each side is nearly, though not quite, longitudinal, sloping somewhat obliquely ventralwards. It thus comes about that the attachment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line shortly behind the anus.
The embryonic muscle-plates, as I have elsewhere shewn, grow into the bases of the fins ; and the cells derived from these ingrowths, which are placed on the dorsal and ventral surfaces in immediate contact with the epiblast, probably give rise to the dorsal and ventral muscular layers of the limb, which are shewn in section in Plate 33, fig. I m, and in Plate 33, fig. 7 m.
The cartilaginous skeleton of the limbs is developed in the indifferent mesoblast cells between the two layers of muscles. Its early development in both the pectoral and the pelvic fins is very similar. When first visible it differs histologically from the adjacent mesoblast simply in the fact of its cells being more concentrated ; while its boundary is not sharply marked.
At this stage it can only be studied by means of sections. It arises simultaneously and continuously with the pectoral and pelvic girdles, and consists, in both fins, of a bar springing at
/26 DEVELOPMENT OF THE SKELETON
right angles from the posterior side of the pectoral or pelvic girdle, and running parallel to the long axis of the body along the base of the fin. The outer side of this bar is continued into a thin plate, which extends into the fin.
The structure of the skeleton of the fin slightly after its first differentiation will be best understood from Plate 33, fig. T, and Plate 33, fig. 7. These figures represent transverse sections through the pelvic and pectoral fins of the same embryo on the same scale. The basal bar is seen at bp, and the plate at this stage (which is considerably later than the first differentiation) already partially segmented into rays at br. Outside the region of the cartilaginous plate is seen the fringe with the horny fibres (h. f.) ; and dorsally and ventrally to the cartilaginous skeleton are seen the already well-differentiated muscles (#2).
The pectoral fin is shewn in horizontal section in Plate 33, fig. 6, at a somewhat earlier stage than that to which the transverse sections belong. The pectoral girdle (p. g^) is cut transversely, and is seen to be perfectly continuous with the basal bar (vp) of the fin. A similar continuity between the basal bar of the pelvic fin and the pelvic girdle is shewn in Plate 33, fig. 2, at a somewhat later stage. The plate continuous with the basal bar of the fin is at first, to a considerable extent in the pectoral, and to some extent in the pelvic fin, a continuous lamina, which subsequently segments into rays. In the parts of the plate which eventually form distinct rays, however, almost from the first the cells are more concentrated than in those parts which will form the tissue between the rays ; and I am not inclined to lay any stress whatever upon the fact of the cartilaginous fin-rays being primitively part of a continuous lamina, but regard it as a secondary phenomenon, dependent on the mode of conversion of embryonic mesoblast cells into cartilage. In all cases the separation into distinct rays is to a large extent completed before the tissue of which the plates are formed is sufficiently differentiated to be called cartilage by an histologist.
The general position of the fins in relation to the body, and their relative sizes, may be gathered from Plate 33, figs. 4 and 5 which represent transverse sections of the same embryo as that from which the transverse sections shewing the fin on a larger scale were taken.
OF THE PAIRED FINS OF ELASMOBRANCHS. 727
During the first stage of its development the skeleton of both fins may thus be described as consisting of a longitudinal bar running along the base of the fin, and giving off at right angles series of rays which pass into the fin. The longitudinal bar may be called the basipterygium ; and it is continuous in front with the pectoral or pelvic girdle, as the case may be.
The further development of the primitive skeleton is different in the case of the two fins.
The Pelvic Fin. The changes in the pelvic fin are comparatively slight. Plate 33, fig. 2, is a. representation of the fin and its skeleton in a female of Scyllium stellare shortly after the primitive tissue is converted into cartilage, but while it is still so soft as to require the very greatest care in dissection. The fin itself forms a simple projection of the side of the body. The skeleton consists of a basipterygium (bp}, continuous in front with the pelvic girdle. To the outer side of the basipterygium a series of cartilaginous fin-rays are attached the posterior ray forming a direct prolongation of the basipterygium, while the anterior ray is united rather with the pelvic girdle than with the basipterygium. All the cartilaginous fin-rays except the first are completely continuous with the basipterygium, their structure in section being hardly different from that shewn in Plate 33, fig. i.
The external form of the fin does not change very greatly in the course of the further development ; but the hinder part of the attached border is, to some extent, separated off from the wall of the body, and becomes the posterior border of the adult fin. With the exception of a certain amount of segmentation in the rays, the character of the skeleton remains almost as in the embryo. The changes which take place are illustrated by Plate 33, fig. 3, shewing the fin of a young male of Scyllium stellare. The basipterygium has become somewhat thicker, but is still continuous in front with the pelvic girdle, and otherwise retains its earlier characters. The cartilaginous fin-rays have now become segmented off from it and from the pelvic girdle, the posterior end of the basipterygial bar being segmented off as the terminal ray.
The anterior ray is directly articulated with the pelvic girdle, and the remaining rays continue articulated with the basipterygium. Some of the latter are partially segmented.
728 DEVELOPMENT OF THE SKELETON
As may be gathered by comparing the figure of the fin at the stage just described with that of the adult fin (woodcut, fig. 2), the remaining changes are very slight. The most important is the segmentation of the basipterygial bar from the pelvic girdle.
The pelvic fin thus retains in all essential points its primitive structure.
The Pectoral Fin. The earliest stage of the pectoral fin differs, as I have shewn, from that of the pelvic fin only in minor points (PL 33, fig. 6). Therq is the same longitudinal or basipterygial bar (bp], to which the fin-rays are attached, which is continuous in front with the pectoral girdle (p g). The changes which take place in the course of the further development, however, are very much more considerable in the case of the pectoral than in that of the pelvic fin.
The most important change in the external form of the firi is caused by a reduction in the length of its attachment to the body. At first (PL 33, fig. 6), the base of the fin is as long as the greatest breadth of the fin; but it gradually becomes shortened by being constricted off from the body at its hinder end. In connection with this process the posterior end of the basipterygial bar is gradually rotated outwards, its anterior end remaining attached to the pectoral girdle. In this way this bar comes to form the posterior border of the skeleton of the fin (PL 33, figs. 8 and 9), constituting the metapterygium (mp\ It becomes eventually segmented off from the pectoral girdle, simply articulating with its hinder edge.
The plate of cartilage, which is continued outwards from the basipterygium, or, as we may now call it, the metapterygium, into the fin, is not nearly so completely divided up into fin-rays as the homologous part of the pelvic fin; and this is especially the case with the basal part of the plate. This basal part becomes, in fact, at first only divided into two parts (PL 33, fig. 8) a small anterior part at the front end (me. /), and a larger posterior along the base of the metapterygium (mp) ; and these two parts are not completely segmented from each other. The anterior part directly joins the pectoral girdle at its base, resembling in this respect the anterior fin-ray of the pelvic girdle. It constitutes the (at this stage undivided) rudiment of the meso
OF THE PAIRED FINS OF ELASMOBRANCHS. 729
pterygium and propterygium of Gegenbaur. It bears in my specimen of this age four fin-rays at its extremity, the anterior not being well marked. The remaining fin-rays are prolongations outwards of the edge of the plate continuous with the metapterygium. These rays are at the stage figured more or less transversely segmented; but at their outer edge they are united together by a nearly continuous rim of cartilage. The spaces between the fin-rays are relatively considerably larger than in the adult.
The further changes jn the cartilages of the pectoral limb are, morphologically speaking, not important, and are easily understood by reference to PL 33, fig. 9 (representing the skeleton of the limb of a nearly ripe embryo). The front end of the anterior basal cartilage becomes segmented off as a propterygium (//), bearing a single fin-ray, leaving the remainder of the cartilage as a mesopterygium (mes). The remainder of the now considerably segmented fin-rays are borne by the metapterygium.
General Conclusions. From the above observations, conclusions of a positive kind may be drawn as to the primitive structure of the skeleton ; and the observations have also, it appears to me, important bearings on the theories of my predecessors in this line of investigation.
The most obvious of the positive conclusions is to the effect that the embryonic skeleton of the paired fins consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft parts of the fins, which have the form of a longitudinal ridge ; and they are continuous at their base with a longitudinal bar. This bar, from its position at the base of the fin, can clearly never have been a median axis with the rays on both sides. It becomes the basipterygium in the pelvic fin, which retains its embryonic structure much more completely than the pectoral fin; and the metapterygium in the pectoral fin. The metapterygium of the pectoral fin is thus clearly homologous with the basipterygium of the pelvic fin, as originally supposed by Gegenbaur, and as has since been maintained by Mivart. The propterygium and mesopterygium are obviously relatively unimportant parts of the skeleton as compared with the metapterygium.
B. 47
730 DEVELOPMENT OF THE SKELETON
My observations on the development of the skeleton of the fins certainly do not of themselves demonstrate that the paired fins are remnants of a once continuous lateral fin ; but they support this view in that they shew the primitive skeleton of the fins to have exactly the character which might have been anticipated if the paired fins had originated from a continuous lateral fin. The longitudinal bar of the paired fins is believed by both Thacker and Mivart to be due to the coalescence of the bases of the primitively independent rays of which they believe the fin to have been originally composed. This view is probable enough in itself, and is rendered more so by the fact, pointed out by Mivart, that a longitudinal bar supporting the cartilaginous rays of unpaired fins is occasionally formed ; but there is no trace in the embryo Scylliums of the bar in question being formed by the coalescence of rays, though the fact of its being perfectly continuous with the bases of the fin-rays is somewhat in favour of such coalescence.
Thacker and Mivart both hold that the pectoral and pelvic girdles are developed by ventral and dorsal growths of the anterior end of the longitudinal bar supporting the fin-rays.
There is, so far as I see, no theoretical objection to be taken to this view ; and the fact of the pectoral and pelvic girdles originating continuously and long remaining united with the longitudinal bars of their respective fins is in favour of it rather than the reverse. The same may be said of the fact that the first part of each girdle to be formed is that in the neighbourhood of the longitudinal bar (basipterygium) of the fin, the dorsal and ventral prolongations being subsequent growths.
On the whole my observations do not throw much light on the theories of Thacker and Mivart as to the genesis of the skeleton of the paired fin ; but, so far as they bear on the subject, they are distinctly favourable to those theories.
The main results of my observations appear to me to be decidedly adverse to the views recently put forward on the structure of the fin by Gegenbaur and Huxley, both of whom, as stated above, consider the primitive type of fin to be most nearly retained in Ceratodus, and to consist of a central multisegmented axis with numerous lateral rays.
OF THE PAIRED FINS OF ELASMOBRANCHS. 731
Gegenbaur derives the Elasmobranch pectoral fin from a form which he calls the archipterygium, nearly like that of Ceratodiis, with a median axis and two rows of rays but holds that in addition to the rays attached to the median axis, which are alone found in Ceratodus, there were other rays directly articulated to the shoulder-girdle. He considers that in the Elasmobranch fin the majority of the lateral rays on the posterior (or median according to his view of the position of the limb) side have become aborted, and that the central axis is represented by the metapterygium ; while the pro- and mesopterygium and their rays are, he believes, derived from those rays of the archipterygium which originally articulated directly with the shoulder-girdle.
This view appears to me to be absolutely negatived by the facts of development of the pectoral fin in Scyllium not so much because the pectoral fin in this form is necessarily to be regarded as primitive, but because what Gegenbaur holds to be the primitive axis of the biserial fin is demonstrated to be really the base, and it is only in the adult that it is conceivable that a second set of lateral rays could have existed on the posterior side of the metapterygium. If Gegenbaur's view were correct, we should expect to find in the embryo, if anywhere, traces of the second set of lateral rays ; but the fact is that, as may easily be seen by an inspection of figs. 6 and 7, such a second set of lateral rays could not possibly have existed in a type of fin like that found in the embryo. With this view of Gegenbaur's it appears to me that the theory held by this anatomist to the effect that the limbs are modified gill-arches also falls, in that his method of deriving the limbs from gill-arches ceases to be admissible, while it is not easy to see how a limb, formed on the type of the embryonic limb of Elasmobranchs, could be derived from a gill-arch with its branchial rays.
Gegenbaur's older view, that the Elasmobranch fin retains a primitive uniserial type, appears to me to be nearer the truth than his more recent view on this subject ; though I hold the ' fundamental point established by the development of these parts in Scyllimn to be that the posterior border of the adult Elasmobranch pectoral fin is the primitive base-line, i.e. line of attachment of the fin to the side of the body.
472
732 DEVELOPMENT OF FINS OF ELASMOBRANCHS.
Huxley holds that the mesopterygium is the proximal piece of the axial skeleton of the limb of Ceratodus, and derives the Elasmobranch fin from that of Ceratodus by the shortening of its axis and the coalescence of some of its elements. The entirely secondary character of the mesopterygium, and its total absence in the young embryo Scyllium, appear to me as conclusive against Huxley's view as the character of the embryonic fin is against that of Gegenbaur ; and I should be much more inclined to hold that the fin of Ceratodus has been derived from a fin like that of the Elasmobranchs by a series of steps similar to those which Huxley supposes to have led to the establishment of the Elasmobranch fin, but in exactly the reverse order.
There is one statement of Davidoff's which I cannot allow to pass without challenge. In comparing the skeletons of the paired and unpaired fins he is anxious to prove that the former are independent of the axial skeleton in their origin and that the latter have been segmented from the axial skeleton, and thus to shew that an homology between the two is impossible. In support of his view he states 1 that he has satisfied himself, from embryos of Acanthias and Scyllium, that the rays of the unpaired fins are undoubtedly products of the segmentation of tJie dorsal and ventral spinous processes.
This statement is wholly unintelligible to me. From my examination of the development of the first dorsal and the anal fins of Scyllium I find that their rays develop at a considerable distance from, and quite independently of, the neural and haemal arches, and that they are at an early stage of development distinctly in a more advanced state of histological differentiation than the neural and haemal arches of the same region. I have also found exactly the same in the embryos of Lepidosteus.
I have, in fact, no doubt that the skeleton of both the paired and the unpaired fins of Elasmobranchs and Lepidosteus is in its development independent of the axial skeleton. The phylogenetic mode of origin of the skeleton both of the paired and of the unpaired fins cannot, however, be made out without further
investigation.
1 Loc. til. p. 514.
EXPLANATION OF PLATE 33. 733
EXPLANATION OF PLATE 33.
Fig. i. Transverse section through the pelvic fin of an embryo of Scy Ilium belonging to stage P 1 , magnified 50 diameters, bp. basipterygium. br. fin ray. m. muscle, hf. horny fibres supporting the peripheral part of the fin.
Fig. 2. Pelvic fin of a very young female embryo of Scyllium stellare, magnified 1 6 diameters, bp. basipterygium. pu. pubic process of pelvic girdle (cut across below), il. iliac process of pelvic girdle, fa. foramen.
Fig. 3. Pelvic fin of a young male embryo of Scyllium stellare, magnified 16 diameters, bp. basipterygium. mo. process of basipterygium continued into clasper. il. iliac process of pelvic girdle, pu. pubic section of pelvic girdle.
Fig. 4. Transverse section through the ventral part of the trunk of an embryo Scyllium of stage P, in the region of the pectoral fins, to shew how the fins are attached to the body, magnified 18 diameters, br. cartilaginous fin-ray, bp. basipterygium. m. muscle of fin. mp. muscle-plate.
Fig. 5. Transverse section through the ventral part of the trunk of an embryo Scyllium of stage P, in the region of the pelvic fin, on the same scale as fig. 4. bp. basipterygium. br. cartilaginous fin-rays, m. muscle of the fins. mp. muscleplate.
Fig. 6. Pectoral fin of an embryo of Scyllium canicula, of a stage between O and P, in longitudinal and horizontal section (the skeleton of the fin was still in the condition of embryonic cartilage), magnified 36 diameters, bp. basipterygium (eventual metapterygium). fr. cartilaginous fin-rays, p g. pectoral girdle in transverse section. fo. foramen in pectoral girdle, pe. epithelium of peritoneal cavity.
Fig. 7. Transverse section through the pectoral fin of a Scyllium embryo of stage P, magnified 50 diameters, bp. basipterygium. br. cartilaginous fin-ray, m. muscle. hf. horny fibres.
Fig. 8. Pectoral fin of an embryo of Scyllium stellare, magnified 16 diameters. mp. metapterygium (basipterygium of earlier stage), me.p. rudiment of future proand mesopterygium. sc. cut surface of a scapular process, cr. coracoid process. fr. foramen, hf. horny fibres.
Fig. 9. Skeleton of the pectoral fin and part of pectoral girdle of a nearly ripe embryo of Scyllium stellare, magnified 10 diameters, mp. metapterygium. mes. mesopterygium. pp. propterygium. cr. coracoid process.
1 I employ here the same letters to indicate the stages as in my "Monograph on Elasmobranch Fishes."
XXI. On the evolution of the Placenta, and on the possibility of employing the characters of the Placenta in the classification of the Mammalia
FROM Owen's observations on the Marsupials it is clear that the yolk-sack in this group plays an important (if not the most important) part, in absorbing the maternal nutriment destined for the foetus. The fact that in Marsupials both the yolk-sack and the allantois are concerned in rendering the chorion vascular, makes it a priori probable that this was also the case in the primitive types of the Placentalia ; and this deduction is supported by the fact that in the Rodentia, Insectivora, and Cheiroptera this peculiarity of the foetal membranes is actually found. In the primitive Placentalia it is also probable that from the discoidal allantoic region of the chorion simple foetal villi, like those of the Pig, projected into uterine crypts ; but it is not certain how far the umbilical region of the chorion, which was no doubt vascular, may also have been villous. From such a primitive type of fcetal membranes divergencies in various directions have given rise to the types of foetal membranes found at the present day.
In a general way it may be laid down that variations in any direction which tended to increase the absorbing capacities of the chorion would be advantageous. There are two obvious ways in which this might be done, viz. (i) by increasing the complexity of the foetal villi and maternal crypts over a limited area, (2) by increasing the area of the part of the chorion covered by the placental villi. Various combinations of the two processes would also, of course, be advantageous.
1 From the Proceedings of the Zoological Society of London, t88i.
THE EVOLUTION OF THE PLACENTA. 735
The most fundamental change which has taken place in all the existing Placentalia is the exclusion of the umbilical vesicle from any important function in the nutrition of the foetus.
The arrangement of the foetal parts in the Rodentia, Insectivora, and Cheiroptera may be directly derived from the primitive form by supposing the villi of the discoidal placental area to have become more complex, so as to form a deciduate discoidal placenta, while the yolk-sack still plays a part, though physiologically an unimportant part, in rendering the chorion vascular.
In the Carnivora, again, we have to start from the discoidal placenta, as evinced by the fact that in the growth of the placenta the allantoic region of the placenta is at first discoidal, and only becomes zonary at a later stage. A zonary deciduate placenta indicates an increase both in area and in complexity. The relative diminution of the breadth of the placental zone in late foetal life in the zonary placenta of the Carnivora is probably due to its being on the whole advantageous to secure the nutrition of the foetus by insuring a more intimate relation between the foetal and maternal parts, than by increasing their area of contact. The reason of this is not obvious, but, as shewn below, there are other cases where it is clear that a diminution in the area of the placenta has taken place, accompanied by an increase in the complexity of its villi.
The second type of differentiation from the primitive form of placenta is illustrated by the Lemuridae, the Suidae, and Manis. In all these cases the area of the placental villi appears to have increased so as to cover nearly the whole subzonal membrane, without the villi increasing to any great extent in complexity. From the diffused placenta covering the whole surface of the chorion, differentiations appear to have taken place in various directions. The placenta of Man and Apes, from its mode of ontogeny, is clearly derived from a diffused placenta (very probably similar to that of Lemurs) by a concentration of the foetal villi, which are originally spread over the whole chorion, to a disk-shaped area, and by an increase in their arborescence. Thus the discoidal placenta of Man has no connexion with, and ought not to be placed in, the same class as those of the Rodentia, Cheiroptera, and Insectivora.
736 THE EVOLUTION OF THE PLACENTA.
The polycotyledonary forms of placenta are due to similar .concentrations of the fcetal villi of an originally diffused placenta.
In the Edentata we have a group with very varying types of placenta. Very probably these may all be differentiations within the group itself from a diffused placenta such as that found in Manis. The zonary placenta of Orycteropus is capable of being easily derived from that of Manis by the disappearance of the fcetal villi at the two poles of the ovum. The small size of the umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that of the Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion. The discoidal and dome-shaped placentae of the Armadillos, Myrmecophaga, and the Sloths may easily have been .formed from a diffused placenta, just as the discoidal placenta of the Simiidse and Hominidae appears to have been formed from a diffused placenta like that of the Lemuridae.
The presence of zonary placentae in Hyrax and ElepJias does not necessarily afford any proof of affinity of these types with the Carnivora. A zonary placenta may be quite as easily derived from a diffused placenta as from a discoidal placenta ; and the presence of two villous patches at the poles of the chorion in
- Elephas very probably indicates that its placenta has been evolved
from a diffused placenta.
Although it would not be wise to attempt to found a classification upon the placental characters alone, it may be worth while to make a few suggestions as to the affinities of the orders of Mammalia indicated by the structure of the placenta. We clearly, of course, have to start with forms which could not be grouped with any of the existing orders, but which might be called the Protoplacentalia. They probably had the primitive type of placenta described above : the nearest living representatives of the group are the Rodentia, Insectivora/and Cheiroptera. Before, however, these three groups had become dis.tinctly differentiated, there must have branched off from the .primitive stock the ancestors of the Lemuridae, the Ungulata, and the Edentata.
It is obvious on general anatomical grounds that the Monkeys and Man are to be derived from a primitive Lemurian type ; and
THE EVOLUTION OF THE PLACENTA. 737
with this conclusion the form of the placenta completely tallies. The primitive Edentata and Ungulata had no doubt a diffused placenta which was probably not very different from that of the primitive Lemurs ; but how far these groups arose quite independently from the primitive stock, or whether they may have had a nearer common ancestor, cannot be decided from the structure of the placenta. The Carnivora were certainly an offshoot from the primitive placental type which was quite independent of the three groups just mentioned ; but the character of the placenta of the Carnivora does not indicate at what stage in the evolution of the placental Mammalia a primitive type of Carnivora was first differentiated.
No important light is thrown by the placenta on the affinities of the Proboscidea, the Cetacea, or the Sirenia ; but the character of the placenta in the latter group favours the view of their being related to the Ungulata.
XXII. ON THE STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS 1 . By F. M. BALFOUR and W. N. PARKER.
(With Plates 3442.) TABLE OF CONTENTS.
PAGE
INTRODUCTION 739
GENERAL DEVELOPMENT 74
BRAIN
Adult brain 759
Development of the brain . . _ 7^4
Comparison of the larval and adult brain of Lepidosteiis, together with some observations on the systematic value of the characters of the Ganoid brain 767
SENSE ORGANS
Olfactory organ 77 '
Anatomy of the eye H>.
Development of the eye 771
SUCTORIAL Disc 774
MUSCULAR SYSTEM 775
SKELETON
Vertebral column and ribs of the adult 77^
Development of the vertebral column and ribs 778
Comparison of the vertebral column of Lepidosteus with that of other
forms 79 3
The ribs of Fishes 793
The skeleton of the ventral lobe of the tail fin, and its bearing on the
nature of the tail fin of the various types of Pisces . . . 80 1
EXCRETORY AND GENERATIVE ORGANS
Anatomy of the excretory and generative organs of the female . 810
Anatomy of the excretory and generative organs of the male . 813
Development of the excretory and generative organs . . . . 815
Theoretical considerations 822
1 From the Philosophical Transactions of the Royal Society, 1882.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 739
THE ALIMENTARY CANAL AND ITS APPENDAGES PAGE
Topographical anatomy of the alimentary canal 828
Development of the alimentary canal and its appendages . . . 831
THE GILL ON THE HYOID ARCH 835
THE SYSTEMATIC POSITION OF LEPIDOSTEUS . . . . . . 836
LIST OF MEMOIRS ON THE ANATOMY AND DEVELOPMENT OF LEPIDOSTEUS 840
LIST OF REFERENCE LETTERS . . . . 841
EXPLANATION OF PLATES 842
INTRODUCTION.
THE following paper is the outcome of the very valuable gift of a series of embryos and larvae of Lepidostens by Professor Alex. Agassiz, to whom we take this opportunity of expressing our most sincere thanks. The skull of these embryos and larvae has been studied by Professor Parker, and forms the subject of a memoir already presented to the Royal Society.
Considering that Lepidosteus is one of the most interesting of existing Ganoids, and that it is very closely related to species of Ganoids which flourished during the Triassic period, we naturally felt keenly anxious to make the most of the opportunity of working at its development offered to us by Professor Agassiz' gift. Professor Agassiz, moreover, most kindly furnished us with four examples of the adult Fish, which have enabled us to make this paper a study of the adult anatomy as well as of the development.
The first part of our paper is devoted to the segmentation, formation of the germinal layers, and general development of the embryo and larva. The next part consists of a series of sections on the organs, in which both their structure in the adult and their development are dealt with. This part is not, however, in any sense a monograph, and where already known, the anatomy is described with the greatest possible brevity. In this part of the paper considerable space is devoted to a comparison of the organs of Lepidosteus with those of other Fishes, and to a statement of the conclusions which follow from such comparison.
The last part of the paper deals with the systematic position of Lepidosteus and of the Ganoids generally.
74 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
GENERAL DEVELOPMENT.
The spawning of Lepidosteus takes place in the neighbourhood of New York about May 2Oth. Agassiz (No. i) 1 gives an account of the process from Mr S. W. Carman's notes, which we venture to quote in full.
" Black Lake is well stocked with Bill-fish. When they appear, they are said to come in countless numbers. This is only for a few days in the spring, in the spawning season, between the 1 5th of May and the 8th of June. During the balance of the season they are seldom seen. They remain in the deeper parts .of the lake, away from the shore, and, probably, are more or less nocturnal in habits. Out of season, an occasional one is caught on a hook baited with a minnow. Commencing with the 2Oth of April, until the I4th of May we were unable to find the Fish, or to find persons who had seen them during this time. Then a fisherman reported having seen one rise to the surface. Later, others were seen. On the afternoon of the i8th, a few were found on the points, depositing the spawn. The temperature at the time was 68 to 69 on the shoals, while out in the lake the mercury stood at 62 to 63. The points on which the eggs were laid. were of naked granite, which had been broken by the frost and heat into angular blocks of 3 to 8 inches in diameter. The blocks were tumbled upon each other like loose heaps of brickbats, and upon and between them the eggs were dropped. The points are the extremities of small capes that make out into the lake. The eggs were laid in water varying in depth from 2 to 14 inches. At the time of approaching the shoals, the Fish might be seen to rise quite often to the surface to take air. This they did by thrusting the bill out of the water as far as the corners of the mouth, which was then opened widely and closed with a snap. After taking the air, they seemed more able to remain at the surface. Out in the lake they are very timid, but once buried upon the shoals they become quite reckless as to what is going on about them. A few moments after being driven
1 The numbers refer to the list of memoirs of the anatomy and development given at the end of this memoir.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS; 741
off, one or more of the males would return as if scouting. If frightened, he would retire for some time ; then another scout would appear. If all promised well, the females, with the attendant males, would come back. Each female was accompanied by from one to four males. Most often, a male rested against each side, with their bills reaching up toward the back of her head. Closely crowded together, the little party would pass back and forth over the rocky bed they had selected, sometimes passing the same spot half-a-dozen times without dropping an egg, then suddenly would indulge in an orgasm ; and, lashing and plashing the water in all directions with their convulsive movements, would scatter at the same instant the eggs and the sperm. This ended, another season of moving slowly back and forth was observed, to be in turn followed by another of excitement. The eggs were excessively sticky. To whatever they happened to touch, they stuck, and so tenaciously that it was next to impossible to release them without tearing away a portion of their envelopes. It is doubtful whether the eggs would hatch if removed. As far as could be seen at the time, upon or under the rocks to which the eggs were fastened there was an utter absence of anything that might serve as food for the young Fishes.
" Other Fishes, Bull-heads, &c., are said to follow the Bill-fish to eat the spawn. It may be so. It was not verified. Certainly the points under observations were unmolested. During the afternoon of the i8th of May a few eggs were scattered on several of the beds. On the igth there were more. With the spear and the snare, several dozens of both sexes of the Fish were taken. Taking one out did not seem greatly to startle the others. They returned very soon. The males are much smaller than the average size of the females ; and, judging from those taken, would seem to have as adults greater uniformity in size. The largest taken was a female, of 4 feet ii inch in length. Others of 2 feet 6 inches contained ripe ova. With the igth of May all disappeared, and for a time the weather being meanwhile cold and stormy there were no signs of their continued existence to be met with. Nearly two weeks later, on the 3ist of May, as stated by Mr Henry J. Perry, they again came up, not in small detachments on scattered points as before, but in
742 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
multitudes, on every shoal at all according with their ideas of spawning beds. They remained but two days. During the summer it happens now and then that one is seen to come up for his mouthful of air ; beyond this there will be nothing to suggest the ravenous masses hidden by the darkness of the waters."
Egg membranes, The ova of Lepidosteus are spherical bodies of about 3 millims. in diameter. They have a double investment consisting of (i) an outer covering formed of elongated, highly refractive bodies, somewhat pyriform at their outer ends (Plate 34, fig. i/,/*.), which are probably metamorphosed follicular cells 1 , and (2) of an inner membrane, divided into two zones, viz. : an outer and thicker zone, which is radially striated, and constitutes the zona radiata (s. r.}, and an inner and narrow homogeneous zone (2. r'.\
Segmentation. We have observed several stages in the segmentation, which shew that it is complete, but that it approaches the meroblastic type more nearly than in the case of any other known holoblastic ovum.
Our earliest stage shewed a vertical furrow at the upper or animal pole, extending through about one-fifth of the circumference (Plate 34, fig. I), and in a slightly later stage we found a second similar furrow at right angles to the first (Plate 34, fig. 2). We have not been fortunate enough to observe the next phases of the segmentation, but on the second day after impregnation (Plate 34, fig. 3), the animal pole is completely divided into small segments, which form a disc, homologous to the blastoderm of meroblastic ova ; while the vegetative pole, which subsequently forms a large yolk-sack, is divided by a few vertical furrows, four of which nearly meet at the pole opposite the blastoderm (Plate 34, fig. 4). The majority of the vertical furrows extend only a short way from the edge of the small spheres, and are partially intercepted by imperfect equatorial furrows.
1 We have examined the structure of the ovarian ova in order to throw light on the nature of these peculiar pyriform bodies. Unfortunately, the ovaries of our adult examples of Lepidosteus were so badly preserved, that we could not ascertain anything on this subject. The ripe ova in the ovary have an investment of pyriform bodies similar to those of the just laid ova. With reference to the structure of the ovarian ova we may state that the germinal vesicles are provided with numerous nucleoli arranged in close proximity with the membrane of the vesicle.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 743
Development of the embryo. We have not been able to work out the stages immediately following the segmentation, owing to want of material ; and in the next stage satisfactorily observed, on the third day after impregnation, the body of the embryo is distinctly differentiated. The lower pole of the ovum is then formed of a mass in which no traces of the previous segments or segmentation furrows could any longer be detected.
Some of the dates of the specimens sent to us appear to have been transposed ; so that our statements as to ages must only be taken as approximately correct.
Third day after impregnation. In this stage the embryo is about 3*5 millims. in length, and has a somewhat dumb-bell shaped outline (Plate 34, fig. 5). It consists of (i) an outer area (p. z] with some resemblance to the area pellucida of the Avian embryo, forming the parietal part of the body ; and (2) a central portion consisting of the vertebral and medullary plates and the axial portions of the embryo. In hardened specimens the peripheral part forms a shallow depression surrounding the central part of the embryo.
The central part constitutes a somewhat prominent ridge, the axial part of it being the medullary plate. Along the anterior half of this part a dark line could be observed in all our specimens, which we at first imagined to be caused by a shallow groove. We have, however, failed to find in our sections a groove in this situation except in a single instance (Plate 35, fig. 20, x), and are inclined to attribute the appearance above-mentioned to the presence of somewhat irregular ridges of the outer layer of the epiblast, which have probably been artificially produced in the process of hardening.
The anterior end of the central part is slightly dilated to form the brain (.) ; and there is present a pair of lateral swellings near the anterior end of the brain which we believe to be the commencing optic vesicles. We could not trace any other clear indications of the differentiation of the brain into distinct lobes.
At the hinder end of the central part of the embryo a very distinct dilatation may also be observed, which is probably homologous with the tail swelling of Teleostei. Its structure is more particularly dealt with in the description of our sections of this stage.
744 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
After the removal of the egg-membranes described above we find that there remains a delicate membrane closely attached to the epiblast. This membrane can be isolated in distinct portions, and appears to be too definite to be regarded as an artificial product.
We have been able to prepare several more or less complete series of sections of embryos of this stage (Plate 35, figs. 18 22\ These sections present as a whole a most striking resemblance to those of Teleostean embryos at a corresponding stage of development.
Three germinal layers are already fully established. The epiblast (ep.} is formed of the same parts as in Teleostei, viz. : of an outer epidermic and an inner nervous or mucous stratum. In the parietal region of the embryo these strata are each formed of a single row of cells only. The cells of both strata are somewhat flattened, but those of the epidermic stratum are decidedly the more flattened of the two.
Along the axial line there is placed, as we have stated above, the medullary plate. The epidermic stratum passes over this plate without undergoing any change of character, and the plate is entirely constituted of the nervous stratum of the epidermis.
The medullary plate has, roughly speaking, the form of a solid keel, projecting inwards towards the yolk. There is no trace, at this stage at any rate, of a medullary groove ; and as we shall afterwards shew, the central canal of the cerebro-spinal cord is formed in the middle of the solid keel. The shape of this keel varies according to the region of the body. In the head (Plate 35, fig. 18, m.c.}, it is very prominent, and forming^ as it does, the major part of the axial tissue of the body, impresses its own shape on the other parts of the head and gives rise to a marked ridge on the surface of the head directed towards the yolk. In the trunk (Plate 35, fig's. 19, 20) the keel is much less prominent, but still projects sufficiently to give a convex form to the surface of the body turned towards the yolk.
In the head, and also near the hind end of the trunk, the nervous layer of the epiblast continuous with the keel on each side is considerably thicker than the lateral parts of the layer. The thickening of the nervous layer in the head gives rise to
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 745
what has been called by Gotte l " the special sense plate," owing to its being subsequently concerned in the formation of parts of the organs of special sense. We cannot agree with Gotte in regarding it as part of the brain.
In the keel itself two parts may be distinguished, viz.: a superficial part, best marked in the region of the brain, formed of more or less irregularly arranged polygonal cells, and a deeper part of horizontally placed flatter cells. The upper part is mainly concerned in the formation of the cranial nerves, and of the dorsal roots of the spinal nerves.
The mesoblast (ms.) in the trunk consists of a pair of independent plates which are continued forwards into the head, and in the prechordal region of the latter, unite below the medullary keel.
The mesoblastic plates of the trunk are imperfectly divided into vertebral and lateral regions. Neither longitudinal sections nor surface views shew at this stage any trace of a division of the mesoblast into somites. The mesoblast cells are polygonal, and no indication is as yet present of a division into splanchnic and somatic layers.
The notochord (nc.) is well established, so that its origin could not be made out. It is, however, much more sharply separated from the mesoblastic plates than from the hypoblast, though the ventral and inner corners of the mesoblastic plates which run in underneath it on either side, are often imperfectly separated from it. It is formed of polygonal cells, of which between 40 and 50 may as a rule be seen in a single section. No sheath is present around it. It has the usual extension in front.
The hypoblast (/y.) has the form of a membrane, composed of a single row of oval cells, bounding the embryo on the side adjoining the yolk.
In the region of the caudal swelling the relations of the germinal layers undergo some changes. This region may, from the analogy of other Vertebrates be assumed to constitute the lip of the blastopore. We find accordingly that the layers become more or less fused. In the anterior part of the tail
1 " Ueb. d. Entwick. d. Central Nerven Systems d. Teleoslier," Arc/iiv fur inikr. Anat. Vol. xv. 1878.
B. . 48
746 STRUCTURE AND DEVELOPMENT OF I.EPIDOSTEUS.
swelling, the boundary between the notochord and hypoblast becomes indistinct. A short way behind this point (Plate 35, fig. 21), the notochord unites with the medullary keel, and a neurenteric cord, homologous with the neurenteric canal of other Ichthyopsida, is thus established. In the same region the boundary between the lateral plates of mesoblast and the notochord, and further back (Plate 35, fig. 22), that between the mesoblast and the medullary keel, becomes obliterated.
Fifth day after impregnation. Between the stage .last described and the next stage of which we have specimens, a considerable progress has been made. The embryo (Plate 34, figs. 6 and 7) has grown markedly in length and embraces more than half the circumference of the ovum. Its general appearance is, however, much the same as in the earlier stage, but in the cephalic region the medullary plate is divided by constrictions into three distinct lobes, constituting the regions of the forebrain, the mid-brain, and the hind-brain. The fore-brain (Plate 34, fig. 6,f.b.} is considerably the largest of the three lobes, and a pair of lateral projections forming the optic vesicles are decidedly more conspicuous than in the previous stage. The mid-brain (m.b.} is the smallest of the three lobes, while the hind-brain (h.b) is decidedly longer, and passes insensibly into the spinal cord behind.
The medullary keel, though retaining to a great extent the shape it had in the last stage, is no longer completely solid. Throughout the whole region of the brain and in the anterior part of the trunk (Plate 35, figs. 23, 24, 25) a slit-like lumen has become formed. We are inclined to hold that this is due to the appearance of a space between the cells, and not, as supposed by Oellacher for Teleostei, to an actual absorption of cells, though we must admit that our sections are hardly sufficiently well preserved to be conclusive in settling this point. Various stages in its growth may be observed in different regions of the cerebrospinal cord. When first formed, it is a very imperfectly defined cavity, and a few cells may be seen passing right across from one side of it to the other. It gradually becomes more definite, and its wall then acquires a regular outline.
The optic vesicles are now to be seen in section (Plate 35, fig. 23, op.} as flattish outgrowths of the wall of the fore-brain,
STRUCTURE AND DEVELOPMENT OF I.KHDOSTKUS. 747
into which the lumen of the third ventricle is prolonged for a short distance.
The brain has become to some extent separate from the superjacent epiblast, but the exact mode in which this is effected is not clear to us. In some sections it appears that the separation takes place in such a way that the nervous keel is only covered above by the epidermic layer of the epiblast, and that the nervous layer, subsequently interposed between the two, grows in from the two sides. Such a section is represented in Plate 35, fig. 24. Other sections again favour the view that in the isolation of the nervous keel, a superficial layer of it remains attached to the nervous layer of the epidermis at the two sides, and so, from the first, forms a continuous layer between the nervous keel and the epidermic layer of the epiblast (Plate 35, fig. 25). In the absence of a better series of sections we do not feel able to determine this point. The posterior part of the nervous keel retains the characters of the previous stage.
At the sides of the hind-brain very distinct commencements of the auditory vesicles are apparent. They form shallow pits (Plate 35, fig. 24, au.} of the thickened part of the nervous layer adjoining the brain in this region. Each pit is covered over by the epidermic layer above, which has no share in its formation.
In many parts of the lateral regions of the body the nervous layer of the epidermis is more than one cell deep.
The mesoblastic plates are now divided in the anterior part of the trunk into a somatic and a splanchnic layer (Plate 35, fig. 25, so., sp.), though no distinct cavity is as yet present between these two layers. Their vertebral extremities are somewhat wedge-shaped in section, the base of the wedge being placed at the sides of the medullary keel. The wedge-shaped portions are formed of a superficial layer of 'palisade-like cells and an inner kernel of polygonal cells. The superficial layer on the dorsal side is continuous with the somatic mesoblast, while the remainder pertains to the splanchnic layer.
The diameter of the notochord has diminished, and the cells have assumed a flattened form, the protoplasm being confined to an axial region. In consequence of this, the peripheral layer appears clear in transverse sections. A delicate cuticular sheath
48-2
748 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
is formed around it. This sheath is probably the commencement of the permanent sheath of later stages, but at this stage it cannot be distinguished in structure from a delicate cuticle which surrounds the greater part of the medullary cord.
The hypoblast has undergone no changes of importance.
The layers at the posterior end of the embryo retain the characters of the last stage.
Sixth day after impregnation. At this stage (Plate 34, fig. 8) the embryo is considerably more advanced than at the last stage. The trunk has decidedly increased in length, and the head forms a relatively smaller portion of the whole. The regions of the brain are more distinct. The optic vesicles (op.} have grown outwards so as to nearly reach the edges of the area which forms the parietal part of the body. The fore-brain projects slightly in front, and the mid-brain is seen as a distinct rounded prominence. Behind the latter is placed the hind-brain, which passes insensibly into the spinal cord. On either side of the mid- and hind-brain a small region is slightly marked off from the rest of the parietal part, and on this are seen two more or less transversely directed streaks, which, by comparison with the Sturgeon 1 . we are inclined to regard as the two first visceral clefts (br.c.}. We have, however, failed to make them out in sections, and owing to the insufficiency of our material, we have not even studied them in surface views as completely as we could have wished.
The body is now laterally compressed, and more decidedly raised from the yolk than in the previous stages. In the lateral regions of the trunk the two segmental or archinephric ducts (sg.} are visible in surface views : the front end of each is placed at the level of the hinder border of the head, and is marked by a flexure inwards towards the middle line. The remainder of each duct is straight, and extends backwards for about half the length of the embryo. The tail has much the same appearance as in the last stage.
The vertebral regions of the mesoblastic plates are now segmented for the greater part of the length of the trunk, and the
1 Salensky, " Recherches s. le Developpement du Sterlet." Archives de Biol. Vol. n. 1881, pi. xvii. fig. 27.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 749
somites of which they are composed (Plate 36, fig. 30, pr.) are very conspicuous in surface views.
Our sections of this stage are not so complete as could be desired : they shew, however, several points of interest.
The central canal of the nervous system is large, with welldefined walls, and in hardened specimens is filled with a coagulum. It extends nearly to the region of the tail.
The optic vesicles, which are so conspicuous in surface views, appear in section (Plate 35, fig. 26, op.} as knob-like outgrowths of the fore-brain, and very closely resemble the figures given by Oellacher of these vesicles in Teleostei 1 .
From the analogy of the previous stage, we are inclined to think that they have a lumen continuous with that of the forebrain. In our only section through them, however, they are solid, but this is probably due to the section merely passing through them to one side.
The auditory pits (Plate 35, fig. 27, au.} are now well marked, and have the form of somewhat elongated grooves, the walls of which are formed of a single layer of columnar cells belonging to the nervous layer of the epidermis, and extending inwards so as nearly to touch the brain.
In an earlier stage it was pointed out that the dorsal part of the medullary keel was different in its structure from the remainder, and that it was destined to give rise to the nerves. The process of differentiation is now to a great extent completed, and may best be seen in the auditory region (Plate 35, fig. 27, VIII.). In this region there was present during the last stage a great rhomboidal mass of cells at the dorsal region of the brain (Plate 35, fig. 24, VIII.). In the present stage, this, which is the rudiment of the seventh and auditory nerves, is seen growing down on each side from the roof of the hind-brain, between the brain and the auditory involution, and abutting against the wall of the latter.
Rudiments of the spinal nerves are also seen at intervals as projections from the dorsal angles of the spinal cord (Plate 36, fig. 29, sp.1t.}. They extend only for a short distance outwards, gradually tapering off to a point, and situated
1 "Beitrage zur Entwick. d. Knochenfische," Zeit.f. wiss. Zool. Vol. xxm. 1873, taf. m. fig. ix. 2.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
between the epiblast and the dorsal angles of the mesoblastic somites.
The process of formation of the cranial nerves and dorsal roots of the spinal nerves is, it will be seen, essentially the same as that already known in the case of Elasmobranchii, Aves, &c. The nerVes afise as outgrowths of a special crest of cells, the neural crest of Marshall, which is placed along the dorsal angle of the cord. The peculiar position of the dorsal roots of the spinal nerves is also very similar to what has been met with in the early stages of these structures by Marshall in Birds 1 , and by one of us in Elasmobranchs 2 .
In the parietal region a cavity has now appeared in part of the trunk betweeri the splanchnic and somatic layers of the mesoblast (Plate 36, fig. 29, b.c^), the somatic layer (so.) consisting of a single row of columnar cells on the dorsal side, while the remainder of each somite is formed of the splanchnic layer (j/'.). In many of the sections the somatic layer is separated by a considerable interval from the epiblast.
We have been able to some extent to follow the development of the segmental duct. The imperfect preservation of our specimens has, as in other instances, rendered the study of the point somewhat difficult, but we believe that the figure representing the development of the duct some way behind its front end (Plate 36, fig. 29) is an accurate representation of 'what may be seen in a good many of our sections.
It appears from these sections that the duct (Plate 36, fig. 29, .$.) is developed as a hollow ridge-like outgrowth of the somatic layer of mesoblast, directed towards the epiblast, in which it causes a slight bulging. The cavity of the ridge freely communicates with the body-cavity. The anterior part of this ridge appears to be formed first. Very soon, in fact, in an older embryo belonging to this stage, the greater part of the groove becomes segmented off as a duct lying between the epiblast and somatic mesoblast (Plate 36, fig. 28, sg.}, while the front end still remains, as we believe, in communication with the body-cavity by an anterior pore.
1 Journal of Anat and Physiol. Vol. xi. p. 491, plates xx. and xxi.
2 " Elasmobranch Fishes," p. 156, plates 10 and 13. [This edition, p. 378, pi. ii, 14-]
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 75 1
This mode of development corresponds in every particular with that observed in Teleostei by Rosenberg and Oellacher.
The structure of the notochord (nc.) at this stage is very similar to that observed by one of us in Elasmobranchii 1 . The cord is formed of transversely arranged flattened cells, the outer parts of which are vacuolated, while the inner parts are granular, and contain the nuclei. This structure gives rise to the appearance in transverse sections of an axial darker area and a peripheral lighter portion.
The hypoblast retains for the most part its earlier constitution, but underneath the notochord, in the trunk, it is somewhat thickened, and the cells at the two sides spread in to some extent under the thickened portion (Plate 36, fig. 29, s.nc.}. This thickening, as is shewn in transverse sections at the stage when the segmental duct becomes separated from the somatic mesoblast (Plate 36, fig. 28, s.nc.), is the commencement of the subnotochordal rod.
The tail end' of the embryo still retains its earlier characters. Seventh day after impregnation. Our series of specimens of this stage is very imperfect, and we are only able to call attention to the development of a certain number of organs.
Our sections clearly establish the fact that the optic vesicles are now hollow processes of the fore-brain. Their outer ends are dilated, and are in contact with the external skin. The formation of the optic cup has not, however, commenced. The nervous layer of the skin adjoining the outer wall of the optic cup is very slightly thickened, constituting the earliest rudiment of the lens.
In one of our embryos of this day the developing auditory vesicle still has the form of a pit, but in the other it is a closed vesicle, already constricted off from the nervous layer of the epidermis.
With reference to the development of the excretory duct we cannot add much to what we have already stated in describing the last stage.
The duct is considerably dilated anteriorly (Plate 36, fig. 31, .$#.); but our sections throw no light on the nature of the abdominal pore. The posterior part of the duct has still the form
1 " Elasmobranch Fishes," p. 136, plate 11, fig. 10. [This edition, p. 354, pi. 12.]
752 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
of a hollow ridge united with somatic mesoblast (Plate 36, fig. 32, sg.).
During this stage, the embryo becomes to a small extent folded off from the yolk-sack both in front and behind, and in the course of this process the anterior and posterior extremities of the alimentary tract become definitely established.
We have not got as clear a view of the process of formation of these two sections of the alimentary tract as we could desire, but our observations appear to shew that the process is in many respects similar to that which takes place in the formation of the anterior part of the alimentary tract in Elasmobranchii 1 . One of us has shewn that in Elasmobranchs the ventral wall of the throat is formed not by a process of folding in of the hypoblastic sheet as in Birds, but by a growth of the ventral face of the hypoblastic sheet on each side of and at some little distance from the middle line. Each growth is directed inwards, and the two eventually meet and unite, thus forming a complete ventral wall for the gut. Exactly the same process would seem to take place in Lepidosteus, and after the lumen of the gut is in this way established, a process of mesoblast on each side also makes its appearance, forming a mesoblastic investment on the ventral side of the alimentary tract. Some time after the alimentary tract has been thus formed, the epiblast becomes folded in, in exactly the same manner as in the Chick, the embryo becoming thereby partially constricted off from the yolk (Plate 36, figs. 33, 34).
The form of the lumen of the alimentary tract differs somewhat in front and behind. In front, the hypoblastic sheet remains perfectly flat during the formation of the throat, and thus the lumen of the latter has merely the form of a slit. The lumen of the posterior end of the alimentary tract is, however, narrower and deeper (Plate 36, figs. 33, 34, a/.). Both in front and behind, the lateral parts of the hypoblastic sheet become separated from the true alimentary tract as soon as the lumen of the latter is established.
It is quite possible that at the extreme posterior end of the embryo a modification of the above process may take place, for
1 F. M. Balfour, "Monograph on the Development of Elasmobranch Fishes," p. 87, plate 9, fig. 2. [This edition, p. 303, pi. 10.]
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 753
in this region the hypoblast appears to us to have the form of a solid cord.
We could detect no true neurenteric canal, although a more or less complete fusion of the germinal layers at the tail end of the embryo may still be traced.
During this stage the protoplasm of the notochordal cells, which in the last stage formed a kind of axial rod in the centre of the notochord, begins to spread outwards toward the sheath of the notochord.
Eighth day after impregnation. The external form of the embryo (Plate 34, fig. 9) shews a great advance upon the stage last figured. Both head and body are much more compressed laterally and raised from the yolk, and the head end is folded off for some distance. The optic vesicles are much less prominent externally. A commencing opercular fold is distinctly seen. Our figure of this stage is not, however, so satisfactory as we could wish.
A thickening of the nervous layer of the external epiblast which will form the lens (Plate 36, fig. 35, /.) is more marked than in the last stage, and presses against the slightly concave exterior wall of the optic vesicle (op.). The latter has now a large cavity, and its stalk is considerably narrowed.
The auditory vesicles (Plate 36, fig. 36, au.) are closed, appearing as hollow sacks one on each side of the brain, and are no longer attached to the epiblast.
The anterior opening of the segmental duct can be plainly seen close behind the head. The lumen of the duct is considerably larger.
The two vertebral portions of the mesoblast are now separated by a considerable space from the epiblast on one side and from the notochord on the other, and the cells composing them have become considerably elongated from side to side (Plate 36, fig. 37, MS.).
In some sections the aorta can be seen (Plate 36, fig. 37, ##.) lying close under the sub- notochordal rod, between it and the hypoblast, and on either side of it a slightly larger cardinal vein (cd. v.}.
The protoplasm of the notochord has now again retreated towards the centre, shewing a clear space all round. This is
754 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
most marked in the region of the trunk (Plate 36, fig. 37). The sub-notochordal rod (s. nc.) lies close under it.
A completely closed fore-gut, lined by thickened hypoblast, extends about as far back as the auditory sacks (Plate 36, figs. 35 and 36, /.'). In the trunk the hypoblast, which will form the walls of the alimentary tract, is separated from the notochord by a considerable interval.
Ninth day after impregnation : External characters. Very considerable changes have taken place in the external characters of the embryo. It is about 8 millims. in length, and has assumed a completely piscine form. The tail especially has grown in length, and is greatly flattened from side to 'side : it is wholly detached from the yolk, and bends round towards the head, usually with its left side in contact with the yolk. It is provided with well-developed dorsal and ventfal fin-folds, which meet each other round the end of the tail, the tail fin so formed being, nearly symmetrical. The head is not nearly so much folded off from the yolk as the tail. At its front end is placed a disc with numerous papillae, of which we shall say more hereafter. This disc is somewhat bifid, and is marked in the centre by a deep depression.
Dorsal to it, on the top of the head, are two widely separated nasal pits. On the surface of the yolk, in front of the head, is to be seen the heart, just as in Sturgeon embryos. Immediately below the suctorial disc is a slit-like space, forming the mouth. It is bounded below by the two mandibular arches, which meet' ventrally in the median line. A shallow but well-marked depression on each side of the head indicates the posterior boundary of the mandibular arch. Behind this is placed the very conspicuous hyoid arch with its rudimentary opercular flap ; and in the depression, partly covered over by the latter, may be seen a ridge, the external indication of the first branchial arch.
Eleventh day after impregnation : External characters. The embryo (Plate 34, fig. 10) is now about 10 millims. in length, and in several features exhibits an advance upon the embryo of the previous stage.
The tail fin is now obviously not quite symmetrical, and the dorsal fin-fold is continued for nearly the whole length of the trunk. The suctorial disc (Plate 34, fig. 1 1, s.d.} is much more
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 755
prominent, and the papillae (about 30 in number) covering it are more conspicuous from the surface. It is not obviously composed of two symmetrical halves. The opercular flap is larger, and the branchial arches behind it (two of which may be made out without dissection) are more prominent.
The anterior pair of limbs is now visible in the form of two longitudinal folds projecting in a vertical direction from the surface of the yolk-sack at the sides of the body.
The stages subsequent to hatching have been investigated with reference to the external features and to the habits by Agassiz, and we shall enrich our own account by copious quotations from his memoir.
He states that the first batch were hatched on the eighth 1 day after being laid. " The young Fish possessed a gigantic yolk-bag, and the posterior part of the body presented nothing specially different from the general appearance of a Teleostean embryo, with the exception of the great size of the chorda. The anterior part, however, was most remarkable ; and at first, on seeing the head of this young Lepidosteus, with its huge mouthcavity extending nearly to the gill-opening, and surmounted by a hoof-shaped depression edged with a row of protuberances acting as suckers, I could not help comparing this remarkable structure, so utterly unlike anything in Fishes or Ganoids, to the Cyclostomes, with which it has a striking analogy. This organ is also used by Lepidostetts as a sucker, and the moment the young Fish is hatched he attaches himself to the sides of the disc, and there remains hanging immovable; so firmly attached, indeed, that it requires considerable commotion in the water to make him loose his hold. Aerating the water by pouring it from a height did not always produce sufficient disturbance to loosen the young Fishes. The eye, in this stage, is rather less advanced than in corresponding stages in bony Fishes ; the brain is also comparatively smaller, the otolith ellipsoidal, placed obliquely in the rear above the gill-opening. . . . Usually the gill-cover is pressed closely against the sides of the body, but in breathing an opening is seen through which water is constantly passing, a
1 This statement of Agassi/, does not correspond with the dates on the specimens sent to us a fact no doubt due to the hatching not taking place at the same time for all the larva;.
756 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
strong current being made by the rapid movement of the pectorals, against the base of which the extremity of the gill-cover is closely pressed. The large yolk-bag is opaque, of a bluish-gray colour. The body of the young Lepidostens is quite colourless and transparent. The embryonic fin is narrow, the dorsal part commencing above the posterior end of the yolk-bag ; the tail is slightly rounded, the anal opening nearer the extremity of the tail than the bag. The intestine is narrow, and the embryonic fin extending from the vent to the yolk-bag is quite narrow. In a somewhat more advanced stage, hatched a few hours earlier, the upper edge of the yolk-bag is covered with black pigment cells, and minute black pigment cells appear on the surface of the alimentary canal. There are no traces of embryonic fin-rays either in this stage or the one preceding ; the structure of the embryonic fin is as in bony Fishes previous to the appearance of these embryonic fin-rays finely granular. Seen in profile, the yolk-bag is ovoid ; as seen from above, it is flattened, rectangular in front, with rounded corners, tapering to a rounded point towards the posterior extremity, with re-entering sides."
We have figured an embryo of 1 1 millims. in length, shortly after hatching (Plate 34, fig. 12), the most important characters of which are as follows : The yolk-sack, which has now become much reduced, forms an appendage attached to the ventral surface of the body, and has a very elongated form as compared with its shape just before hatching. The mouth, as also noticed by Agassiz, has a very open form. It is (Plate 34, fig. 13, m.} more or less rhomboidal, and is bounded behind by the mandibular arch (?;/.) and laterally by the superior maxillary processes (s. mx). In front of the mouth is placed the suctorial disc (s. </.), the central papillae of which are arranged in groups. The opercular fold (Ji. op.} is very large, covering the arches behind. A wellmarked groove is present between the mandibular and opercular arches, but so far as we can make out it is not a remnant of the hyomandibular cleft.
The pectoral fins (Plate 34, fig. \2,pc.f?} are very prominent longitudinal ridges, which, owing to their being placed on the surface of the yolk-sack, project in a nearly vertical direction : a feature which is also found in many Teleostean embryos with large yolk-sacks.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 757
No traces of the pelvic fins have yet become developed.
The positions of the permanent dorsal, anal, and caudal fins, as pointed out by Agassiz, are now indicated by a deposit of pigment in the embryonic fin.
In an embryo on the sixth day after hatching, of about 15 millims. in length, of which we have also given a figure (Plate 34, fig. 14), the following fresh features deserve special notice.
In the region of the head there is a considerable elongation of the pre-oral part, forming a short snout, at the end of which is placed the suctorial disc. At the sides of the snout are placed the nasal pits, which have become somewhat elongated anteriorly.
The mouth has lost its open rhomboidal shape, and has become greatly narrowed in an antero-posterior direction, so that its opening is reduced to a slit. The mandibles and maxillary processes are nearly parallel, though both of them are very much shorter than in the adult. The operculum is now a very large flap, and has extended so far backwards as to cover the insertion of the pectoral fin. The two opercular folds nearly meet ventrally.
The yolk-sack is still more reduced in size, one important consequence of which is that the pectoral fins (pc.f.) appear to spring out more or less horizontally from the sides of the body, and at the same time their primitive line of attachment to the body becomes transformed from a longitudinal to a more or less transverse one.
The first traces of the pelvic fins are now visible as slight longitudinal projections near the hinder end of the yolk-sack
The pigmentation marking the regions of the permanent fins has become more pronounced, and it is to be specially noted that the ventral part of the caudal fin (the permanent caudal) is considerably more prominent than the dorsal fin opposite to it.
The next changes, as Agassiz points out, " are mainly in the lengthening of the snout ; the increase in length both of the lower and upper jaw ; the concentration of the sucker of the sucking disc ; and the adoption of the general colouring of somewhat older Fish. The lobe of the pectoral has become specially prominent, and the outline of the fins is now indicated by a fine milky granulation. Seen from above, the gill-cover is
STRUCTURE AND DEVELOPMENT OF I.EPIDOSTEUS.
seen to leave a large circular opening leading to the gill-arches, into which a current of water is constantly passing, by the lateral expansion and contraction of the gill-cover; the outer extremity of the gill-cover covers the base of the pectorals. In a somewhat older stage the snout has become more elongated, the sucker more concentrated, and the disproportionate size of the terminal sucking-disc is reduced ; the head, when seen from above, becoming slightly elongated and pointed."
In a larva of about 18 days old and 21 millims. in length, of which we have not given a figure, the snout has grown greatly in length, carrying with it the nasal organs, the openings of which now appear to be divided into two parts. The suctorial disc is still a prominent structure at the end of the snout. The lower jaw has elongated correspondingly with the upper, so that the gape is very considerable, though still very much less than in the adult.
The opercular flaps overlap ventrally, the left being superficial. They still cover the bases of the pectoral fins. The latter are described by Agassiz as being " kept in constant rapid motion, so that the fleshy edge is invisible, and the vibration seems almost involuntary, producing a constant current round the opening leading into the cavity of the gills." The pelvic fins are somewhat more prominent The yolk-sack, as pointed out by Agassiz, has now disappeared as an external appendage.
After the stage last described the young Fish rapidly approaches the adult form. To shew the changes effected we have figured the head of a larva of about a month old and 23 millims. in length (Plate 34, fig. 15). The suctorial disc, though much reduced, is still prominent at the end of the snout. Eventually, as shewn by Agassiz, it forms the fleshy globular termination of the upper jaw.
The most notable feature in which the larva now differs in its external form from the adult is in the presence of an externally heterocercal tail, caused by the persistence of the primitive caudal fin as an elongated filament projecting beyond the permanent caudal (Plate 41, fig. 68).
Delicate dermal fin-rays are now conspicuous in the peripheral parts of all the permanent fins. These rays closely
STRUCTURE AND DEVELOPMENT OF LEHDOSTEUS. 759
resemble the horny fin-rays in the fins of embryo Elasmobranchs in their development and structure. They appear gradually to enlarge to form the permanent rays, and we have followed out some of the stages of their growth, which is in many respects interesting. Our observations are not, however, complete enough to publish, and we can only say here that their early development and structure proves their homology with the horny fibres or rays in fins of Elasmobranchii. The skin is still, however, entirely naked, and without a trace of its future armour of enamelled scales.
The tail of a much older larva, 1 1 centims. in length, in which the scales have begun to be formed, is shewn in Plate 34, fig. 1 6.
We complete this section of our memoir by quoting the following passages from Agassiz as to the habits of the young fish at the stages last described :
" In the stages intervening between plate iii, fig. 19, and plate iii, fig. 30, the young Lepidosteus frequently swim about, and become readily separated from their point of attachment. In the stage of plate iii, fig. 30, they remain often perfectly quiet close to the surface of the water; but, when disturbed, move very rapidly about through the water. . . . The young already have also the peculiar habit of the adult of coming to the surface to swallow air. When they go through the process under water of discharging air again they open their jaws wide, and spread their gill-covers, and swallow as if they were choking, making violent efforts, until a minute bubble of air has become liberated, when they remain quiet again. The resemblance to a Sturgeon in the general appearance of this stage of the young Lepidosteus is quite marked."
BRAIN. I. A natomy.
The brain of Lepidosteus has been figured by Busch (whose figure has been copied by Miklucho-Maclay, and apparently by Huxley), by Owen and by Wilder (No. 15). The figure of the latter author, representing a longitudinal section through the
/60 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
brain, is the most satisfactory, the other figures being in many respects inaccurate ; but even Wilder's figure and description, though taken from the fresh object, appear to us in some respects inadequate. He offers, moreover, fresh interpretations of certain parts of the brain which we shall discuss in the sequel.
We have examined two brains which, though extremely soft, were, nevertheless, sufficiently well preserved to enable us to study the external form. We have, moreover, made a complete series of transverse sections through one of the brains, and our sections, though utterly valueless from a histological point of view, have thrown some light on the topographical anatomy of the brain.
Plate 38, figs. 47 A, B, and C, represent three views of the brain, viz. : from the side, from above, and from below. We will follow in our description the usual division of the brain into forebrain, mid-brain, and hind-brain.
The fore-brain consists of an anterior portion forming the cerebrum, and a posterior portion constituting the thalamencephalon.
The cerebrum at first sight appears to be composed of (a) a pair of posterior and somewhat dorsal lobes, forming what have usually been regarded as the true cerebral hemispheres, but called by Wilder the prothalami, and (b) a pair of anterior and ventral lobes, usually regarded as the olfactory lobes, from which the olfactory nerves spring. Mainly from a comparison with our embryonic brains described in the sequel, we are inclined to think that the usual interpretations are not wholly correct, but that the true olfactory lobes are to be sought for in small enlargements (Plate 38. figs. 47 A, B, and C, o/f.) at the front end of the brain 1 from which the olfactory nerves spring. The cerebrum proper would then consist of a pair of anterior and ventral lobes (ce.}, and of a pair of posterior lobes (ce'.\ both pairs uniting to form a basal portion behind.
The two pairs of lobes probably correspond with the two parts of the cerebrum of the Frog, the anterior of which, like that of Lepidosteus, was held to be the olfactory lobe, till Gotte's researches shewed that this view was not tenable.
1 The homoiogies of the olfactory lobes throughout the group of Fishes require further investigation.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 761
The anterior lobes of the cerebrum have a conical form, tapering anteriorly, and are completely separated from each other. The posterior lobes, as is best shewn in side views, have 'a semicircular form. Viewed from above they appear as rounded prominences, and their dorsal surface is marked by two conspicuous furrows (Plate 38, fig. 47 B, ce'.}, which have been noticed by Wilder, and are similar to those present in many Teleostei. Their front ends overhang the base of the anterior cerebral lobes. The basal portion of the cerebrum is an undivided lobe, the anterior wall of which forms the lamina terminalis.
What we have above described as the posterior cerebral lobes have been described by Wilder as constituting the everted dorsal border of the basal portion of the cerebrum.
The portion of the cerebro-spinal canal within the cerebrum presents certain primitive characters, which are in some respects dissimilar to those of higher types, and have led Wilder to hold the posterior cerebral lobes, together with what we have called the basal portion of the cerebrum, to be structures peculiar to Fishes, for which he has proposed the name " prothalami."
In the basal portion of the cerebrum there is an unpaired slit-shaped ventricle, the outer walls of which are very thick. It is provided with a floor formed of nervous matter, in part of which, judging from Wilder's description, a well-marked commissure is placed. We have found in the larva a large commissure in this situation (Plate 37, figs. 44 and 45, a.c.) ; and it may be regarded as the homologue of the anterior commissure of higher types. This part of the ventricle is stated by Wilder to be without a roof. This appears to us highly improbable. We could not, however, determine the 'nature of the roof from our badly preserved specimens, but if present, there is no doubt that it is extremely thin, as indeed it is in the larva (Plate 37, fig. 46 B). In a dorsal direction the unpaired ventricle extends so as to separate the two posterior cerebral lobes. Anteriorly the ventricle is prolonged into two horns, which penetrate for a short distance, as the lateral ventricles, into the base of the anterior cerebral lobes. The front part of each anterior cerebral lobe, as well as of the whole of the posterior lobes, appears solid in our sections ; but Wilder describes the anterior horns of the B. 49
762 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
ventricle as being prolonged for the whole length of the anterior lobes.
In the embryos of all Vertebrates the cerebrum is not at first divided into two lobes, so that the fact of the posterior part of the cerebrum in Lepidosteus and probably other Ganoids remaining permanently in the undivided condition does not appear to us a sufficient ground for giving to the lobes of this part of the cerebrum the special name of prothalami, as proposed by Wilder, or for regarding them as a section of the brain peculiar to Fishes.
The thalamencephalon (///.) contains the usual parts, but is is some respects peculiar. Its lateral walls, forming the optic thalami, are thick, and are not sharply separated in front from the basal part of the cerebrum ; between them is placed the third ventricle. The thalami are of considerable extent, though partially covered by the optic lobes and the posterior lobes of the cerebrum. They are not, however, relatively so large as in other Ganoid forms, more especially the Chondrostei and Polypterus.
On the roof of the thalamencephalon is placed a large thinwalled vesicle (Plate 38, figs. 47 A and B, v.tk.), which undoubtedly forms the most characteristic structure connected with this part of the brain. Owing to the wretched state of preservation of the specimens, we have found it impossible to determine the exact relations of this body to the remainder of the thalamencephalon; but it appears to be attached to the roof of the thalamencephalon by a narrow stalk only. It extends forwards so as to overlap part of the cerebrum in front, and is closely invested by a highly vascular layer of the pia mater.
No mention is made by Wilder of this body ; nor is it represented in his figures or in those of the other anatomists who have given drawings of the brain of Lepidosteus. It might at first be interpreted as a highly-developed pineal gland, but a comparison with the brain of the larva (vide p. 764) shews that this is not the case, but that the body in question is represented in the larva by a special outgrowth of the roof of the thalamencephalon. The vesicle of the roof of the thalamencephalon is therefore to be regarded as a peculiar development of the tela choroidea of the third ventricle.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 763
How far this vesicle has a homologue in the brains of other Ganoids is not certain, since negative evidence on this subject is all but valueless. It is possible that a vesicular sack covering over the third ventricle of the Sturgeon described by Stannius 1 , and stated by him to be wholly formed of the membranes of the brain, is really the homologue of our vesicle.
Wiedersheim 2 has recently described in Protopterns a body which is undoubtedly homologous with our vesicle, which he describes in the following way :
" Dorsalwarts ist das Zwischenhirn durch ein tiefes, von Hirnschlitz eingenommenes Thai von Vorderhirn abgesetzt ; dasselbe ist jedoch durch eine hautige, mit der Pia mater zusammenhangende Kuppel oder Kapsel uberbruckt."
This " Kuppel " has precisely the same relations and a very similar appearance to our vesicle. The true pineal gland is placed behind it. It appears to us possible that the body found by Huxley 3 in Ceratodus, which he holds to be the pineal gland, is in reality this vesicle. It is moreover possible that what has usually been regarded as the pineal gland in Petromyzon may in reality be the homologue of the vesicle we have found in Lepidosteus.
We have no observations on the pineal gland of the adult, but must refer the reader for the structure and relations of this body to the embryological section.
The infundibulum (Plate 38, fig. 47 A, in.) is very elongated. Immediately in front of it is placed the optic chiasma (Plate 38, figs. 47 A and C, op.c/i.) from which the optic fibres can be traced passing along the sides of the optic thalami and to the optic lobes, very much as in M tiller's figure of the brain of Polypterus,
On the sides of the infundibulum are placed two prominent bodies, the lobi inferiores (l.in), each of which contains a cavity continuous with the prolongation of the third ventricle
1 " Ueb. d. Gehirn des Stors," Mailer's Arc/iiv, 1843, and Lehrbuch d. vergl. Anat. d. Wirbelthiere. Cattie, Archives de Biologie, Vol. in. 1882, has recently described in Acipenser sturio a vesicle on the roof of the thalamencephalon, whose cavity is continuous with the third ventricle. This vesicle is clearly homologous with that in Lepidosteus. (June 28, 1882.)
2 R. Wiedersheim, Morphol. Studien, 1880, p. 71.
3 "On Ceratodus Forstcri" &.C., Proc. Zool. Soc. 1876.
492
764 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
into the infundibulum. The apex of the infundibulum is enlarged, and to it is attached a pituitary body (//.).
The mid-brain is of considerable size, and consists of a basal portion connecting the optic thalami with the medulla, and a pair of large optic lobes (op.l.}. The iter a tertio ad quartum ventriculum, which forms the ventricle of this part of the brain, is prolonged into each optic lobe, and the floor of each prolongation is taken up by a dome-shaped projection, the homologue of the torus semicircularis of Teleostei.
The hind-brain consists of the usual parts, the medulla oblongata and the cerebellum. The medulla presents no peculiar features. The sides of the fourth ventricle are thickened and everted, and marked with peculiar folds (Plate 38, figs. 47 A and B, m.o.).
The cerebellum is much larger than in the majority of Ganoids, and resembles in all essential features the cerebellum of Teleostei. In side views it has a somewhat S-shaped form, from the presence of a peculiar lateral sulcus (Plate 38, fig. 47 A, cd.). As shewn by Wilder, its wall actually has in longitudinal section this form of curvature, owing to its anterior part projecting forwards into the cavity of the iter 1 . This forward projection is not, however, so conspicuous as in most Teleostei. The cerebellum contains a -large unpaired prolongation of the fourth ventricle.
1 1 . Development.
The early development of the brain has already been described ; and, although we do not propose to give any detailed account of the later stages of its growth, we have thought it worth while calling attention to certain developmental features which may probably be regarded as to some extent characteristic of the Ganoids. With this view we have figured (Plate 37, figs. 44, 45) longitudinal sections of the brain at two stages, viz.: of larvae of 15 and 26 millims., and transverse sections (Plate 37, figs. 46 A G) of the brain of a larva at about the latter stage (25 millims.).
1 In Wilder's figure the walls of the cerebellum are represented as much too thin.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 765
The original embryonic fore-brain is divided in both embryos into a cerebrum (ce.) in front and a thalamencephalon (th.) behind. In the younger embryo the cerebrum is a single lobe, as it is in the brains of all Vertebrate embryos ; but in the older larva it is anteriorly (Plate 37, fig. 46 A) completely divided into two hemispheres. The roof of the undivided posterior part of the cerebrum is extremely thin (Plate 37, fig. 46 B). Near the posterior border of the base of the cerebrum there is a great development of nervous fibres, which may probably be regarded as in part equivalent to the anterior commissure (Plate 37, figs. 44, 45 a.c.).
Even in the oldest of the two brains the olfactory lobes are very slightly developed, constituting, however, small lateral and ventral prominences of the front end of the hemispheres. From each of them there springs a long olfactory nerve, extending for the whole length of the rostrum to the olfactory sack.
The thalamencephalon presents a very curious structure, and is relatively a more important part of the brain than in the embryo of any other form which we know of. Its roof, instead of being, as usual, compressed antero-posteriorly 1 , so as to be almost concealed between the cerebral hemispheres and the optic Jpbes (mid-brain), projects on the surface for a length quite equal to that of the cerebral hemispheres (Plate 37, figs. 44 and 45, th.}. In the median line the roof of the thalamencephalon is thin and folded ; at its posterior border is placed the opening of the small pineal gland. This body is a papilliform process of the nervous matter of the roof of this part of the brain, and instead of being directed forwards, as in most Vertebrate types, tends somewhat backwards, and rests on the mid-brain behind (Plate 37, figs. 44, 45, and 46 C and D, /.). The roof of the thalamencephalon immediately in front of the pineal gland forms a sort of vesicle, the sides of which extend laterally as a pair of lobes, shewn in transverse sections in Plate 37, figs. 46 C and D, as th.L This vesicle becomes, we cannot doubt, the vesicle on the roof of the thalamencephalon which we have described in the adult brain. Immediately in front of the pineal gland the roof of the thalamencephalon contains a transverse commissure
1 Vide F. M. Balfour, Comparative Embryology, Vol. II. figs. 248 and 250.
766 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
(Plate 37, fig. 46 C, z.}, which is the homologue of a similarly situated commissure present in the Elasmobranch brain 1 , while behind the pineal gland is placed the posterior commissure. The sides of the thalamencephalon are greatly thickened, forming the optic thalami (Plate 37, figs. 46 C and D, op.th^, which are continuous in front with the thickened outer walls of the hemispheres. Below, the thalamencephalon is produced into a very elongated infundibulum (Plate 37, figs. 44, 45, 46 E, in.}, the apex of which is trilobed as in Elasmobranchii and Teleostei. The sides of the infundibulum exhibit two lobes, the lobi inferiores (Plate 37, fig. 46 E, /./.), which are continued posteriorly into the crura cerebri.
The pituitary body 2 (Plate 37, figs. 44, 45, 46 E,/A) is small, not divided into lobes, and provided with a very minute lumen.
In front of the infundibulum is the optic chiasma (Plate 37, fig. 46 D, op.ch.}, which is developed very early. It is, as stated by Mtiller, a true chiasma.
The mid-brain (Plate 37, figs. 44 and 45, m. b.} is large, and consists in both stages of (i) a thickened floor forming the crura cerebri, the central canal of which constitutes the iter a tertio ad quartum ventriculum ; and (2} the optic lobes (Plate 37, figs. 46 E, F, G, op. /.) above 5 each of which is provided with a cavity continuous with the median iter. The optic lobes are separated dorsally and in front by a well-marked median longitudinal groove. Posteriorly they largely overlap the cerebellum. In the anterior part of the optic lobes, at the point where the iter joins the third ventricle, there may be seen slight projections of the floor into the lumen of the optic lobes (Plate 37, fig. 46 E). These masses probably become in the adult the more conspicuous
1 Vide F. M. Balfour, Comparative Embryology, Vol. II. pp. 355 6 [the original edition], where it is suggested that this commissure is the homologue of the grey commissure of higher types.
8 We have not been able to work out the early development of the pituitary body ns satisfactorily as we could have wished. In Plate 37, fig. 40, there is shewn an invagination of the oral epithelium to form it ; in Plate 37, figs. 41 and 42, it is represented in transverse section in two consecutive sections. Anteriorly it is still connected with the oral epithelium (fig. 41), while posteriorly it is free. It is possible that an earlier stage of it is shewn in Plate 36, fig. 35. Were it not for the evidence in other types of its being derived from the epiblast we should be inclined to regard it as hypoblaslic in origin.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 767
prominences of the floor of the ventricles of the optic lobes, which we regard as homologous with the tori semicirculares of the brain of the Teleostei.
The hind-brain is formed of the usual divisions, viz. : cerebellum and medulla oblongata (Plate 37, figs. 44 and 45, cb., md.). The former constitutes a bilobed projection of the roof of the hind-brain. Only a small portion of it is during these stages left uncovered by the optic lobes, but the major part extends forwards for a considerable distance under the optic lobes, as shewn in the transverse sections (Plate 37, figs. 46 F and G, cb.) ; and its two lobes, each with a prolongation .of its cavity, are continued forwards beyond the opening of the iter into the fourth ventricle.
It is probable that the anterior horns of the cerebellum are equivalent to the prolongations of the cerebellum into the central cavity of the optic lobes of Teleostei, which are continuous with the so-called fornix of Gottsche.
III. Comparison of the larval and adult brain of Lepidosteus, together with some observations on the systematic value of the characters of the Ganoid brain.
The brain of the older of the two larvae, which we have described, sufficiently resembles in most of its features that of the adult to render material assistance in the interpretation of certain of the parts of the latter. It will be remembered that in the adult brain the parts usually held to be olfactory lobes were described as the anterior cerebral lobes. The grounds for this will be apparent by a comparison of the cerebrum of the larva and adult. In the larva the cerebrum is formed of (i) an unpaired basal portion with a thin roof, and (2) of a pair of anterior lobes, with small olfactory bulbs at their free extremities.
The basal portion in the larva clearly corresponds in the adult with the basal portion, together with the two posterior cerebral lobes, which are merely special outgrowths of the dorsal edge of the primitive basal portion. The pair of anterior lobes have exactly the same relations in the larva as in the adult, except that in the former the ventricles are prolonged for their
768 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
whole length instead of being confined to their proximal portions. If, therefore, our identifications of the larval parts of the brain are correct, there can hardly be a question as to our identifications of the parts in the adult. As concerns these identifications, the comparison of the brain of our two larvae appears conclusive in favour of regarding the anterior lobes as parts of the cerebrum, as distinguished from the olfactory lobes, in that they are clearly derived from the undivided anterior portion of the cerebrum of the younger larva.
The comparison of the larval brain with that of the adult again appears to us to leave no doubt that the vesicle attached to the roof of the thalamencephalon in the adult is the same structure as the bilobed outgrowth of this roof in the larva ; and since there is in addition a well-developed pineal gland in the larva with the usual relations, there can be no ground for identifying the vesicle in the adult with the pineal gland.
Miiller, in his often quoted memoir (No. 1 3), states that the brains of Ganoids are peculiar and distinct from those both of Teleostei and Elasmobranchii ; but in addition to pointing out that the optic nerves form a chiasma he does not particularly mention the features, to which he alludes in general terms. More recently Wilder (No. 15) has returned to this subject; and though, as we have already had occasion to point out, we cannot accept all his identifications of the parts of the Ganoid brain, yet he has called attention to certain characteristic features of the cerebrum which have an undoubted systematic value.
The distinctive characters of the Ganoid brain are, in our opinion, (i) the great elongation of the region of the thalamencephalon ; and (2) the unpaired condition of the posterior part of the cerebrum, and the presence of so thin a roof to the ventricle of this part as to cause it to appear open above.
The immense length of the region of the thalamencephalon is a feature in the Ganoid brain which must at once strike any one who examines figures of the brains of Chondrostei, Polypterus, or Amia. It is less striking in the adult Lepidosteus, though here also we have shewn that the thalamencephalon is really very greatly developed ; but in the larva of Lepidosteus this feature is still better marked, so that the brain of the larva may be described as being more characteristically Ganoid than that of the adult.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 769
The presence of a largely developed thalamencephalon at once distinguishes a Ganoid brain from that of a Teleostean Fish, in which the optic thalami are very much reduced ; but Lepidosteus shews its Teleostean affinities by a commencing reduction of this part of the brain.
The large size of the thalamencephalon is also characteristic of the Ganoid brain in comparison with the brain of the Dipnoi ; but is not however so very much more marked in the Ganoids than it is in some Elasmobranchii.
On the whole, we may consider the retention of a large thalamencephalon as a primitive character.
The second feature which we have given as characteristic of the Ganoid brain is essentially that which has been insisted upon by Wilder, though somewhat differently expressed by him.
The simplest condition of the cerebrum is that found in the larva of Lepidosteus, where there is an anterior pair of lobes, and an undivided posterior portion with a simple prolongation of the third ventricle, and a very thin roof. The dorsal edges of the posterior portion, adjoining the thin roof, usually become somewhat everted (cf. Wilder), and in Lepidosteus these edges have in the adult a very great development, and form (vide Plate 38, fig. 47 A C, ce.) two prominent lobes, which we have spoken of as the posterior cerebral lobes.
These characters of the cerebrum are perhaps even more distinctive than those of the thalamencephalon.
In Teleostei the cerebrum appears to be completely divided into two hemispheres, which are, however, all but solid, the lateral ventricles being only prolonged into their bases. In Dipnoi again there is either (Protopterus, Wiedersheim 1 ) a completely separated pair of oval hemispheres, not unlike those of the lower Amphibia, or the oval hemispheres are not completely separated from each other (Ceratodus, Huxley 2 , Lepidosiren, Hyrtl 3 ) ; in either case the hemispheres are traversed for the whole length by lateral ventricles which are either completely or nearly completely separated from each other.
1 Alorphol. Studicn, in. Jena, 1880.
2 "On Ceratodus Forsteri," Proc. Zool. Soc. 1876.
3 Lepidosiren paradqxa. Prag. 1845.
770 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
In Elasmobranchii the cerebrum is an unpaired though bilobed body, but traversed by two completely separated lateral ventricles, and without a trace of the peculiar membranous roof found in Ganoids.
Not less interesting than the distinguishing characters of the Ganoid brain are those cerebral characters which indicate affinities between Lepidostens and other groups. The most striking of these are, as might have been anticipated, in the direction of the Teleostei.
Although the foremost division of the brain is very dissimilar in the two groups, yet the hind-brain in many Ganoids and the mid-brain also in Lepidosteus approaches closely to the Teleostean type. The most essential feature of the cerebellum in Teleostei is its prolongation forwards into the ventricles of the optic vesicles as the valvula cerebelli. We have already seen that there is a homologous part of the cerebellum in Lepidosteus ; Stannius also describes this part in the Sturgeon, but no such part is represented in M tiller's figure of the brain of Polypterus, or described by him in the text.
The cerebellum is in most Ganoids relatively smaller, and this is even the case with Amia; but the cerebellum of Lepidosteus is hardly less bulky than that of most Teleostei.
The presence of tori semicirculares on the floor of the midbrain of Lepidosteus again undoubtedly indicates its affinities with the Teleostei, and such processes are stated by Stannius to be absent in the Sturgeon, and have not, so far as we are aware, been described in other Ganoids. Lastly we may point to the presence of well-developed lobi inferiores in the brain of Lepidosteus as an undoubted Teleostean character.
On the whole, the brain of Lepidosteus, though preserving its true Ganoid characters, approaches more closely to the brain of the Teleostei than that of any other Ganoid, including even A mia.
It is not easy to point elsewhere to such marked resemblances of the Ganoid brain, as to the brain of the Teleostei.
The division of the cerebrum into anterior and posterior lobes, which is found in Lepidosteus, probably reappears again, as already indicated, in the higher Amphibia. The presence of the peculiar vesicle attached to the roof of the thalamencephalon
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 7/1
has its parallel in the brain of Protopterus, and as pointing in the same direction a general similarity in the appearance of the brain of Polypterus to that of the Dipnoi may be mentioned.
There appears to us to be in no points a close resemblance between the brain of Ganoids and that of Elasmobranchii.
SENSE ORGANS.
Olfactory organ.
Development. The nasal sacks first arise during the late embryonic period in the form of a pair of thickened patches of the nervous layer of the epiblast on the dorsal surface of the front end of the head (Plate 37, fig. 39, ol.). The patches very soon become partially invaginated ; and a small cavity is developed between them and the epidermic layer of the epiblast (Plate 37, figs. 42 and 43, ol.}. Subsequently, the roof of this space, formed by the epidermic layer of the epiblast, is either broken through or absorbed ; and thus open pits, lined entirely by the nervous layer of the epidermis, are formed.
We are not acquainted with any description of an exactly similar mode of origin of the olfactory pits, though the process is almost identical with that of the other sense organs.
We have not worked out in detail the mode of formation of the double openings of the olfactory pits, but there can be but little doubt that it is caused by the division of the single opening into two.
The olfactory nerve is formed very early (Plate 37, fig. 39, I), and, as Marshall has found in Aves and Elasmobranchii, it arises at a stage prior to the first differentiation of an olfactory bulb as a special lobe of the brain.
The Eye.
Anatomy. We have not made a careful histological examination of the eye of Lepidosteus, which in our specimens was not sufficiently well preserved for such a purpose ; but we have
772 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
found a vascular membrane enveloping the vitreous humour on its retinal aspect, which, so far as we know, is unlike anything which has so far been met with in the eye of any other adult Vertebrate.
The membrane itself is placed immediately outside the hyaloid membrane, i.e. on the side of the hyaloid membrane bounding the vitreous humour. It is easily removed from the retina, to which it is only adherent at the entrance of the optic nerve. In both the eyes we examined it also adhered, at one point, to the capsule of the lens, but we could not make out whether this adhesion was natural, or artificially produced by the coagulation of a thin layer of albuminous matter. In one instance, at any rate, the adhesion appeared firmer than could easily be produced artificially.
The arrangement of the vessels in the membrane is shewn diagrammatically in Plate 38, fig. 49, while the characteristic form of the capillary plexus is represented in Plate 38, fig. 50.
The arterial supply appears to be derived from a vessel perforating the retina close to the optic nerve, and obviously homologous with the artery of the processus falciformis and pecten of Teleostei and Birds, and with the arteria centralis retinae of Mammals. From this vessel branches diverge and pursue a course towards the periphery. They give off numerous branches, the blood from which enters a capillary plexus (Plate 38, figs. 49 and 50) and is collected again by veins, which pass outwards and finally bend over and fall into (Plate 38, fig. 49) a circular vein (cr. z>.) placed at the outer edge of the retina along the insertion of the iris (ir). The terminal branches of some of the main arteries appear also to fall directly into this vein.
The membrane supporting the vessels just described is composed of a transparent matrix, in which numerous cells are embedded (Plate 38, fig. 50).
Development. In the account of the first stages of development of LepidosteuS) the mode of formation of the optic cup, the lens, &c., have been described (vide Plates 35 and 36, figs. 23, 26, 35). With reference to the later stages in the development of the eye, the only subject with which we propose to deal is the growth of the mesoblastic processes which enter the cavity of the vitreous humour through the choroid slit.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 773
Lepidosteus is very remarkable for the great number of mesoblast cells which thus enter the cavity of the vitreous humour, and for the fact that these cells are at first unaccompanied by any vascular structures (Plate 37, fig. 43, v.h). The mesoblast cells are scattered through the vitreous humour, and there can be no doubt that during early larval life, at a period however when the larva is certainly able to see, every histologist would consider the vitreous humour to be a tissue formed of scattered cells, with a large amount of intercellular substance ; and the fact that it is so appears to us to demonstrate that Kessler's view of the vitreous humour being a mere transudation is not tenable.
In the larva five or six days after hatching, and about 15 millims. in length, the choroid slit is open for its whole length. The edges of the slit near the lens are folded, so as to form a ridge projecting into the cavity of the vitreous humour, while nearer the insertion of the optic nerve they cease to exhibit any such structure. The mesoblast, though it projects between the lips of the ridge near the lens, only extends through the choroid slit into the cavity of the vitreous humour in -the neighbourhood of the optic nerve. Here it forms a lamina with a thickened edge, from which scattered cells in the cavity of the vitreous humour seem to radiate.
At a slightly later stage than that just described, bloodvessels become developed within the cavity of the vitreous humour, and form the vascular membrane already described in the adult, placed close to the layer of nerve-fibres of the retina, but separated from this layer by the hyaloid membrane (Plate 38, fig. 48, v.s/1.). The artery bringing the blood to the above vascular membrane is bound up in the same sheath as the optic nerve, and passes through the choroid slit very close to the optic nerve. Its entrance into the cavity of the vitreous humour is shewn in Plate 38, fig. 48 (vs.); its relation to the optic nerve in Plate 37, fig. 46, C and D (vs.).
The above sheath has, so far as we know, its nearest analogue in the eye of Alytes, where, however, it is only found in the larva.
The reader who will take the trouble to refer to the account of the imperfectly-developed processus falcifprmis of the Elas
774 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
mobranch eye in the treatise On Comparative Embryology, by one of us 1 , will not fail to recognize that the folds of the retina at the sides of the choroid slit, and the mesoblastic process passing through this slit, are strikingly similar in Lepidosteus and Elasmobranchii ; and that, if we are justified in holding them to be an imperfectly-developed processus falciformis in the one case, we are equally so in the other.
Johannes Miiller mentions the absence of a processus falciformis as one of the features distinguishing Ganoids and Teleostei. So far as the systematic separation of the two groups is concerned, he is probably perfectly justified in this course ; but it is interesting to notice that both in Ganoids and Elasmobranchii we have traces of a structure which undergoes a very special development in the Teleostei, and that the processus falciformis of Teleostei is therefore to be regarded, not as an organ peculiar to them, but as the peculiar modification within the group of a primitive Vertebrate organ.
SUCTORIAL Disc.
One of the most remarkable organs of the larval Lepidosteus is the suctorial disc, placed at the front end of the head, to which we have made numerous allusions in the first section of this memoir.
The external features of the disc have been fully dealt with by Agassiz, and he also explained its function by observations on the habits of the larva. We have already quoted (p. 755) a passage from Agassiz' memoir shewing how the young Fishes use the disc to attach themselves firmly to any convenient object. The discs appear in fact to be highly efficient organs of attachment, in that the young Fish can remain suspended by them to the sides of the jar, even after the water has been lowered below the level at which they are attached.
The disc is formed two or three days before hatching, and from Agassiz' statements, it appears to come into use immediately the young Fish is liberated from the egg membranes.
We have examined the histological structure of the disc at various ages of its growth, and may refer the reader to Plate 34,
1 Vol. II. p. 414 [the original edition].
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 775
figs. 1 1 and 13, and Plate 37, figs. 40 and 44. The result of our examination has been to shew that the disc is provided with a series of papillae often exhibiting a bilateral arrangement. The papillae are mainly constituted of highly modified cells of the mucous layer of the epidermis. These cells have the form of elongated columns, the nucleus being placed at the base, and the main mass of the cells being filled with a protoplasmic reticulum. They may' probably be regarded as modified mucous cells. In the mesoblast adjoining the suctorial disc there are numerous sinus-like vascular channels.
It does not appear probable that the disc has a true sucking action. It is unprovided with muscular elements, and there appears to be no mechanism by which it could act as a sucking organ. We must suppose, therefore, that its adhesive power depends upon the capacity of the cells composing its papillae to pour out a sticky secretion.
MUSCULAR SYSTEM.
There is a peculiarity in the muscular system of Lepidosteus, which so far as we know has not been previously noticed. It is that the lateral muscles of each side are not divided, either in the region of the trunk or of the tail, into a dorso-lateral and ventro-lateral division.
This peculiarity is equally characteristic of the older larvae as of the adult, and is shewn in Plate 41, figs. 67, 72, and 73, and Plate 42, figs. 74 76. In the Cyclostomata the lateral muscles are not divided into dorsal and ventral sections ; but except in this group such a division has been hitherto considered as invariable amongst Fishes.
This character must, without doubt, be held to be the indication of a very primitive arrangement of the muscular system. In the embryos of all Fishes with the usual type of the lateral muscles, the undivided condition of the muscles precedes the divided condition ; and in primitive forms such as the Cyclostomata and Amphioxus the embryonic condition is retained, as it is in Lepidosteus.
776 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
SKELETON. PART I. Vertebral column and ribs of the adult.
A typical vertebra from the trunk of Lepidosteus has the following characters (Plate 42, figs. 80 and 81).
The centrum is slightly narrower in the middle than at its two extremities. It articulates with adjacent vertebrae by a convex face in front and a concave face behind, being thus, according to Owen's nomenclature, opisthoccelous. It presents on its under surface a well-marked longitudinal ridge, which in many vertebrae is only united at its two extremities with the main body of the vertebra.
From the lateral borders of the centrum there project, at a point slightly nearer the front than the hind end, a pair of prominent haemal processes (h.a.} } to the ends of which are articulated the ribs. These processes have a nearly horizontal direction in the greater part of the trunk, though bent downwards in the tail.
The neural arches (n.a.) have a somewhat complicated form. They are mainly composed of two vertical plates, the breadth of the basal parts of which is nearly as great as the length of the vertebrae, so that comparatively narrow spaces are left between the neural arches of successive vertebrae for the passage of the spinal nerves. Some little way from its dorsal extremity each neural arch sends a horizontal process inwards, which meets its fellow and so forms a roof for the spinal canal. These processes appear to be confined to the posterior parts of the vertebrae, so that at the front ends of the vertebrae, and in the spaces between them, the neural canal is without an osseous roof. Above the level of this osseous roof there is a narrow passage, bounded laterally by the dorsal extremities of the neural plates. This passage is mainly filled up by a series of cartilaginous elements (Plate 42, figs. 80 and 81, t.c.) (probably fibro-cartilage), which rest upon the roof of the neural canal. Each element is situated intervertebrally, its anterior end being wedged in between the two dorsal processes of the neural arch of the vertebra in front, and its posterior end extending for some
STRUCTURE AND DEVELOPMENT OF LEFIDOSTEUS. 777
distance over the vertebra behind. The successive elements are connected by fibrous tissue, and are continuous dorsally with a fibrous band, known as the ligamentum longitudinale superius (Plate 42, figs. 80 and 81, /./.), characteristic of Fishes generally, and running continuously for the whole length of the vertebral column. Each of the cartilaginous elements is, as will be afterwards shewn, developed as two independent pieces of cartilage, and might be compared with the dorsal element which usually forms the keystone of the neural arch in Elasmobranchs, were not the latter vertebral instead of intervertebral in position. More or less similar elements are described by Gotte in the neural arches of many Teleostei, which also, however, appear to be vertebral ly placed, and he has compared them and the corresponding elements in the Sturgeon with the Elasmobranch cartilages forming the keystone of the neural arch. Gotte does not, however, appear to have distinguished between the cartilaginous elements, and the osseous elements forming the roof of the spinal canal, which are true membrane bones ; it is probable that the two are not so clearly separated in other types as in Lepidosteus.
The posterior ends of the neural plates of the neural arches are continued into the dorsal processes directed obliquely upwards and backwards, which have been somewhat unfortunately described by Stannius as rib-like projections of the neural arch. The dorsal processes of the two sides do not meet, but between them is placed a median free spinous element, also directed obliquely upwards and backwards, which forms a kind of roof for the groove in which the cartilaginous elements and the ligamentum longitudinale are placed.
The vertebrae are wholly formed of a very cellular osseous tissue, in which a distinction between the bases of the neural and haemal processes and the remainder of the vertebra is not recognizable. The bodies of the vertebras are, moreover, directly continuous with the neural and haemal arches.
The ribs in the region of the trunk are articulated to the ends of the long haemal processes. They envelop the bodycavity, their proximal parts being placed immediately outside the peritoneal membrane, along the bases of the intermuscular septa. Their distal ends do not, however, remain close to the B. 50
778 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
peritoneal membrane, but pass outwards along the intermuscular septa till their free ends come into very close proximity with the skin. This peculiarity, which holds good in the adult for all the free ribs, is shewn in one of the anterior ribs of an advanced larva in Plate 41, fig. 72 (rb.}. We are not aware that this has been previously noticed, but it appears to us to be a point not without interest in all questions which concern the homology of rib-like structures occupying different positions in relation to the muscles. Its bearings are fully dealt with in the section of this paper devoted to the consideration of the homologies of the ribs in Fishes.
As regards the behaviour of the ribs in the transitional region between the trunk and the tail, we cannot do better than translate the description given by Gegenbaur of this region (No. 6, p. 411): "Up to the 34th vertebra the ribs borne by the laterally and posteriorly directed processes present nothing remarkable, though they have gradually become shorter. The ribs of the 35th vertebra exhibit a slight curvature outwards of their free ends, a peculiarity still more marked in the 36th. The last named pair of ribs converge somewhat in their descent backwards so that both ribs decidedly approach before bending outwards. The 37th vertebra is no longer provided with freely terminating ribs, but on the contrary, the same pair of processes which in front was provided with ribs, bears a short forked process as the haemal arch. The two, up to this point separated ribs, have here formed a haemal arch by the fusion of their lower ends, which arch is movable just like the ribs, and, like them, is attached to the vertebral column ' \ !
In the region of the tail-fin the haemal arches supporting the caudal fin-rays are very much enlarged.
PART II. Development of the vertebral column and ribs.
The first development and early histological changes of the notochord have already been given, and we may take up the history of the vertebral column at a period when the notochord forms a large circular rod, whose cells are already highly vacuolated, while the septa between the vacuoles form a delicate
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 779
wide-meshed reticulum. Surrounding the notochord is the usual cuticular sheath, which is still thin.
The first indications of the future vertebral column are to be found in the formation of a distinct mesoblastic investment of the notochord. On the dorsal aspect of the notochord, the mesoblast forms two ridges, one on each side, which are prolonged upwards so as to meet above the neural canal, for which they form a kind of sheath. On the ventral side of the notochord there are also two ridges, which are, however, except on the tail, much less prominent than the dorsal ridges.
The changes which next ensue are practically identical with those which take place in Teleostei. Around the cuticular sheath of the notochord there is formed an elastic membrane the membrana elastica externa. At the same time the basal parts of the dorsal, or as we may perhaps more conveniently call them, the neural ridges of the notochord become enlarged at each intermuscular septum, and the tissue of these enlargements soon becomes converted into cartilage, thus forming a series of independent paired neural processes riding on the membrana elastica externa surrounding the notochord, and extending about two-thirds of the way up the sides of the medullary cord. They are shewn in transverse section in Plate 41, fig. 67 (n.a.), and in a side view in fig. 68 (n.a.}.
Simultaneously with the neural arches, the haemal arches also become established, and arise by the formation of similar enlargements of the ventral or haemal ridges. In the trunk they are very small, but in the region of the tail their condition is very different. At the front end of the anal fin the paired haemal arches suddenly enlarge and extend ventralwards (Plate 41, fig. 67, h.a.}.
Each succeeding pair of arches becomes larger than the one in front, and the two elements of each arch first nearly meet below the caudal vein (Plate 41, fig. 67) and finally actually do so, forming in this way a completely closed haemal canal. At the point where they first meet the permanent caudal fin commences, and here (Plate 41, fig. 68) we find that not only do the. haemal arches meet and coalesce below the caudal vein, but they are actually produced into long spines supporting the fin-rays of the caudal fin, which thus differs from the other fins in being
502
780 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
supported by parts of the true vertebral column and not by independently formed elements of the skeleton.
Each of the large caudal haemal arches, including the spine, forms a continous whole, and arises at an earlier period of larval life than any other part of the vertebral column. We noticed the first indications of the neural arches in the larva of about a week old, while they are converted into fully formed cartilage in the larva of three weeks.
The neural and haemal arches, resting on th'e membrana elastica externa, do not at this early stage in the least constrict the notochord. They grow gradually more definite, till the larva is five or six weeks old and about 26 millims. in length, but otherwise for a long time undergo no important changes. During the same period, however, the true sheath of the notochord greatly increases in thickness, and the membrana elastica externa becomes more definite. So far it would be impossible to distinguish the development of the vertebral column of Lepidosteus from that of a Teleostean Fish.
Of the stages immediately following we have unfortunately had no examples, but we have been fortunate enough to obtain some young specimens of Lepidosteus^, which have enabled us to work out with tolerable completeness the remainder of the developmental history of the vertebral column. In the next oldest larva, of about 5 '5 centims., the changes which have taken place are already sufficient to differentiate the vertebral column of Lepidosteus from that of a Teleostean, and to shew how certain of the characteristic features of the adult take their origin.
In the notochord the most important and striking change consists in the appearance of a series of very well marked vertebral constrictions opposite the insertions of the neural and hcemal arches. The first constrictions of the notochord are thus, as in other Fishes, vertebral; and although, owing to the growth of the intervertebral cartilage, the vertebral constrictions are subsequently replaced by intervertebral constrictions, yet at the same time the primitive occurrence of vertebral constrictions demonstrates that the vertebral column of Lepidosteus is a modification of a type of vertebral column with biconcave vertebrae.
1 These specimens were given to us by Professor W. K. Parker, who received them from Professor Burl G. Wilder.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
The structure of the gelatinous body of the notochord has undergone no important change. The sheath, however, exhibits certain features which deserve careful description. In the first place the attention of the observer is at once struck by the fact that, in the vertebral regions, the sheath is much thicker ('014 millim.) than in the intervertebral ('005 millim.), and a careful examination of the sheath in longitudinal sections shews that the thickening is due to the special differentiation of a superficial part (Plate 41, fig. 69, s/i.~) of the sheath in each vertebral region. This part is somewhat granular as compared to the remainder, especially in longitudinal sections. It forms a cylinder (the walj of which is about *oi millim. thick) in each vertebral region, immediately within the membrana elastica externa. Between it and the gelatinous tissue of the notochord within there is a very thin unmodified portion of the sheath, which is continuous with the thinner intervertebral parts of the sheath. This part of the sheath is faintly, but at the same time distinctly, concentrically striated a probable indication of concentric fibres. The inner unmodified layer of the sheath has the appearance in transverse sections through the vertebral regions of an inner membrane, and may perhaps be Kolliker's "membrana elastica interna."
We are not aware that any similar modification of the sheath has been described in other forms.
The whole sheath is still invested by a very distinct membrana elastica externa (m.e/).
The changes which have taken place in the parts which form the permanent vertebrae will be best understood from Plate 41, figs. 69 71. From the transverse section (fig. 70) it will be seen that there are still neural and haemal arches resting upon the membrana elastica externa ; but longitudinal sections (fig. 69) shew that laterally these arches join a cartilaginous tube, embracing the intervertebral regions of the notochord, and continuous from one vertebra to the next.
It will be convenient to treat separately the neural arches, the haemal arches with their appendages, and the intervertebral cartilaginous rings.
The neural arches, except in the fact of embracing a relatively smaller part of the neural tube than in the earlier stage, do not
782 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
at first sight appear to have undergone any changes. Viewed from the side, however, in dissected specimens, they are seen to be prolonged upwards so as to unite above with bars of cartilage directed obliquely backwards. An explanation of this appearance is easily found in the sections. The cartilaginous neural arches are invested by a delicate layer of homogeneous bone, developed in the perichondrium, and this bone is prolonged beyond the cartilage and joins a similar osseous investment of the dorsal bars above mentioned. The whole of these parts may, it appears to us, be certainly reckoned as parts of the neural arches, so that at this stage each neural arch consists of: (i) a pair of basal portions resting on the notochord consisting of cartilage invested by bone, (2) of a pair of dorsal cartilaginous bars invested in bone (n.a'.}, and (3) of osseous bars connecting (i) and (2).
Though, in the absence of the immediately preceding stages, it is not perfectly certain that the dorsal pieces of cartilage are developed independently of the ventral, there appears to us every probability that this is so ; and thus the cartilage of each neural arch is developed discontinuously, while the permanent bony neural arch, which commences as a deposit of bone partly in the perichondrium and partly in the intervening membrane, forms a continuous structure.
Analogous occurrences have been described by Gotte in Teleostei.
The dorsal portion of each neural arch becomes what we have called the dorsal process of the adult arch.
Between the dorsal processes of the two sides there is placed a median rod of cartilage (Plate 41, fig. 70, i. s.), which in its development is wholly independent of the true neural arches, and which constitutes the median spinous element of the adult. In tracing these backwards it becomes obvious that they are homologous with the interspinous elements supporting the dorsal fin, in that they are replaced by these interspinous elements in the region of the dorsal fin, and that the interspinous bones occupy the same position as the median spinous processes. This homology was first pointed out by Gotte in the case of the Teleostei.
Immediately beneath this rod is placed the longitudinal
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 783
ligament (Plate 41, fig. 70, /./.), but there is as yet no trace of a junction between the neural arches of the two sides in the space between the longitudinal ligament and the spinal cord.
The basal parts of the neural arches of the two sides are united dorsally by a thin cartilaginous layer resting on the sheath of the notochord, but they are not united ventrally with the haemal arches.
The haemal processes in the trunk are much more prominent than in the preceding stage, and their bases are united ventrally by a tolerably thick layer of cartilage. In the trunk they are continuous with the so-called ribs of the adult (Plate 41, fig. 70) ; but in order to study the nature of these ribs it is necessary to trace the modifications undergone by the haemal arches in passing from the tail to the trunk.
It will -be remembered that at an earlier stage the haemal arches in the region of the tail-fin were fully formed, and that through the anterior part of the caudal region the haemal processes were far advanced in development, and just in front of the caudal fin had actually met below the caudal vein.
The mode of development of the haemal arches in the tail as unjointed cartilaginous bars investing the caudal arteries and veins is so similar to that of the caudal haemal arches of Elasmobranchii, that it appears to us impossible to doubt their identity in the two groups 1 .
The changes which have taken place by this stage with reference to the haemal arches of the tail are not very considerable.
In the case of a few more vertebrae the haemal processes
1 Gegenbaur (No. 6) takes a different view on this subject, as is clear from the following passage in this memoir (pp. 369 370) :" Each vertebra of Lepidosteus thus consists of a section of the notochord, and of the cartilaginous tissue surrounding its sheath, which gives origin to the upper arches for the whole length of the vertebral column, and in the caudal region to that of the lower arches also. The latter do not however complete the enclosure of a lower canal, but this is effected by special independent elements, which are to be interpreted as homologues of the ribs." (The italics are ours.) While we fully accept the homology between the ribs and the lower elements of the kemal arches of the tail, the view expressed in the italicised section, to the effect that the lower parts of the caudal arches are not true haemal arches but are independently formed elements, is entirely opposed to our observations, and has we believe only arisen from the fact that Gegenbaur had not the young larvae to work with by which alone this question could be settled.
784 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
have united into an arch, and the spinous processes of the arches in the region of the caudal fin have grown considerably in length. A more important change is perhaps the commencement of a segmentation of the distal parts of the haemal arches from the proximal. This process has not, however, as yet resulted in a complete separation of the two, such as we find in the adult.
If the haemal processes are traced forwards (Plate 42, figs. 75 and 76) from the anterior segment where they meet ventrally, it will be found that each haemal process consists of a basal portion, adjoining the notochord, and a peripheral portion. These two parts are completely continuous, but the line of a future separation is indicated by the structure of the cartilage, though not shewn in our figures. As the true body-cavity of the trunk replaces the obliterated body-cavity of 'the caudal region, no break of continuity will be found in the structure of the haemal processes (Plates 41 and 42, figs. 73 and 74), but while the basal portions grow somewhat larger, the peripheral portions gradually elongate and take the form of delicate rods of cartilage extending ventralwards, on each side of the bodycavity, immediately outside the peritoneal membrane, and along the lines of insertion of the intermuscular septa. These rods obviously become the ribs of the adult.
As one travels forwards the ribs become continually longer and more important, and though they are at this stage united' with the haemal processes in every part of the trunk, yet they are much more completely separated from these processes in front than behind (Plate 41, fig. 72).
In front (Plate 41, fig. 72), each rib (rb.} t after continuing its ventral course for some distance, immediately outside the peritoneal membrane, turns outwards, and passes along one of the intermuscular septa till it reaches the epidermis. This feature in the position of the ribs is, as has been already pointed out in the anatomical part of this section, characteristic of all the ribs of the adult.
It is unfortunate that we have had no specimens shewing the ribs at an earlier stage of development ; but it appears hardly open to doubt that iJie ribs are originally continuous with tlie hcenial processes, and that the indications of a separation between
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 785
those two parts at this stage are not due to a secondary fusion, but to a commencing segmentation.
It further appears, as Miiller, Gegenbaur and others have stated, that the ribs and haemal processes of the tail are serially homologous structures ; but that the view maintained by Gotte in his very valuable memoirs on the Vertebrate skeleton is also correct to the effect that the h&mal arches of the tail are homologous throughout the series of Fishes.
To this subject we shall return again at the end of the section.
Before leaving the haemal arches it may be mentioned that behind the region of the ventral caudal fin the two haemal processes merge into one, and form an unpaired knob resting on the ventral side of the notochord, and not perforated by a canal.
There are now present well -developed intervertebral rings of cartilage, each of which eventually becomes divided into two parts, and converted into the adjacent faces of the contiguous vertebrae. These rings are united with the neural and haemal arches of the vertebrae in front and behind.
Each ring, as shewn by the transverse section (Plate 41, fig. 71), is not uniformly thick, but exhibits four projections, two dorsal and two ventral. These four projections are continuous with the bases of the neural and haemal arches of the adjacent vertebrae, and afford presumptive evidence of the derivation of the intervertebral rings from the neural and haemal arches; in that had they so originated, it would be natural to anticipate the presence of four thickenings indicating the four points from which the cartilage had spread, while if the rings had originated independently, it would not be easy to give any explanation of the presence of such thickenings. Gegenbaur (No. 6), from the investigation of a much older larva than that we are now describing, also arrived at the conclusion that the intervertebral cartilages were derived from the neural and haemal arches ; but as doubts have been thrown upon this conclusion by Gotte, and as it obviously required further confirmation, we have considered it important to attempt to settle this point. From the description given above, it is clear that we have not, however, been able absolutely to trace the origin of this cartilage, but at the same
786 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
time we think that we have adduced weighty evidence in corroboration of Gegenbaur's view.
As shewn in longitudinal section (Plate 41, fig. 69, iv.r.}, the intervertebral rings are thicker in the middle than at the two ends. In this thickened middle part the division of the cartilage into two parts to form the ends of two contiguous vertebrae is subsequently effected. The curved line which this segmentation will follow is, however, already marked out, and from surface views it might be supposed that this division had actually occurred.
The histological structure of the intervertebral cartilage is very distinct from that of the cartilage of the bases of the arches, the nuclei being much more closely packed. In parts, indeed, the intervertebral cartilage has almost the character of -fibre-cartilage. On each side of the line of division separating two vertebrae it is invested by a superficial osseous deposit.
The next oldest larva we have had was 1 1 centims. in length. The filamentous dorsal lobe of the caudal fin still projected far beyond the permanent caudal fin (Plate 34; fig. 16).
The vertebral column was considerably less advanced in development than that dissected by Gegenbaur, though it shews a great advance on the previous stage. Its features are illustrated by two transverse sections, one through the median plane of a vertebral region (Plate 42, fig. 78) and the other through that of an intervertebral region (Plate 42, fig. 79), and by a horizontal section (Plate 42, fig. 77).
In the last stage the notochord was only constricted vertebrally. Now, however, by the great growth of intervertebral cartilage there have appeared (Plate 42, fig. 77) very wellmarked intervertebral constrictions, by the completion of which the vertebrae of Lcpidosteus acquire their unique character amongst Fishes.
These constrictions still, however, coexist with the earlier, though at this stage relatively less conspicuous, vertebral constrictions.
The gelatinous body of the notochord retains its earlier condition. The sheath has, however, undergone some changes. In the vertebral regions there is present in any section of the sheath (i) externally, the membrana elastica externa
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 787
then (2) the external layer of the sheath (sh.), which is, however, less thick than before, and exhibits a very faint form of radial striation ; and (3) internally, a fairly thick and concentrically striated layer. The whole thickness is, on an average, O'l8 millim.
In the intervertebral regions the membrana elastica externa is still present in most parts, but has become absorbed at the posterior border of each vertebra, as shewn in longitudinal section in Plate 42, fig. 77. It is considerably puckered transversely. The sheath of the notochord within the membrana elastica externa is formed of a concentrically striated layer, continuous with the innermost layer of the sheath in the vertebral regions. It is puckered longitudinally. Thus, curiously enough, the membrana elastica externa and the sheath of the notochord in the intervertebral regions are folded in different directions, the folds of the one being only visible in transverse sections (Plate 42, fig. 79), and those of the other in longitudinal sections (Plate 42, fig. 77).
The osseous and cartilaginous structures investing the notochord may conveniently be dealt with in the same order as before, viz. : the neural arches, the haemal arches, and the intervertebral cartilages.
The cartilaginous portions of the neural arches are still unossified, and form (Plate 42, fig. 78, n.a.) small wedge-shaped masses resting on the sheath of the notochord. They are invested by a thick layer of bone prolonged upwards to meet the dorsal processes (n.a'.}, which are still formed of cartilage invested by bone.
It will be remembered that in the last stage there was no key-stone closing in the neural arch above. This deficiency is now however supplied, and consists of (i) two bars of cartilage repeated for each vertebra, but intervertebral ly placed, which are directly differentiated from the ligamentum longitudinale superius, into which they merge above ; and (2) two osseous plates placed on the outer sides of these cartilages, which are continuous with the lateral osseous bars of the neural arch. The former of these elements gives rise to the cartilaginous elements above the osseous bridge of the neural arch in the adult. The two osseous plates supporting these cartilages clearly form what we
788 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
have called in our description of the adult the osseous roof of the spinal canal.
A comparison of the neural arch at this stage with the arch in the adult, and in the stage last described, shews that the greater part of the neural arch of the adult is formed of membrane-bone, there being preformed in cartilage only a small basal part, a dorsal process, and paired key-stones below the ligamentum longitudinale superius.
The haemal arches (Plate 42, fig. 78) are still largely cartilaginous, and rest upon the sheath of the notochord. They are invested by a thick layer of bone. The bony layer investing the neural and haemal arches is prolonged to form a continuous investment round the vertebral portions of the notochord (Plate 42, fig. 78). This investment is at the sides prolonged outwards into irregular processes (Plate 42, fig. 78), which form the commencement of the outer part of the thick but cellular osseous cylinder forming the middle part of the vertebral body.
The intervertebral cartilages are much larger than in the earlier stage (Plate 42, figs. 77 and 79), and it is by their growth that the intervertebral constrictions of the notochord are produced. They have ceased to be continuous with the cartilage of the arches, the intervening portion of the vertebral body between the two being only formed of bone. They are not yet divided into two masses to form the contiguous ends of adjacent vertebrae.
Externally, the part of each cartilage which will form the hinder end of a vertebral body is covered by a tube of bone, having the form of a truncated funnel, shewn in longitudinal section in Plate 42, fig. 77, and in transverse section in Plate 42,
fig- 79 At each end, the intervertebral cartilages are becoming penetrated and replaced by beautiful branched processes from the homogeneous bone which was first of all formed in the perichondrium (Plate 42, fig. 77).
This constitutes the latest stage which we have had.
Gegenbaur (No. 6) has described the vertebral column in a somewhat older larva of 18 centims.
The chief points in which the vertebral column of this larva differed from ours are : (i) the disappearance of all trace of the
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 789
primitive vertebral constriction of the notochord ; (2) the nearly completed constriction of the notochord in the intervertebral regions ; (3) the complete ossification of the vertebral portions of the bodies of the vertebrae, the terminal so-called intervertebral portions alone remaining cartilaginous ; (4) the complete ossification of the basal portions of the haemal and neural processes included within the bodies of the vertebrae, so that in the case of the neural arch all trace of the fact that the greater part was originally not formed in cartilage had become lost. The cartilage of the dorsal spinous processes was, however, still persistent.
The only points which remain obscure in the later history of the vertebral column are the history of the notochord and of its sheath. We do not know how far these are either simply absorbed or partially or wholly ossified.
Gotte in his memoir on the formation of the vertebral bodies of the Teleostei attempts to prove (i) that the so-called membrana elastica externa of the Teleostei is not a homogeneous elastica, but is formed of cells, and (2) that in the vertebral regions ossification first occurs in it.
In Lepidosteus we have met with no indication that the membrana elastica externa is composed of cells ; though it is fair to Gotte to state that we have not examined such isolated portions of it as he states are necessary in order to make out its structure. But further than this we have satisfied ourselves that during the earlier stage of ossification this membrane is not ossified, and indeed in part becomes absorbed in proximity to the intervertebral cartilages ; and Gegenbaur met with no ossification of this membrane in the later stage described by him.
Summary of the development of the vertebral column and ribs,
A mesoblastic investment is early formed round the notochord, which is produced into two dorsal and two ventral ridges, the former uniting above the neural canal. Around the cuticular sheath of the notochord an elastic membrane, the membrana elastica externa, is next developed. The neural ridges become enlarged at each inter-muscular septum, and these enlargements
7QO STRUCTURE 'AND DEVELOPMENT OF LEPIDOSTEUS.
soon become converted into cartilage, thus forming a series of neural processes riding on the membrana elastica externa, and extending about two-thirds of the way up the sides of the neural canal. The haemal processes arise simultaneously with, and in the same manner as, the neural. They are small in the trunk, but at the front end of the anal fin they suddenly enlarge and extend ventralwards. Each succeeding pair of hsemal arches becomes larger than the one in front, each arch finally meeting its fellow below the caudal vein, thus forming a completely closed haemal canal. These arches are moreover produced into long spines supporting the fin-rays of the caudal fin, which thus differs from the other unpaired fins in being supported by parts of the vertebral column, and not by separately formed skeletal elements.
In the next stage which we have had the opportunity of studying (larva of 5^ centims.), a series of very well-marked vertebral constrictions are to be seen in the notochord. The sheath is now much thicker in the vertebral than in the intervertebral regions : this is due to a special differentiation of a superficial part of the sheath, which appears more granular than the remainder. This granular part of the sheath thus forms a cylinder in each vertebral region. Between it and the gelatinous tissue of the notochord there remains a thin unmodified portion of the sheath, which is continuous with the intervertebral parts of the sheath. The neural and haemal arches are seen to be continuous with a cartilaginous tube embracing the intervertebral regions of the notochord, and continuous from one vertebra to the next. A delicate layer of bone, developed in the perichondrium, invests the cartilaginous neural arches, and this bone grows upwards so as to unite above with the osseous investment of separately developed bars of cartilage, which are directed obliquely backwards. These bars, or dorsal processes, may be reckoned as parts of the neural arches. Between the dorsal processes of the two sides is placed a median rod of cartilage, which is developed separately from the true neural arches, and which constitutes the median spinous element of the adult. Immediately below this rod is placed the ligamentum longitudinale superius. There is now a commencement of separation between the dorsal and ventral parts of the haemal arches, not only in the tail, but also
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 79 1
in the trunk, where they pass ventralwards on each side of the body-cavity, immediately outside the peritoneal membrane, along the lines of insertion of the intermuscular septa. These are obviously the ribs of the adult, and there is no break of continuity of structure between the haemal processes of the tail and the ribs. In the anterior part of the trunk the ribs pass outwards along the intermuscular septa till they reach the epidermis. Thus the ribs are originally continuous with the haemal processes. Behind the region of the ventral caudal fin the two haemal processes merge into one, which is not perforated by a canal.
Each of the intervertebral rings of cartilage becomes eventually divided into two parts, and converted into the adjacent faces of contiguous vertebrae, the curved line where this will be effected being plainly marked out. These rings are united with the neural and haemal arches of the vertebrae next in front and behind. As these rings are formed originally by the spreading of the cartilage from the primitive neural and haemal processes, the intervertebral cartilages are clearly derived from the neural and haemal arches. The intervertebral cartilages are thicker in the middle than at their two ends.
In our latest stage (11 centims.), the vertebral constrictions of the notochord are rendered much less conspicuous by the growth of the intervertebral cartilages giving rise to marked intervertebral constrictions. In the intervertebral regions the membrana elastica externa has become aborted at the posterior border of each vertebra, and the remaining part is considerably puckered transversely. The inner sheath of the notochord is puckered longitudinally in the intervertebral regions. The granular external layer of the sheath in the vertebral regions is less thick than in the last stage, and exhibits faint radial striations.
Two closely approximated cartilaginous elements now form a key-stone to the neural arch above : these are directly differentiated from the ligamentum longitudinale superius, into which they merge above. An osseous plate is formed on the outer side of each of these cartilages. These plates are continuous with the lateral osseous bars of the neural arches, and also give rise to the osseous roof of the spinal canal of the adult.
792 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
Thus the greater part of the neural arches is formed of membrane bone. The haemal arches are invested by a thick layer of bone, and there is also a continuous osseous investment round the vertebral portions of the notochord. The intervertebral cartilages become penetrated by branched processes of bone.
Comparison of the vertebral column of Lepidosteus with that of
other forms.
The peculiar form of the articulatory faces of the vertebrae of Lepidosteus caused L. Agassiz (No. 2) to compare them with the vertebrae of Reptiles, and subsequent anatomists have suggested that they more nearly resemble the vertebrae of some Urodelous Amphibia than those of any other form.
If, however, Gotte's account of the formation of the amphibian vertebrae is correct, there are serious objections to a comparison between the vertebrae of Lepidosteus and Amphibia on developmental grounds. The essential point of similarity supposed to exist between them consists in the fact that in both there is a great development of intervertebral cartilage which constricts the notochord intervertebrally, and forms the articular faces of contiguous vertebrae.
In Lepidosteus this cartilage is, as we have seen, derived from the bases of the arches ; but in Amphibia it is held by Gotte to be formed by a special thickening of a cellular sheath round the notochord which is probably homologous with the cartilaginous sheath of the notochord of Elasmobranchii, and therefore with part of the notochordal sheath placed within the membrana elastica externa.
If the above statements with reference to the origin of the intervertebral cartilage in the two types are true, it is clear that no homology can exist between structures so differently developed. Provisionally, therefore, we must look elsewhere than in Lepidosteus for the origin of the amphibian type of vertebrae.
The researches which we have recorded demonstrate, however, in a very conclusive manner that the vertebrae of Lepidosteus have very close affinities with those of Teleostei.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 793
In support of this statement we may point: (i) To the structure of the sheath of the notochord ; (2) to the formation of the greater part of the bodies of the vertebrae from ossification in membrane around the notochord ; (3) to the early biconcave form of the vertebras, only masked at a later period by the development of intervertebral cartilages ; (4) to the character of the neural arches.
This latter feature will be made very clear if the reader will compare our figures of the sections of later vertebrae (Plate 42, fig. 78) with Gotte's 1 figure of the section of the vertebra of a Pike (Plate 7, fig. i). In Gotte's figure there are shewn similar intercalated pieces of cartilage to those which we have found, and similar cartilaginous dorsal processes of the vertebras. Thus we are justified in holding that whether or no the opisthoccelous form of the vertebrae of Lepidostens. is a commencement of a type of vertebrae inherited by the higher forms, yet in any case the vertebrae are essentially built on the type which has become inherited by the Teleostei from the bony Ganoids.
PART III. The ribs of Fishes.
The nature and homologies of the ribs of Fishes have long been a matter of controversy ; but the subject has recently been brought forward in the important memoirs of Gotte 2 on the Vertebrate skeleton. The alternatives usually adopted are, roughly speaking, these : Either the haemal arches of the tail are homologous throughout the piscine series, while the ribs of Ganoids and Teleostei are not homologous with those of Elasmobranchii ; or the ribs are homologous in all the piscine groups, and the haemal arches in the tail are differently formed in the different types. Gotte has brought forward a great body of evidence in favour of the first view; while Gegenbaur 3 may
1 "Beitrage zur vergl. Morphol. d. Skeletsystems d. Wirbelthiere." Archiv f. Mikr. Anat. Vol. xvi. 1879.
2 " Beitrage z. vergl. Morph. d. Skeletsystems d. Wirbelthiere. II. Die Wirbelsaule u. ihre Anhange." Archvo /. Mikr. Anat., Vol. xv., 1878, and Vol. xvi., 1879.
3 " Ueb. d. Entwick. d. Wirbelsaule d. Lepidosteus, mit. vergl. Anat. Bemerkungen. "Jena ische Zeitschrift, Bd. in., 1863.
B. 51
794 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
be regarded as more especially the champion of the second view.
One of us held in a recent publication 1 that the question was not yet settled, though the view that the ribs are homologous throughout the series was provisionally accepted.
It is admitted by both Gegenbaur and Gotte that in Lepidosteus the ribs, in the transition from the trunk to the tail, bend inwards, and finally unite in the region of the tail to form the ventral parts of the haemal arches, and our researches have abundantly confirmed this conclusion.
Are the haemal arches, the ventral parts of which are thus formed by the coalescence of the ribs, homologous with the haemal arches in Elasmobranchii ? The researches recorded in the preceding pages appear to us to demonstrate in a conclusive manner that they are so. .
The development of the haemal arches in the tail in these two groups is practically identical ; they are formed in both as simple elongations of the primitive haemal processes, which meet below the caudal vein. In the adult there is an apparent difference between them, arising from the fact that in Lepidosteus the peripheral parts of the haemal processes are only articulated with the basal portions, and not, as in Elasmobranchii, continuous with them. This difference does not, however, exist in the early larva, since in the larval Lepidosteus the haemal arches of the tail are unsegmented cartilaginous arches, as they permanently are in Elasmobranchii. If, however, the homology between the haemal arches of the two types should still be doubted, the fact that in both types the haemal arches are similarly modified to support the fin-rays of the ventral lobe of the caudal fin, while in neither type are they modified to support the anal fin, may be pointed out as a very strong argument in confirmation of their homology.
The demonstration of the homology of the haemal arches of the tail in Lepidosteus and Elasmobranchii might at first sight be taken as a conclusive argument in favour of Gotte's view, that the ribs of Elasmobranchii are not homologous with those of Ganoidei. This view is mainly supported by two facts :
1 Comparative Embryology, Vol. II., pp. 462, 463 [the original edition].
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 795
(1) In the first place, the ribs in Elasmobranchii do not at first sight appear to be serially homologous with the ventral parts of the haemal arches of the tail, but would rather seem to be lateral offshoots of the haemal processes, while the haemal arches of the tail appear to be completed by the coalescence of independent ventral prolongations of the haemal processes.
(2) In the second place, the position of the ribs is different in the two groups. In Elasmobranchii they are situated between the dorso-lateral and ventro- lateral muscles (woodcut, fig. i, rb.},
FIG. i.
II,
m.el
Diagrammatic section through the trunk of an advanced embryo of Scyllium, to shew the position of the ribs.
ao., aorta; c. sh., cartilaginous notochordal sheath; cv., cardinal vein; hp., hremal process; k., kidney; /.j., ligamentum longitudinale superius ; m.el., membrana elastica externa; na., neural arch; no., notochord ; //., lateral line; rb., rib; sp.c., spinal cord.
while in Lepidosteus and other Ganoids they immediately girth the body-cavity.
There is much, therefore, to be said in favour of Gotte's view. At the same time, there is another possible interpretation of the facts which would admit the homology of the ribs as well as of the haemal arches throughout the Pisces.
512
796 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
Let us suppose, to start with, that the primitive arrangement of the parts is more or less nearly that found in Lepidosteus, where we have well-developed ribs in the region of the trunk, girthing the body-cavity, and uniting in the caudal region to form the ventral parts of the haemal arches. It is easy to conceive that the ribs in the trunk might somewhat alter their position by passing into the muscles, along the inter-muscular septa, till they come to lie between the dorso-lateral and ventrolateral muscles, as in Elasmobranchii. Lepidosteus itself affords 'a proof that such a change in the position of the ribs is not impossible, in that it differs from other Ganoids and from Teleostci in the fact that the free ends of the ribs leave the neighbourhood of the body-cavity and penetrate into the muscles.
If it be granted that the mere difference in position between the ribs of Ganoids and Elasmobranchii is not of itself sufficient to disprove their homology, let us attempt to picture what would take place at the junction of the trunk and tail in a type in which the ribs had undergone the above change in position. On nearing the tail it may be supposed that the ribs would gradually become shorter, and at the same time alter their position, till finally they shaded off into ordinary haemal processes. If, however, the haemal canal became prolonged forwards by the formation of some additional complete or nearly complete haemal arches, an alteration in the relation of the parts would necessarily take place. Owing to the position of the ribs, these structures could hardly assist in the new formation of the anterior part of the haemal canal, but the continuation forwards of the canal would be effected by prolongations of the haemal processes supporting the ribs. The new arches so formed would naturally be held to be homologous with the haemal arches of the tail, though really not so, while the true nature of the ribs would also be liable to be misinterpreted, in that the ribs would appear to be lateral outgrowths of the haemal processes of a wholly different nature to the ventral parts of the haemal arches of the tail.
In some Elasmobranchii, as shewn in the accompanying woodcut (fig. 2), in the transitional vertebrae between the trunk and the tail, the ribs are supported by lateral outgrowths of the haemal processes, while the wholly independent prolongations of
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 797
the haemal processes appear to be about to give rise to the haemal arches of the tail.
This peculiar state of things led Gotte, and subsequently one of us, to deny for Elasmobranchs all homology between the ribs and any part of the haemal arches of the tail ; but in view of the explanation just suggested, this denial was perhaps too hasty.
FIG. 2.
r.p
. . . V. etuis.
Transverse section through the ventral part of the notochord, and adjoining structures of an advanced Scyllium embryo at the root of the tail.
Vb., cartilaginous sheath of the notochord; ka., haemal process; r.p., process to which the rib is articulated ; m.el., membrana elastica externa ; ck., notochord ; ao., aorta; V.cau., caudal vein.
We are the more inclined to take this view because the researches of Gotte appear to shew that an occurrence, in manyrespects analogous, has taken place in some Teleostei.
In Teleostei, Johannes Muller, and following him Gegenbaur, do not admit that the haemal arches of the tail are in any part formed by the ribs. Gegenbaur (Elements of Comp. Anat., translation, p. 431) says, "In the Teleostei, the costiferous transverse processes" (what we have called the haemal processes) "gradually
798 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
converge in the caudal region, and form inferior arches, which are not homologous with those of Selachii and Ganoidei, although they also form spinous processes."
The opposite view, that the haemal arches of the tail in Teleostei contain parts serially homologous with the basal parts of the haemal processes as well as with the ribs, has been also maintained by many anatomists, e.g., Meckel, Aug. Muller, &c., and has recently found a powerful ally in Gotte.
In many cases, the relations of the parts appear to be fundamentally those found in Lepidosteus and Amia, and Gotte has shewn by his careful embryological investigations on Esox and Anguilla, that in these two forms there is practically conclusive evidence that the ribs as well as the haemal costiferous processes of Gegenbaur, which support them, enter into the formation of the haemal arches of the tail.
In a great number of Teleostei, e.g., the Salmon and most Cyprinoids, &c., the haemal arches in the region of transition from the trunk to the tail have 'a structure which at first sight appears to support Johannes Miiller's and Gegenbaur's view. The haemal processes grow larger and meet each other ventrally; while the ribs articulated to them gradually grow smaller and disappear.
The Salmon is typical in this respect, and has been carefully studied by Gotte, who attempts to shew (with, in our opinion, complete success) that the anterior haemal arches are really not entirely homologous with the true haemal arches behind, but that in the latter, the closure of the arch below is effected by the haemal spine, which is serially homologous with a pair of coalesced ribs, while in the anterior haemal arches, i.e., those of the trunk, the closure of the arch is effected by a bridge of bone uniting the haemal processes.
The arrangement of the parts just described, as well as the view of Gotte with reference to them, will be best understood from the accompanying woodcut (fig. 3), copied from Gotte's memoir.
Gotte sums up his own results on this point in the following words (p. 138): "It follows from this, that the half rings, forming the haemal canal in the hindermost trunk vertebrae of the Salmon, are not (with the exception of the last) completely homo
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 799
logous with those of the tail, but are formed by a connecting piece between the basal stumps (haemal processes), which originates as a paired median process of these stumps."
The incomplete homology between the anterior haemal arches and the true caudal haemal arches which follow them is exactly what we suggest may be the case in Elasmobranchii, and if it be admitted in the one case, we see no reason why it should not also be admitted in the other.
If this admission is made, the only ground for not regarding the ribs of Elasmobranchii as homologous with those of Ganoids
FIG. 3 .
Semi-diagrammatic transverse sections through the first caudal vertebra (A), the last
trunk vertebra (B), and the two trunk vertebrae in front (C and D), of a Salmon
embryo of 2-3 centims. (From Gotte.)
ub., haemal arch; ub'., haemal process; ud"., rib; c., notochord ; a., aorta; v. , vein; ^., connecting pieces between haemal processes ; u., kidney ; d., intestine ; sp'., haemal spine ; m',, muscles.
is their different position, and we have already attempted to prove that this is not a fundamental point.
The results of our researches appear to us, then, to leave two alternatives as to the ribs of Fishes. One of these, which may be called Gotte's view, may be thus stated: The haemal arches
800 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
are homologous throughout the Pisces: in Teleostei, Ganoidei, and Dipnoi 1 , the ribs, placed on the inner face of the body-wall, are serially homologous with the ventral parts of the haemal arches of the tail ; in Elasmobranchii, on the other hand, the ribs are neither serially homologous with the haemal arches of the tail nor homologous with the ribs of Teleostei and Ganoidei, but are outgrowths of the haemal processes into the space between the dorso-lateral and ventro-lateral muscles, which may perhaps have their homologues in Teleostei and Ganoids in certain accessory processes of the vertebrae.
The other view, which we are inclined to adopt, and the arguments for which have been stated in the preceding pages, is as follows: The Teleostei, Ganoidei, Dipnoi, and Elasmobranchii are provided with homologous haemal arches, which are formed by the coalescence below the caudal vein of simple prolongations of the primitive haemal processes of the embryo. The canal enclosed by the haemal arches can be demonstrated embryologically to be the aborted body-cavity.
In the region of the trunk the haemal processes and their prolongations behave somewhat differently in the different types.
In Ganoids and Dipnoi, in which the most primitive arrangement is probably retained, the ribs are attached to the haemal processes,and are placed immediately without the peritoneal membrane at the insertions of the intermuscular septa. These ribs are in many instances (Lepidosteus, Acipenser], and very probably in all, developed continuously with the haemal processes, and become subsequently segmented from them. They are serially homologous with the ventral parts of the haemal arches of the tail, which, like them, are in many instances (Ceratodus, Lepidosteus, Polypterus, and to some extent in Amia) segmented off from the basal parts of the haemal arches.
In Teleostei the ribs have the same position and relations as those in Ganoids and Dipnoi, but their serial homology with the ventral parts of the haemal processes of the tail, is often (e.g., the Salmon) obscured by some of the anterior haemal arches in the posterior part of the trunk being completed, not by the ribs, but
1 We .find the serial homology of the ribs and ventral parts of the haemal arches to be very clear in Ceratodus. Wiedersheim states that it is not clear in Protopterus, although he holds that the facts are in favour of this view.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 8oi
by independent outgrowths of the basal parts of the haemal processes.
In Elasmobranchii a still further divergence from the primitive arrangement is present. The ribs appear to have passed outwards along the intermuscular septa into the muscles, and are placed between the dorso-lateral and ventro-lateral muscles (a change of position of the ribs of the same nature, but affecting only their ends, is observable in Lepidosteus). This change of position, combined probably with the secondary formation of a certain number of anterior haemal arches similar to those in the Salmon, renders their serial homology with the ventral parts of the haemal processes of the tail far less clear than in other types, and further proof is required before such homology can be considered as definitely established.
This is not the place to enter into the obscure question as to how far the ribs of the Amphibia and Amniota are homologous with those of Fishes. It is to be remarked, however, that the ribs of the Urodela (i) occupy the same position in relation to the muscles as the Elasmobranch ribs, (2) that they are connected with the neural arches, and (3) that they coexist in the tail with the haemal arches, and seem, therefore, to be as different as possible from the ribs of the Dipnoi.
PART IV. The skeleton of the ventral lobe of the tail fin, and its bearing on the nature of the tail fin of the various types of Pisces.
In the embryos or larvae of all the Elasmobranchii, Ganoidei, and Teleostei which have up to this time been studied, the unpaired fins arise as median longitudinal folds of the integument on the dorsal and ventral sides of the body, which meet at the apex of the tail. The tail at first is symmetrical, having a form which has been called diphycercal or protocercal. At a later stage, usually, though not always, parts of these fins atrophy, while other parts undergo a special development and constitute the permanent unpaired fins.
Since the majority of existing as well as extinct Fishes are provided with discontinuous fins, those forms, such as the Eel (Anguilla), in which the fins are continuous, have probably re
802 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
verted to an embryonic condition : an evolutional process which is of more frequent occurrence than has usually been admitted.
In the caudal region there is almost always developed in the larvae of the above groups a special ventral lobe of the embryonic fin a short distance from the end of the tail. In Elasmobranchii and Chondrostean Ganoids the portion of the embryonic tail behind this lobe persists through life, and a special type of caudal fin, which is usually called heterocercal, is thus produced. This type of caudal fin appears to have been the most usual in the earlier geological periods.
Simultaneously with the formation of the ventral lobe of the heterocercal caudal fin, the notochord with the vertebral tissues surrounding it, becomes bent somewhat dorsalwards, and thus the primitive caudal fin forms a dorsally directed lobe of the heterocercal tail. We shall call this part the dorsal lobe of the tail-fin, and the secondarily formed lobe the ventral lobe.
Lepidosteus and Amia (Wilder, No. 15) amongst the bony Ganoids, and, as has recently been shewn by A. Agassiz 1 , most Teleostei acquire at an early stage of their development heterocercal caudal fins, like those of Elasmobranchii and the Chondrostean Ganoids ; but in the course of their further growth the dorsal lobe partly atrophies, and partly disappears as such, owing to the great prominence acquired by the ventral lobe. A portion of the dorsally flexed notochord and of the cartilage or bone replacing or investing it remains, however, as an indication of the original dorsal lobe, though it does not project backwards beyond the level of the end of the ventral lobe, which in these types forms the terminal caudal fin.
The true significance of the dorsally flexed portion of the vertebral axis was first clearly stated by Huxley 2 , but as A. Agassiz has fairly pointed out in the paper already quoted, this fact does not in any way militate against the view put forward by L. Agassiz that there is a complete parallelism between the embryonic development of the tail in these Fishes and the palseontological development of this organ. We think
1 " On the Young Stages of some Osseous Fishes. I. The Development of the Tail," Proc. of the American Academy of Arts and Sciences, Vol. XIII., 1877.
2 "Observations on the Development of some Parts of the Skeleton of Fishes," Quart. Journ. of Micr. Science, Vol. vil., 1859.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 803
that it is moreover convenient to retain the term homocercal for those types of caudal fin in which the dorsal lobe has atrophied so far as not to project beyond the ventral lobe.
We have stated these now well-known facts to enable the reader to follow us in dealing with the comparison between the skeleton supporting the fin-rays of the ventral lobe of the caudal fin, and that supporting the fin-rays of the remaining unpaired fins.
It has been shewn that in Lepidosteus the unpaired fins fall into two categories, according to the nature of the skeletal parts supporting them. The fin-rays of the true ventral lobe of the caudal fin are supported by the spinous processes of certain of the haemal arches. The remaining unpaired fins, including the anal fin, are supported by the so-called interspinous bones, which are developed independently of the vertebral column and its arches.
The question which first presents itself is, how far does this distinction hold good for other Fishes ? This question, though interesting, does not appear to have been greatly discussed by anatomists. Not unfrequently the skeletal supports of the ventral lobe of the caudal fin are assumed to be the same as those of the other fins.
Davidoff 1 , for instance, in speaking of the unpaired fins of Elasmobranch embryos, says (p. 514): "The cartilaginous rays of the dorsal fins agreed not only in number with the spinous processes (as indeed is also found in the caudal fin of the fullgrown Dog-fish)," &c.
Thacker 2 , again, in his memoir on the Median and Paired Fins, states at p. 284 : " We shall here consider the skeleton of the dorsal and anal fins alone. That of the caudal fin has undergone peculiar modifications by the union of fin-rays with haemal spines."
Mivart 3 goes into the question more fully. He points out (p. 471) that there is an essential difference between the dorsal and ventral parts of the caudal fin in Elasmobranchs, in that in
1 " Beitrage z. vergl. Anat. d. hinteren Gliedmassen d. Fische," Morph. Jahrbuch, Vol. v., 1879.
- Trans, of the Connecticut Acad., Vol. in., 1877.
3 St George Mivart, "Fins of Elasmobranchs, " Zool, Trans., Vol. x.
804 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
the former the radials are more numerous than the vertebrae and unconformable to them, while in the latter they are equal in number to the vertebras and continuous with them. "This," he goes on to say, "seems to point to a difference in nature between the dorsal and ventral portions of the caudal fin, in at least most Elasmobranchs." He further points out that Polyodon resembles Elasmobranchs. As to Teleostei, he does not express himself decidedly except in the case of Murcena, to which we shall return.
Mivart expresses himself as very doubtful as to the nature of the supports of the caudal fin, and thinks " that the caudal fin of different kinds of Fishes may have arisen in different ways in different cases."
An examination of the ventral part of the caudal fin in various Ganoids, Teleostei, and Elasmobranchii appears to us to shew that there can be but little doubt that, in the majority of the members of these groups at any rate, and we believe in all, the same distinction between the ventral lobe of the caudal fin and the remaining unpaired fins is found as in Lepidosteus.
In the case of most Elasmobranchii, a simple inspection of the caudal fin suffices to prove this, and the anatomical features involved in this fact have usually been recognized ; though, in the absence of embryological evidence, the legitimate conclusion has not always been drawn from them.
The difference between the ventral lobe of the caudal fin and the other fins in the mode in which the fin-rays are supported is as obvious in Chondrostean Ganoids as it is in Elasmobranchii ; it would appear also to hold good for Amia. Polypterus we have had no opportunity of examining, but if, as there is no reason to doubt, the figure of its skeleton given by Agassiz (Poissons Fossiles) is correct, there can be no question that the ventral lobe of the caudal fin is supported by the haemal arches, and not by interspinous bones. In Calamoicthys, the tail of which we have had an opportunity of dissecting through the kindness of Professor Parker, the fin- rays of the ventral lobe of the true caudal fin are undoubtedly supported by true haemal arches.
There is no unanimity of opinion as to the nature of the elements supporting the fin-rays of the caudal fin of Teleostei.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 805
Huxley 1 in his paper on the development of the caudal fin of the Stickleback, holds that these elements are of the nature of interhsemal bones. He says (p. 39) : " The last of these rings lay just where the notochord began to bend up. It was slightly longer than the bony ring which preceded it, and instead of having its posterior margin parallel with the anterior, it sloped from above downwards and backwards. Two short osseous plates, attached to the anterior part of the inferior surface of the penultimate ring, or rudimentary vertebral centrum, passed downwards and a little backwards, and abutted against a slender elongated mass of cartilage. Similar cartilaginous bodies occupy the same relation to corresponding plates of bone in the anterior vertebrae in the region of the anal fin ; and it is here seen, that while the bony plates coalesce and form the inferior arches of the caudal vertebrae, the cartilaginous elements at their extremities become the interhaemal bones. The cartilage connected with the inferior arch of the penultimate centrum is therefore an " interhsemal " cartilage. The anterior part of the inferior surface of the terminal ossification likewise has its osseous inferior arch, but the direction of this is nearly vertical, and though it is connected below with an element which corresponds in position with the interhaemal cartilage, this cartilage is five or six times as large, and constitutes a broad vertical plate, longer than it is deep, and having its longest axis inclined downwards and backwards. . . .
" Immediately behind and above this anterior hypural apophysis (as it may be termed) is another very much smaller vertical cartilaginous plate, which may be called the posterior hypural apophysis."
We have seen that Mivart expresses himself doubtful on the subject. Gegenbaur 2 appears to regard them as haemal arches.
The latter view appears to us without doubt the correct one. An examination of the tail of normal Teleostei shews that the fin-rays of that part of the caudal fin which is derived from the ventral lobe of the larva are supported by elements serially homologous with the haemal arches, but in no way homologous
1 "Observations on the Development of some parts of the Skeleton of Fishes," Quart. Journ. Micr. Science, Vol. vn., 1859.
2 Elements of Comparative Anatomy. (Translation), p. 431.
806 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
with the interspinous bones of the anal fin. The elements in question formed of cartilage in the larva, become ossified in the adult, and are known as the hypural bones. They may appear in the form of a series of separate haemal arches, corresponding in number with the primitive somites of this region, which usually, however, atrophy in the adult, or more often are from the first imperfectly segmented, and have in the adult the form of two or three or even of a single broad bony plate. The transitional forms between this state of things and that, for instance, in Lepidosteus are so numerous, that there can be no doubt that even the most peculiar forms of the hypural bones of Teleostei are simply modified haemal arches.
This view of the hypural bones is, moreover, supported by embryological evidence, since Aug. MUller 1 (p. 205) describes their development in a manner which, if his statements are to be trusted, leaves no doubt on this point.
There are a considerable number of Fishes which are not provided with an obvious caudal fin as distinct from the remaining unpaired fins, i.e. Chimaera, Eels, and various Eel-like forms amongst Teleostei, and the Dipnoi. Gegenbaur appears to hold that these Fishes ought to be classed together in relation to the structure of the caudal portion of their vertebral column, as he says on p. 431 of his Comparative Anatomy (English Translation): " In the Chimaerae, Dipnoi, and many Teleostei, the caudal portion of the vertebral column ends by gradually diminishing in size, but in most Fishes, &c."
For our purpose it will, however, be advisable to treat them separately.
The tail of Chimsera appears to us to be simply a peculiar modification of the typical Elasmobranch heterocercal tail, in which the true ventral lobe of the caudal fin may be recognized in the fin-fold immediately in front of the filamentous portion of the tail. In the allied genus Callorhynchus this feature is more distinct. The filamentous portion of the tail of Chimaera constitutes, according to the nomenclature adopted above, the true dorsal lobe, and may be partially paralleled in the filamentous dorsal lobe of the tail of the larval Lepidosteus (Plate 34, fig. 16).
1 " Beobachtungen zur vergl. Anat. d. Wirbelsaule," Miiller's Archiv, 1853.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 807
The tail of the eel-like Teleostei is again undoubtedly a modification of the normal form of tail characteristic of the Teleostei, in which, however, the caudal fin has become very much reduced and merged into the prolongations of the anal and dorsal fins.
This can be very clearly seen in Siluroid forms with an Eellike tail, such as Cnidoglanis. Although the dorsal and ventral fins appear to be continuous round the end of the tail, and there is superficially no distinct caudal fin, yet an examination of the skeleton of Cnidoglanis shews that the end of the vertebral column is modified in the usual Teleostean fashion, and that the haemal arches of the modified portion of the vertebral column support a small number of fin-rays ; the adjoining ventral finrays being supported by independent osseous fin-supports (interspinous bones).
In the case of the Eel (Anguilla anguilld) Huxley (loc. cit.} long ago pointed out that the terminal portion of the vertebral column was modified in an analogous fashion to that of other Teleostei, and we have found that the modified haemal arches of this part support a few fin-rays, though a still smaller number than in Cnidoglanis, The fin-rays so supported clearly constitute an aborted ventral lobe of the caudal fin.
Under these circumstances we think that the following statement by Mivart (ZooL Trans. Vol. X., p. 471) is somewhat misleading :
"As to the condition of this part (i.e. the ventral lobe of the tail-fin) in Teleosteans generally, I will not venture as yet to say anything generally, except that it is plain that in siich forms as Murcena, the dorsal and ventral parts of the caudal fin are similar in nattire and homotypal with ordinary dorsal and anal fins 1 ."
The italicized portion of this sentence is only true in respect to that part of the fringe of fin surrounding the end of the body, which is not only homotypal with, but actually part of, the dorsal and anal fins.
Having settled, then, that the tails of Chimaera and of Eellike Teleostei are simply special modifications of the typical form of tail of the group of Fishes to which they respectively
1 The italics are ours.
8o8 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
belong, we come to the consideration of the Dipnoi, in which the tail-fin presents problems of more interest and greater difficulty than those we have so far had to deal with.
The undoubtedly very ancient and primitive character of the Dipnoi has led to the view, implicitly if not definitely stated in most text- books, that their tail-fin retains the character of the piscine tail prior to the formation of the ventral caudal lobe, a stage which is repeated embryologically in the pre-heterocercal condition of the tail in ordinary Fishes.
Through the want of embryological data, and in the absence of really careful histological examination of the tail of any of the Dipnoi, we are not willing to speak with very great confidence as to its nature ; we are nevertheless of the opinion that the facts we can bring forward on this head are sufficient to shew that the tail of the existing Dipnoi is largely aborted, so that it is more or less comparable with that of the Eel.
We have had opportunities of examining the structure of the tail of Ceratodus and Protopterus in dissected specimens in the Cambridge Museum. The vertebral axis runs to the ends of the tail without shewing any signs of becoming dorsally flexed. At some distance from the end of the tail the fin-rays are supported by what are apparently segmented spinous prolongations of the neural and haemal arches. The dorsal elements are placed above the longitudinal dorsal cord, and occupy therefore the same position as the independent elements of the neural arches of Lepidostetis. They are therefore to be regarded as homologous with the dorsal fin-supports or interspinous bones of other types. The corresponding ventral elements are therefore also to be regarded as interspinous bones.
In view of the fact that the fin-supports, whenever their development has been observed, are found to be formed independently of the neural and haemal arches, we may fairly assume that this is also true for what we have identified as the interspinous elements in the Dipnoi.
The interspinous elements become gradually shorter as the end of the tail is approached, and it is very difficult from a simple examination of dissected specimens to make out how far any of the posterior fin-rays are supported by the haemal arches only. To this question we shall return, but we may remark
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 809
that, although there is a prolongation backwards of the vertebral axis beyond the last interspinous elements, composed it would seem of the coalesced neural and haemal arches but without the notochord, yet by far the majority of the fin-rays which constitute the apparent caudal fin are supported by interspinous elements.
The grounds on which we hold that the tail of the Dipnoi is to be regarded as a degenerate rather than primitive type of tail are the following :
(1) If it be granted that a diphycercal or protocercal form of tail must have preceded a heterocercal form, it is also clear that the ventral fin-rays of such a tail must have been supported, as in Polypterus and Calamoicthys, by haemal arches, and not by interspinous elements ; otherwise, a special ventral lobe, giving a heterocercal character to the tail, and provided with fin-rays supported only by haemal arches, could never have become evolved from the protocercal tail-fin. Since the ventral fin-rays of the tail of the Dipnoi are supported by interspinous elements and not by haemal arches, this tail-fin cannot claim to have the character of that primitive type of diphycercal or protocercal tail from which the heterocercal tail must be supposed to have been evolved.
(2) Since the nearest allies of the Dipnoi are to be found in Polypterus and the. Crossopterygidae of Huxley, and since in these forms (as evinced by the structure of the tail-fin of Polypterus, and the transitional type between a heterocercal and diphycercal form of fin observable in fossil Crossopterygidae) the ventral fin-rays of the caudal fin were clearly supported by haemal arches and not by interspinous elements, it is rendered highly probable that the absence of fin-rays so supported in the Dipnoi is a result of degeneration of the posterior part of the tail.
[We use this argument without offering any opinion as to whether the diphycercal character of the tail of many Crossopterygidae is primary or secondary.]
(3) The argument just used is supported by the degenerate and variable state of the end of the vertebral axis in the Dipnoi a condition most easily explained by assuming that the terminal part of the tail has become aborted.
B. 52
8 10 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
(4) We believe that in Ceratodus we have been able to trace a small number of the ventral fin-rays supported by haemal arches only, but these rays are so short as not to extend so far back as some of the rays attached to the interspinous elements in front. These rays may probably be interpreted, like the more or less corresponding rays in the tail of the Eel, as the last remnant of a true caudal fin.
The above considerations appear to us to shew with very considerable probability that the true caudal fin of the Dipnoi has become all but aborted like that of various Teleostei ; and that the apparent caudal fin is formed by the anal and dorsal fins meeting round the end of the stump of the tail.
From the adult forms of Dipnoi we are, however, of opinion that no conclusion can be drawn as to whether their ancestors were provided with a diphycercal or a heterocercal form of caudal fin.
The general conclusions with reference to the tail-fin at which we have arrived are the following :
(1) The ventral lobe of the tail-fin of Pisces differs from the other unpaired fins in the fact that its fin- rays are directly supported by. spinous processes of certain of the haemal arches instead of independently developed interspinous bones.
(2) The presence or absence of fin-rays in the tail-fin supported by haemal arches may be used in deciding whether apparently diphycercal tail-fins are aborted or primitive.
EXCRETORY AND GENERATIVE ORGANS.
I. Anatomy.
The excretory organs of Lepidostens have been described by MUller (No. 13) and Hyrtl (No. n). These anatomists have given a fairly adequate account of the generative ducts in the female, and Hyrtl has also described the male generative ducts and the kidney and its duct, but his description is contradicted by our observations in some of the most fundamental points.
In the female example of 100*5 centims. which we dissected, the kidney forms a paired gland, consisting of a narrow strip of glandular matter placed on each side of the vertebral column, on
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 8ll
the dorsal aspect of the body-cavity. It is covered on its ventral aspect by the oviduct and by its own duct, but is separated from both of these by a layer of the tough peritoneal membrane, through which the collecting tubes pass. It extends forwards from the anus for about three-fifths of the length of the body-cavity, and in our example had a total length of about 28 centims. (Plate 39, fig. 60, k). Anteriorly the two kidneys are separated by a short interval in the median line, but posteriorly they come into contact, and are so intimately united as almost to constitute a single gland.
A superficial examination might lead to the supposition that the kidney extended forwards for the whole length of the bodycavity up to the region of the branchial arches, and Hyrtl appears to have fallen into this error ; but what appears to be its anterior continuation is really a form of lymphatic tissue, something like that of the spleen, filled with numerous cells. This matter (Plate 39, fig. 60, fy.) continues from the kidney forwards without any break, and has a colour so similar to that of the kidney as to be hardly distinguishable from it with the naked eye. The true anterior end of the kidney is placed about 3 centims. in front on the left side, and on the same level on the right side as the wide anterior end of the generative duct (Plate 39, fig. 60, od.}. It is not obviously divided into segments, and is richly supplied with malpighian bodies.
It is clear from the above description that there is no trace of head-kidney or pronephros visible in the adult. To this subject we shall, however, again return.
As will appear from the embryological section, the ducts of the kidneys are probably simply the archinephric ducts, but to avoid the use of terms involving a theory, we propose in the anatomical part of our work to call them kidney ducts. They are thin-walled widish tubes coextensive with the kidneys. If cut open there may be seen on their inner aspect the numerous openings of the collecting tubes of the kidneys. They are placed ventrally to and on the outer border of the kidneys (Plate 39, fig. 60, s.g.}. Posteriorly they gradually enlarge, and approaching each other in the median line, coalesce, forming an unpaired vesicle or bladder (/.) about 6 centims. long in our example opening by a median pore on a more or less
52 2
8l2 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
prominent papilla (u.g.} behind the anus. The dilated portions of the two ducts are called by Hyrtl the horns of the bladder.
The sides of the bladder and its so-called horns are provided with lateral pockets into which the collecting tubes of the kidney open. These pockets, which we have found in two female examples, are much larger in the horns of the bladder than in the bladder itself. Similar pockets, but larger than those we have found, have been described by Hyrtl in the male, but are stated by him to be absent in the female. It is clear from our examples that this is by no means always the case.
Hyrtl states that the wide kidney ducts, of which his description differs in no material point from our own, suddenly narrow in front, and, perforating the peritoneal lining, are continued forwards to supply the anterior part of the kidney. We have already shewn that the anterior part of the kidney has no existence, and the kidney ducts supplying it are, according to our investigations, equally imaginary.
It was first shewn by Miiller, whose observations on this point have been confirmed by Hyrtl, &c., that the ovaries of Lepidosteus are continuous with their ducts, forming in this respect an exception to other Ganoids.
In our example of Lepidosteus the ovaries (Plate 39, fig. 60, ov.) were about 1 8 centims. in length. They have the form of simple sacks, filled with ova, and attached about their middle to their generative duct, and continued both backwards and forwards from their attachment into a blind process.
With reference to these sacks Miiller has pointed out and the importance of this observation will become apparent when we deal with the development that the ova are formed in the thickness of the inner wall of the sack. We hope to shew that the inner wall of the sack is alone equivalent to the genital ridge of, for instance, the ovary of Scyllium. The outer aspect of this wall i.e., that turned towards the interior of the sack is equivalent to the outer aspect of the Elasmobranch genital ridge, on which alone the ova are developed 1 . The sack into which the ova fall is, as we shall shew in the embryological section, a special section of the body-cavity shut off from the remainder,
1 Treatise on Comparative Embryology, Vol. I., p. 43 [the original edition].
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 813
and the dehiscence of the ova into this cavity is equivalent to their discharge into the body-cavity in other forms.
The oviduct (Plate 39, fig. 60, od.} is a thin-walled duct of about 21 centims. in length in the example we are describing, continuous in front with the ovarian sack, and gradually tapering behind, till it ends (od'.} by opening into the dilated terminal section of the kidney duct on 'the inner side, a short distance before the latter unites with its fellow. It is throughout closely attached to the ureter and placed on its inner, and to some extent on its ventral, aspect. The hindermost part of the oviduct which runs beside the enlarged portion of the kidney duct that portion called by Hyrtl the horn of the urinary bladder is so completely enveloped by the wall of the horn of the urinary bladder as to appear like a projection into the lumen of the latter structure, and the somewhat peculiar appearance which it presents in Hyrtl's figure is due to this fact. In our examples the oviduct was provided with a simple opening into the kidney duct, on a slight papilla ; the peculiar dilatations and processes of the terminal parts of the oviduct, which have been described by Hyrtl, not being present.
The results we have arrived at with reference to the male organs are very different indeed from those of our predecessor, in that we find the testicular products to be carried off by a series of vasa efferentia, which traverse the mesorchium, and are continuous with the uriniferous tubuli ; so that the semen passes through the uriniferous tubuli into the kidney duct and so to the exterior. We have moreover been unable to find in tJu male a duct homologous with the oviduct of the female.
This mode of transportation outwards of the semen has not hitherto been known to occur in Ganoids, though found in all Elasmobranchii, Amphibia, and Amniota. It is not, however, impossible that it exists in other Ganoids, but has hitherto been overlooked.
Our male example of Lepidosteus was about 60 centims. in length, and was no doubt mature. It was smaller than any of our female examples, but this according to Garman (vide, p. 361) is usual. The testes (Plate 39, fig. 58 A. A) occupied a similar position to the ovaries, and were about 21 centims. long. They were, as is frequently the case with piscine testes,
8 14 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
divided into a series of lobes (10 12), and were suspended by a delicate mesentery (mesorchium) from the dorsal' wall of the abdomen on each side of the dorsal aorta. Hyrtl (No. n) states that air or quicksilver injected between the limbs of the mesentery, passed into a vas deferens 'homologous with the oviduct which joins the ureter. We have been unable to find such a vas deferens ; but we have found in the mesorchium a number of tubes of a yellow colour, the colour being due to a granular substance quite unlike coagulated blood, but which appeared to us from microscopic examination to be the remains of spermatozoa 1 . These tubes to the number of 40 50 constitute, we believe, the vasa efferentia. Along the line of suspension of the testis on its inner border these tubes unite to form an elaborate network of tubes placed on the inner face of the testis an arrangement very similar to that often found in Elasmobranchii (vide F. M. Balfour, Monograph on tJie Development of Elasmobranch Fishes, plate 20, figs. 4 and 8).
We have figured this network on the posterior lobe of the testis (fig. 58 B), and have represented a section through it (fig. 59 A, n.v.e.}, and through one of the vasa efferentia (v.e.) in the mesorchium. Such a section conclusively demonstrates the real nature of these passages : they are filled with sperm like that in the body of the testis, and are, as may be seen from the section figured, continuous with the seminal tubes of the testis itself.
At the attached base of the mesorchium the vasa efferentia unite into a longitudinal canal, placed on the inner side of the kidney duct (Plate 39, fig. 58 A, t.c., also shewn in section in Plate 39, fig. 59 B, I.e.). From this canal tubules pass off which are continuous with the tubuli uriniferi, as may be seen from fig. 59 B, but the exact course of these tubuli through the kidney could not be made out in the preparations we were able to make of the badly conserved kidney. Hyrtl describes the arrangement of the vascular trunks in the mesorchium in the following way (No. 11, p. 6): "The mesorchium contains vascular trunks, viz., veins, which through their numerous anasto 1 The females we examined, which were no doubt procured at the same time as the male, had their oviducts filled with ova : and it is therefore not surprising that the vasa efferentia should be naturally injected with sperm.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 815
moscs form a plexus at the hilus of the testis, whose efferent trunks, 13 in number, again unite into a plexus on the vertebral column, which is continuous with the cardinal veins." The arrangement (though not the number) of Hyrtl's vessels is very similar to that of our vasa efferentia, and we cannot help thinking that a confusion of the two may have taken place ; which, in badly conserved specimens, not injected with semen, would be very easy.
We have, as already stated, been unable to find in our dissections any trace of a duct homologous with the oviduct of the female, and our sections through the kidney and its ducts equally fail to bring to light such a duct. The kidney ducts are about 19 centims. in length, measured from the genital aperture to their front end. These ducts are generally similar to those in the female ; they unite about 2 centims. from the genital pore to form an unpaired vesicle. Their posterior parts are considerably enlarged, forming what Hyrtl calls the horns of the urinary bladder. In these enlarged portions, and in the wall of the unpaired urinary bladder, numerous transverse partitions are present, as correctly described by Hyrtl, which are similar to those in the female, but more numerous. They give rise to a series of pits, at the blind ends of which are placed the openings of the kidney tubules. The kidney duct without doubt serves as vas deferens, and we have found in it masses of yellowish colour similar to the substance in the vasa efferentia identified by us as remains of spermatozoa.
1 1 . Development.
In the general account of the development we have already called attention to the earliest stages of the excretory system.
We may remind the reader that the first part of the system to be formed is the segmental or archinephric duct (Plate 36, figs. 28 and 29, .$-.). This duct arises, as in Teleostei and Amphibia, by the constriction of a hollow ridge of the somatic mesoblast into a canal, which is placed in contiguity with the epiblast, along the line of junction between the mesoblastic somites and the lateral plates of mesoblast. Anteriorly the duct
8l6 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
does not become shut off from the body-cavity, and also bends inwards towards the middle line. The inflected part of the duct is the first rudiment of the pronephros, and very soon becomes considerably dilated relatively to the posterior part of the duct.
The posterior part of each segmental duct acquires an opening into the cloacal section of the alimentary tract. Apart from this change, the whole of the ducts, except their pronephric sections, remain for a long time unaltered, and the next changes we have to speak of concern the definite establishment of the pronephros.
The dilated incurved portion of each segmental duct soon becomes convoluted, and by the time the embryo is about 10 milling in length, but before the period of hatching, an important change is effected in the relations of their peritoneal openings 1 .
Instead of leading into the body-cavity, they open into an isolated chamber on each side (Plate 38, fig. $i,pr. c.}, which we will call t\\Q pronephric chamber. The pronephric chamber is not, however, so far as we can judge, completely isolated from the body-cavity. We have not, it is true, detected with certainty at this stage a communication between the two ; but in later stages, in larvae of from 1 1 to 26 millims., we have found a richly ciliated passage leading from the body-cavity into the pronephros on each side (Plate 38, fig. 52, p.f.pl). We have not succeeded in determining with absolute certainty the exact relations between this passage and the tube of the pronephros, but we are inclined to believe that it opens directly into the pronephric chamber just spoken of.
As we hope to shew, this chamber soon becomes largely filled by a vascular glomerulus. On the accomplishment of these changes, the pronephros is essentially provided with all the parts typically present in a segment of the mesonephros (woodcut, fig. 4). There is a peritoneal tube (/) 2 , opening into a vesicle (v) ; from near the neck of the peritoneal tube there
1 The change is probably effected somewhat earlier than would appear from our description, but our specimens were not sufficiently well preserved to enable us to speak definitely as to the exact period.
2 We feel fairly confident that there is only one pronephric opening on each side, though we have no single series of sections sufficiently complete to demonstrate this fact with absolute certainty.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 817
comes off a convoluted tube (pr.n.}, forming the main mass of the pronephros, and ending in the segmental duct (sd.\
Diagrammatic views of the pronephros of Lepidosteus.
A, pronephros supposed to be isolated and seen from the side ; B, section through the vesicle of the pronephros and the ciliated peritoneal funnel leading into it ; pr.n., coiled tube of pronephros; sd., segmental or archinephric duct ; f., peritoneal funnel ; v., vesicle of pronephros ; bv., blood vessel of glomerulus ; /., glomerulus.
The different parts do not, however, appear to have the same morphological significance as those in the mesonephros.
Judging from the analogy of Teleostei, the embryonic structure of whose pronephros is strikingly similar to that of Lepidosteus, the two pronephric chambers into which the segmental ducts open are constricted off sections of the body-cavity.
8l8 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
With the formation of the convoluted duct opening into the isolated section of the body-cavity we may speak of a definite pronephros as having become established. The pronephros is placed, as can be made out in later stages, on the level of the opening of the air-bladder into the throat.
The pronephros increases in size, so far as could be determined, by the further convolution of the duct of which it is mainly formed ; and the next change of importance which we have noticed is the formation of a vascular projection into the pronephric chamber, forming the glomerulus already spoken of (vide woodcut, fig. 4,gl.), which is similar to that of the pronephros of Teleostei. We first detected these glomeruli in an embryo of about 15 millims., some days after hatching (Plate 38, fig. 52, gl.}, but it is quite possible that they may be formed considerably earlier.
In the same embryo in which the glomeruli were found we also detected for the first time a mesonephros consisting of a series of isolated segmental or nephridial tubes, placed posteriorly to the pronephros along the dorsal wall ot' the abdomen.
These were so far advanced at this stage that we are not in a position to give any account of their mode of origin. They are, however, formed independently of the segmental ducts, and in the establishment of the junction between the two structures, there is no outgrowth from the segmental duct to meet the segmental tubes. We could not at this stage find peritoneal funnels of the segmental tubes, though we have met with them at a later stage (Plate 38, fig. 53, //.), and our failure to find them at this stage is not to be regarded as conclusive against their existence.
A very considerable space exists between the pronephros and the foremost segmental tube of the mesonephros. The anterior mesonephric tubes are, moreover, formed earlier than the posterior.
In the course of further development, the mesonephric tubules increase in size, so that there ceases to be an interval between them, the mesonephros thus becoming a continuous gland. In an embryo of 26 millims. there was no indication of the formation of segmental tubes to fill up the space between the pronephros and mesonephros.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 819
The two segmental ducts have united behind into an unpaired structure in an embryo of 1 1 millims. This structure is no doubt the future unpaired urinogenital chamber (Plate 39, figs. 58 A, and 60, bl.}. Somewhat later, the hypoblastic cloaca becomes split into two sections, the hinder one receiving the coalesced segmental ducts, and the anterior remaining continuous with the alimentary tract. The opening of the hinder one forms the urinogenital opening, and that of the anterior the anus.
In an older larva of about 5*5 centims. the pronephros did not exhibit any marked signs of atrophy, though the duct between it and the mesonephros was somewhat reduced and surrounded by the trabecular tissue spoken of in connection with the adult. In the region between the pronephros and the front end of the fully developed part of the mesonephros very rudimentary tubules had become established.
The latest stage of the excretory system which we have studied is in a young Fish of about 1 1 centims. in length. The special interest of this stage depends upon the fact that the ovary is already developed, and not only so, but the formation of the oviducts has commenced, and their condition at this stage throws considerable light on the obscure problem of their nature in the Ganoids.
Unfortunately, the head of the young Fish had been removed before it was put into our hands, so that it was impossible for us to determine whether the pronephros was still present ; but as we shall subsequently shew, the section of the segmental duct, originally present between the pronephros and the front end of the permanent kidney or mesonephros, has in any case disappeared.
In addition to an examination of the excretory organs in situ, which shewed little except the presence of the generative ridges, we made a complete series of sections through the excretory organs for their whole length (Plate 39, figs. 54 57).
Posteriorly these sections shewed nothing worthy of note, the excretory organs and their ducts differing in no important particular from these organs as we have described them in the adult, except in the fact that the segmental ducts are not joined by the oviducts.
Some little way in front of the point where the two segmental
820 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
ducts coalesce to form the urinary bladder, the genital ridge comes into view. For its whole extent, except near its anterior part (of which more hereafter) this ridge projects freely into the body-cavity, and in this respect the young Fish differs entirely from the adult. As shewn in Plate 39, figs. 56 and 57 (g.r.), it is attached to the abdominal wall on the ventral side of, and near the inner border of each kidney. The genital ridge itself has a structure very similar to that which is characteristic of young Elasmobranchii, and it may be presumed of young Fishes generally. The free edge of the ridge is swollen, and this part constitutes the true generative region of the ridge, while its dorsal portion forms the supporting mesentery. The ridge itself is formed of a central stroma and a germinal epithelium covering it. The epithelium is thin on the whole of the inner aspect of the ridge, but, just as in Elasmobranchii, it becomes greatly thickened for a band-like strip on the outer aspect. Here, the epithelium is several layers deep, and contains numerous primitive germinal cells (p.o.}.
Though the generative organs were not sufficiently advanced for us to decide the point with certainty, the structure of the organ is in favour of the view that this specimen was a female, and, as will be shewn directly, there can on other grounds be no doubt that this is so. The large size of the primitive germinal cells (primitive ova) reminded us of these bodies in Elasmobranchii.
In the region between the insertion of the genital ridge (or ovary, as we may more conveniently call it) and the segmental duct we detected the openings of a series of peritoneal funnels of the excretory tubes (Plate 39 , fig. 57, /./!), which clearly therefore persist till the young Fish has reached a very considerable size.
As we have already said, the ovary projects freely into the body-cavity for the greater part of its length. Anteriorly, however, we found that a lamina extended from the free ventral edge of the ovary to the dorsal wall of the body-cavity, to which it was attached on the level of the outer side of the segmental duct. A somewhat triangular channel was thus constituted, the inner wall of which was formed by the ovary, the outer by the lamina just spoken of, and the roof by the strip of the peritoneum
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 821
of the abdominal wall covering that part of the ventral surface of the kidney in which the openings of the peritoneal funnels of the excretory tubes are placed. The structure of this canal will be at once understood by the section of it shewn in Plate 39,
% 55 There can be no doubt that this canal is the commencing ovarian sack. On tracing it backwards we found that the lamina forming its outer wall arises as a fold growing upwards from the free edge of the genital ridge meeting a downward growth of the peritoneal membrane from the dorsal wall of the abdomen ; and in Plate 39, fig. 56, these two laminae may be seen before they have met. Anteriorly the canal becomes gradually smaller and smaller in correlation with the reduced size of the ovarian ridge, and ends blindly nearly on a level with the front end of the excretory organs.
It should be noted that, owing to the mode of formation of the ovarian sack, the outer side of the ovary with the band of thickened germinal epithelium is turned towards the lumen of the sack; and thus the fact of the ova being formed on the inner wall of the genital sack in the adult is explained, and the comparison which we instituted in our description of the adult between the inner wall of the genital sack and the free genital ridge of Elasmobranchs receives its justification.
It is further to be noticed that, from the mode of formation of the ovarian sack, the openings of the peritoneal funnels of the excretory organs ought to open into its lumen ; and if these openings persist in the adult, they will no doubt be found in this situation.
Before entering on further theoretical considerations with reference to the oviduct, it will be convenient to complete our description of the excretory organs at this stage.
When we dissected the excretory organs out, and removed them from the body of the young Fish, we were under the impression that they extended for the whole length of the bodycavity. Great was our astonishment to find that slightly in front of the end of the ovary both excretory organs and segmental ducts grew rapidly smaller and finally vanished, and that what we had taken to be the front part of the kidney was nothing else but a linear streak of tissue formed of cells with
822 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
peculiar granular contents supported in a trabecular work (Plate 39, fig. 54). This discovery first led us to investigate histologically what we, in common with previous observers, had supposed to be the anterior end of the kidneys in the adult, and to shew that they were nothing else but trabecular tissue with cells like that of lymphatic glands. The interruption of the segmental duct at the commencement of this tissue demonstrates that if any rudiment of the pronephros still persists, it is quite functionless, in that it is not provided with a duct.
Ill . Theoretical considerations.
There are three points in our observations on the urinogenital system which appear to call for special remark. The first of these concerns the structure and fate of the pronephros, the second the nature of the oviduct, and the third the presence of vasa efferentia in the male.
Although the history we have been able to give of the pronephros is not complete, we have nevertheless shewn that in most points it is essentially similar to the pronephros of Teleostei. In an early stage we find the pronephros provided with a peritoneal funnel opening into the body-cavity. At a later stage we find that there is connected with the pronephros on each side, a cavity the pronephric cavity into which a glomerulus projects. This cavity is in communication on the one hand with the lumen of the coiled tube which forms the main mass of the pronephros, and on the other hand with the body-cavity by means of a richly ciliated canal (woodcut, fig. 4, p. 817).
In Teleostei the pronephros has precisely the same characters, except that the cavity in which the glomerulus is placed is without a peritoneal canal.
The questions which naturally arise in connection with the pronephros are: (i) what is the origin of the above cavity with its glomerulus ; and (2) what is the meaning of the ciliated canal connecting this cavity with the peritoneal cavity ?
We have not from our researches been able to answer the first of these questions. In Teleostei, however, the origin of this
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 823
cavity has been studied by Rosenberg 1 and Gotte*. According to the account of the latter, which we have not ourselves confirmed but which has usually been accepted, the front end of the segmental duct, instead of becoming folded off from the bodycavity, becomes included in a kind of diverticulum of the bodycavity, which only communicates with the remainder of the body-cavity by a narrow opening. On the inner wall of this diverticulum a projection is formed which becomes a glomerulus. At this stage in the development of the pronephros we have essentially the same parts as in the fully formed pronephros of Lepidosteus, the only difference being that the passage connecting the diverticulum containing the glomerulus with the remainder of the body-cavity is short in Teleostei, and in Lepidosteus forms a longish ciliated canal. In Teleostei the opening into the body-cavity becomes soon closed. If the above comparison is justified, and if the development of these parts in Lepidosteus takes place as it is described as doing in Teleostei, there can, we think, be no doubt that the ciliated canal of Lepidosteus , which connects the pronephric cavity with the body-cavity, is a persisting communication between this cavity and the body-cavity; and that Lepidostetis presents in this respect a more primitive type of pronephros than Teleostei.
It may be noted that in Lepidosteus the whole pronephros has exactly the character of a single segmental tube of the mesonephros. The pronephric cavity with its glomerulus is identical in structure with a malpighian body. The ciliated canal is similar in its relations to the peritoneal canal of such a segmental tube, and the coiled portion of the pronephros resembles the secreting part of the ordinary segmental tube. This comparison is no doubt an indication that the pronephros is physiologically very similar to the mesonephros, and so far justifies Sedgwick's 3 comparison between the two, but it does not appear to us to justify the morphological conclusions at
1 Rosenberg, Untersuch. ueb. d. Entwick. d. Teleostiemiere, Dorpat, 1867.
2 Gotte, Entwick. d. Unke, p. 826.
3 Seclgwick, " Early Development of the Wolffian Duct and anterior Wolffian Tubules in the Chick; with some Remarks on the Vertebrate Excretory System," Quart. Journ. of Micros. Science, Vol. xxi., 1881.
824 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
which he has arrived, or to necessitate any modification in the views on this subject expressed by one of us l .
The genital ducts of Ganoids and Teleostei have for some time been a source of great difficulty to morphologists ; and any contributions with reference to the ontogeny of these structures are of interest.
The essential point which we have made out is that the anterior part of the oviduct of Lepidosteus arises by a fold of the peritoneum attaching itself to the free edge of the genital ridge. We have not, unfortunately, had specimens old enough to decide how the posterior part of the oviduct is formed ; and although in the absence of such stages it would be rash in the extreme to speak with confidence as to the nature of this part of the duct, it may be well to consider the possibilities of the case in relation to other Ganoids and Teleostei.
The simplest supposition would be that the posterior part of the genital duct had the same origin as the anterior, i. e., that it was formed for its whole length by the concrescence of a peritoneal fold with the genital ridge, and that the duct so formed opened into the segmental duct.
The other possible supposition is that a true Miillerian duct i.e., a product of the splitting of the segmental duct is subsequently developed, and that the open end of this duct coalesces with the duct which has already begun to be formed in our oldest larva.
In attempting to estimate the relative probability of these two views, one important element is the relation of the oviducts of Lepidosteus to those of other Ganoids.
In all other Ganoids (vide Hyrtl, No. 1 1) there are stated to be genital ducts in both sexes which are provided at their anterior extremities with a funnel-shaped mouth open to the abdominal cavity. At first sight, therefore, it might be supposed that they had no morphological relationship with the oviducts of Lepidosteus, but, apart from the presence of a funnel-shaped mouth, the oviducts of Lepidosteus are very similar to those of Chondrostean Ganoids, being thin-walled tubes opening on a projecting papilla into the dilated kidney ducts (horns of the
1 F. M. Balfour, Comparative Embryology, Vol. n., pp. 600 603 [the original edition].
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 825
urinary bladder, Hyrtl). These relations seem to prove beyond a doubt that the oviduct of Lepidosteus is for its major part homologous with the genital ducts of other Ganoids.
The relationship of the genital ducts to the kidney ducts in Amia and Polypterns is somewhat different from that in the Chondrostei and Lepidosteus. In Amia the ureters are so small that they may be described rather as joining the coalesced genital ducts than vice versa, although the apparent coalesced portion of the genital ducts is shewn to be really part of the kidney ducts by receiving the secretion of a number of mesonephric tubuli. In Polyptenis the two ureters are stated to unite, and open by a common orifice into a sinus formed by the junction of the two genital ducts, which has not been described as receiving directly the secretion of any part of the mesonephros.
It has been usual to assume that the genital ducts of Ganoids are true Mullerian ducts in the sense above defined, on the ground that they are provided with a peritoneal opening and that they are united behind with the kidney ducts. In the absence of ontological evidence this identification is necessarily provisional. On the assumption that it is correct we should have to accept the second of the two alternatives above suggested as to the development of the posterior parts of the oviduct in Lepidosteus.
There appear to us, however, to be sufficiently serious objections to this view to render it necessary for us to suspend our judgment with reference to this point. In the first place, if the view that the genital ducts are Mullerian ducts is correct, the true genital ducts of Lepidosteus must necessarily be developed at a later period than the secondary attachment between their open mouths and the genital folds, which would, to say the least of it, be a remarkable inversion of the natural order of development. Secondly, the condition of our oldest larva shews that the Mullerian duct, if developed later, is only split off from quite the posterior part of the segmental duct ; yet in all types in which the development of the Mullerian duct has been followed, its anterior extremity, with the abdominal opening, is split off from either the foremost or nearly the foremost part of the segmental duct.
B- S3
826 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
Judging from the structure of the adult genital ducts of other Ganoids they must also be developed only from the posterior part of the segmental duct, and this peculiarity so struck one of us that in a previous paper 1 the suggestion was put forward that the true Ganoid genital ducts were perhaps not Miillerian ducts, but enlarged segmental tubes with persisting abdominal funnels belonging to the mesonephros.
If the possibility of the oviduct of Lepidosteus not being a Miillerian duct is admitted, a similar doubt must also exist as to the genital ducts of other Ganoids, and we must be prepared to shew that there is a reasonable ground for scepticism on this point. We would in this connexion point out that the second of the two arguments urged against the view that the genital duct of Lepidosteus is not a Miillerian duct applies with equal force to the case of all other Ganoids.
The short funnel-shaped genital duct of the Chondrostei is also very unlike undoubted Miillerian ducts, and could moreover easily be conceived as originating by a fold of the peritoneum, a slight extension of which would give rise to a genital duct like that of Lepidosteus.
The main difficulty of the view that the genital ducts of Ganoids are not Miillerian ducts lies in the fact that they open into the segmental duct. While it is easy to understand the genesis of a duct from a folding of the peritoneum, and also easy to understand how such a duct might lead to the exterior by coalescing, for instance, with an abdominal pore, it is not easy to see how such a duct could acquire a communication with the segmental duct.
We do not under these circumstances wish to speak dogmatically, either in favour of or against the view that the genital ducts of Ganoids are Miillerian ducts. Their ontogeny would be conclusive on this matter, and we trust that some of the anatomists who have the opportunity of studying the development of the Sturgeon will soon let us know the facts of the case. If there are persisting funnels of the mesonephric segmental tubes in adult Sturgeons, some of them ought to be situated within the genital ducts, if the latter are not Mullerian ducts ;
1 F. M. Balfour, "On the Origin and History of the Urinogenital Organs of Vertebrates," Journ. of Anat. and Phys., Vol. X., 1876 [This edition, No. VII].
STRUCTURE AND DEVELOPMENT OF I-EPIDOSTEUS. 827
and naturalists who have the opportunity ought also to look out for such openings.
The mode of origin of the anterior part of the genital duct of Lepidosteus appears to us to tell strongly in favour of the view, already regarded as probable by one of .us 1 , that the Teleostean genital ducts are derived from those of Ganoids ; and if, as appears to us indubitable, the most primitive type of Ganoid genital ducts is found in the Chondrostei, it is interesting to notice that the remaining Ganoids present in various ways approximations to the arrangement typically found in Teleostei. Lepidosteus obviously approaches Teleostei in the fact of the ovarian ridge forming part of the wall of the oviduct, but differs from the Teleostei in the fact of the oviduct opening into the kidney ducts, instead of each pair of du^ts having an independent opening in the cloaca, and in the fact that the male genital products are not carried to the exterior by a duct homologous with the oviduct. Amia is closer to the Teleostei in the arrangement of the posterior part of the genital ducts, in that the two genital ducts coalesce posteriorly ; while Polypterus approaches still nearer to the Teleostei in the fact that the two genital ducts and the two kidney ducts unite with each other before they join ; and in order to convert this arrangement into that characteristic of the Teleostei we have only to conceive the coalesced ducts of the kidneys acquiring an independent opening into the cloaca behind the genital opening.
The male genital ducts. The discovery of the vasa efferentia in Lepidosteus, carrying off the semen from the testis, and transporting it to the mesonephros, and thence through the mesonephric tubes to the segmental duct, must be regarded as the most important of our results on the excretory system.
It proves in the first place that the transportation outwards of the genital products of both sexes by homologous ducts, which has been hitherto held to be universal in Ganoids, and which, in the absence of evidence to the contrary, must still be assumed to be true for all Ganoids except Lepidosteus, is a secondary arrangement. This conclusion follows from the fact that in Elasmobranchs, &c., which are not descendants of
1 F. M. Balfour, Comparative Embryology, Vol. II., p. 605 [the original edition].
532
828 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
the Ganoids, the same arrangement of seminal ducts is found as in Lepidostens, and it must therefore have been inherited from an ancestor common to the two groups.
If, therefore, the current statements about the generative ducts of Ganoids are true, the males must have lost their vasa efferentia, and the function of vas deferens must have been taken by the homologue of the oviduct, presumably present in the male. The Teleostei must, moreover, have sprung from Ganoidei in which the vasa efferentia had become aborted.
Considerable phylogenetic difficulties as to the relationships of Ganoidei and Elasmobranchii are removed by the discovery that Ganoids were originally provided with a system of vasa efferentia like that of Elasmobranchii.
THE ALIMENTARY CANAL AND ITS APPENDAGES.
I. -Anatomy.
Agassiz (No. 2) gives a short description with a figure of the viscera of Lepidosteus as a whole. Van der Hceven has also given a figure of them in his memoir on the air-bladder of this form (No. 8), and Johannes Muller first detected the spiral valve and gave a short account of it in his memoir (No. 13). Stannius, again, makes several references to the viscera of Lepidosteus in his anatomy of the Vertebrata, and throws some doubt on Miiller's determination of the spiral valve.
The following description refers to a female Lepidosteus of IOO'5 centims. (Plate 40, fig. 66).
With reference to the mouth and pharynx, we have nothing special to remark. Immediately behind the pharynx there comes an elongated tube, which is not divisible into stomach and oesophagus, and may be called the stomach (j/.). It is about 44*6 centims. long, and gradually narrows from the middle towards the hinder or pyloric extremity. It runs straight backwards for the greater part of its length, the last 3*8 centims., however, taking a sudden bend forwards. For about half its length the walls are thin, and the mucous membrane is smooth ;
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 829
in the posterior half the walls are thick, and the mucous membrane is raised into numerous longitudinal ridges. The peculiar glandular structure of the epithelium of this part in the embryo is shewn in Plate 40, fig. 62 (st.}. Its opening into the duodenum is provided with a very distinct pyloric valve (Py}. This valve projects into a kind of chamber, freely communicating with the duodenum, and containing four large pits (c'}, into each of which a group of pyloric caeca opens. These caeca form a fairly compact gland (c.) about 6-5 centims. long, which overlaps the stomach anteriorly, and the duodenum posteriorly.
Close to the pyloric valve, on its right side, is a small papilla, on the apex of which the bile duct opens (b.d'}.
A small, apparently glandular, mass closely connected with the bile duct, in the position in which we have seen the pancreas in the larva (Plate 40, figs. 62 and 63, /.), is almost certainly a rudimentary pancreas, like that of many Teleostei ; but its preservation was too bad for histological examination. We believe that the pancreas of Lepidosteus has hitherto been overlooked.
The small intestine passes straight backwards for about 8 centims., and then presents three compact coils. From the end of these a section, about 5 centims. long, the walls of which are much thicker, runs forwards. The intestine then again turns backwards, making one spiral coil. -This spiral part passes directly, without any sharp line of demarcation, into a short and straight tube, which tapers slightly from before backwards, and ends at the anus. The mucous membrane of the intestine for about the first 3^5 centims. is smooth, and the muscular walls thin : the rest of the small intestine has thick walls, and the mucous membrane is reticulated.
A short spiral valve (sp. v.}, with a very rudimentary epithelial fold, making nearly two turns, begins in about the posterior half of the spiral coil of the intestine, extending backwards for slightly less than half the straight terminal portion of the intestine, and ending 4 centims. in front of the anus. Its total length in one example was about 4'5 centims.
The termination of the spiral valve is marked by a slight constriction, and we may call the straight portion of the intestine behind it the rectum (re.}.
830 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
The posterior part of the intestine, from the beginning of the spiral valve to the anus, is connected with the ventral wall of the abdomen by a mesentery.
The air-bladder (a.b.} is 45 centims. long, and opens into the alimentary canal by a slit-like aperture (a.fr.) on the median dorsal line, immediately behind the epipharyngeal teeth. Each lip of this aperture is largely formed by a muscular cushion, thickest at its posterior end, and extending about 6 millims. behind the aperture itself. A narrow passage is bounded by these muscular walls, which opens dorsally into the air-bladder.
The air-bladder is provided with two short anterior cornua, and tapers to a point behind : it shews no indication of any separation into two parts. A strong band of connective tissue runs along the inner aspect of its whole dorsal region, from which there are given off on each side at intervals of about 12 millims. anteriorly, gradually increasing to 18 millims. posteriorly bands of muscle, which pass outwards towards its side walls, and then spread out into the numerous reticulations with which the air-bladder is lined throughout. By the contraction of these muscles the cavity of the air-bladder can doubtless be very much diminished.
The main muscular bands circumscribe a series of more or less complete chambers, which were about twenty-seven in number on each side in our example. The chambers are confined to the sides, so that there is a continuous cavity running through the central part of the organ. The whole organ has the characteristic structure of a simple lung.
The liver (lr.} consists of a single elongated lobe, about 32 centims. long, tapering anteriorly and posteriorly, the anterior half being on the average twice as thick as the posterior half. The gall-bladder (g.b.} lies at its posterior end, and is of considerable size, tapering gradually so as to pass insensibly into the bile duct. The hepatic duct (kp.d) opens into the gallbladder at its anterior end.
The spleen (s.) is a large, compact, double gland, one lobe lying in the turn of the intestine immediately above the spiral valve, and the other on the opposite side of the intestine, so that the intestine is nearly embraced between the two lobes.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 831
1 1. Development.
We have already described in detail the first formation of the alimentary tract so far as we have been able to work it out, and we need only say here that the anterior and posterior ends of the canal become first formed, and that these two parts gradually elongate, so as to approach each other ; the growth of the posterior part is, however, the most rapid. The junction of the two parts takes place a very short distance behind the opening of the bile duct into the intestine.
For some time after the two parts of the alimentary tract have nearly met, the ventral wall of the canal at this point is not closed ; so that there is left a passage between the alimentary canal and the yolk-sack, which forms a vitelline duct.
After the yolk-sack has ceased to be visible as an external appendage it still persists within the abdominal cavity. It has, however, by this stage ceased to communicate with the gut, so that the eventual absorption of the yolk is no doubt entirely effected by the vitelline vessels. At these later stages of development we have noticed that numerous yolk nuclei, like those met with in Teleostei and Elasmobranchii 1 , are still to be found in the yolk.
It will be convenient to treat the history of sections of the alimentary tract in front of and behind the vitelline duct separately. The former gives rise to the pharyngeal region, the oesophagus, the stomach, and the duodenum.
The pharyngeal region, immediately after it has become established, gives rise to a series of paired pouches. These may be called the branchial pouches, and are placed between the successive branchial arches. The first or hyomandibular pouch, placed between the mandibular and hyoid arches, has rather the character of a double layer of hypoblast than of a true pouch, though in parts a slight space is developed between its two walls. It is shewn in section in Plate 37, fig. 43 (h.m), from an embryo of about 10 millims., shortly before hatching. It
1 For a history of similar nuclei, vide Comp. Embryol., Vol. II., chapters III. and IV.
832 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
does not appear to undergo any further development, and, so far as we can make out, disappears shortly after the embryo is hatched, without acquiring an opening to the exterior.
It is important to notice that this cleft, which in the cartilaginous Ganoids and Polypterus remains permanently open as the spiracle, is rudimentary even in the embryo of Lepidosteus.
The second pouch is the hyobranchial pouch : its outer end meets the epiblast before the larva is hatched, and a perforation is effected at the junction of the two layers, converting the pouch into a visceral cleft.
Behind the hyobranchial pouch there are four branchial pouches, which become perforated and converted into branchial clefts shortly after hatching.
The region of the oesophagus following the pharynx is not separated from the stomach, unless a glandular posterior region (vide description of adult) be regarded as the stomach, a nonglandular anterior region forming the oesophagus. The lumen of this part appears to be all but obliterated in the stages immediately before hatching, giving rise for a short period to a solid oesophagus like that of Elasmobranchii and Teleostei 1 .
From the anterior part of the region immediately behind the pharynx the air-bladder arises as a dorsal unpaired diverticulum. From the very first it has an elongated slit-like mouth (Plate 40, fig. 64, a.b'-.}, and is placed in the mesenteric attachment of the part of the throat from which it springs.
We have first noticed it in the stages immediately after hatching. At first very short and narrow, it grows in succeeding stages longer and wider, making its way backwards in the mesentery of the alimentary tract (Plate 40, fig. 65, a.b.}. In the larva of a month and a half old (26 millims.) it has still a perfectly simple form, and is without traces of its adult lung-like structure ; but in the larva of 1 1 centims. it has the typical adult structure.
The stomach is at first quite straight, but shortly after the larva is hatched its posterior end becomes bent ventralwards and forwards, so that the flexure of its posterior end (present in the adult) is very early established. The stomach is continuous be 1 Vide Coinp. Embryo!., Vol. II., pp. 50 63 [the original edition].
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 833
hind with the duodenum, the commencement of which is indicated by the opening of the bile duct.
The liver is the first-formed alimentary gland, and is already a compact body before the larva is hatched. We have nothing to say with reference to its development, except that it exhibits the same simple structure in the embryo that it does in the adult.
A more interesting glandular body is the pancreas. It has already been stated that in the adult we have recognized a small body which we believe to be the pancreas, but that we were unable to study its histological characters.
In the embryo there is a well-developed pancreas which ' arises in the same position and the same manner as in those Vertebrata in which the pancreas is an important gland in the adult.
We have first noticed the pancreas in a stage shortly after hatching (Plate 40, fig. 6i,/.). It then has the form of a funnelshaped diverticulum of the dorsal wall of the duodenum, immediately behind the level of the opening of the bile duct. From the apex of this funnel numerous small glandular tubuli soon sprout out.
The similarity in the development of the pancreas in Lepidosteus to that of the same gland in Elasmobranchii is very striking 1 .
The pancreas at a later stage is placed immediately behind the end of the liver in a loop formed by the pyloric section of the stomach (Plate 40, fig. 62,/.). During larval life it constitutes a considerable gland, the anterior end of which partly envelopes the bile duct (Plate 40, fig. 63,/.).
Considering the undoubted affinities between Lepidosteus and the Teleostei, the facts just recorded with reference to the pancreas appear to us to demonstrate that the small size and occasional absence (?) of this gland in Teleostei is a result of the degeneration of this gland ; and it seems probable that the pancreas will be found in the larvae of most Teleostei. These conclusions render intelligible, moreover, the great development of the pancreas in the Elasmobranchii.
1 Vide F. M. Balfour, "Monograph on Development of Elasmobranch Fishes," p. 226 [This edition, No. X., p. 454].
834 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
We have first noticed the pyloric caeca arising as outgrowths of the duodenum in larvae of about three weeks old, and they become rapidly longer and more prominent (Plate 40, fig. 62, .).
The portion of the intestine behind the vitelline duct is, as in all the Vertebrata, at first straight. In Elasmobranchs the lumen of the part of the intestine in which a spiral valve is present in the adult, very early acquires a more or less semilunar form by the appearance of a fold which winds in a long spiral. In Lepidosteus there is a fold similar in every respect (Plate 38, fig. 53, sp.v.\ forming an open spiral round the intestine. This fold is the first indication of the spiral valve, but it is relatively very much later in its appearance than in Elasmobranchs, not being formed till about three weeks after hatching. It is, moreover, in correlation with the small extent of the spiral valve of the adult, confined to a much smaller portion of the intestine than in Elasmobranchii, although owing to the relative straightness of the anterior part of the intestine it is proportionately longer in the embryo than in the adult.
The similarity of the embryonic spiral valve of Lepidosteus to that of Elasmobranchii shews that Stannius' hesitation in accepting Miiller's discovery of the spiral valve in Lepidosteus is not justified.
J. Mliller (Ban u. Entwick. d. Myxinoideii) holds that the socalled bursa entiana of Elasmobranchii (i.e., the chamber placed between the part of the intestine with the spiral valve and the end of the pylorus) is the homologue of the more elongated portion of the small intestine which occupies a similar position in the Sturgeon. This portion of the small intestine is no doubt homologous with the still more elongated and coiled portion of the small intestine in Lepidosteus placed between the chamber into which the pyloric caeca, &c., .open and the region of the spiral valve. The fact that the vitelline duct in the embryo Lepidosteus is placed close to the pyloric end of the stomach, and that the greater portion of the small intestine is derived from part of the alimentary canal behind this, shews that Miiller is mistaken in attempting to homologise the bursa entiana of Elasmobranchii, which is placed in front of the vitelline duct, with the coiled part of the small intestine of the above forms. The latter is either derived from an elongation of the very short
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 835
portion of the intestine between the vitelline duct and the primitive spiral valve, or more probably by the conversion of the anterior part of the intestine, originally provided with a spiral valve into a coiled small intestine not so provided.
We have already called attention to the peculiar mesentery present in the adult attaching the posterior straight part of the intestine to the ventral wall of the body. This mesentery, which together with the dorsal mesentery divides the hinder section of the body-cavity into two lateral compartments is, we believe, a persisting portion of the ventral mesentery which, as pointed out by one of us 1 , is primitively present for the whole length of the body-cavity. The persistence of such a large section of it as that found in the adult Lcpidosteus is, so far as we know, quite exceptional. This mesentery is shewn in section in the embryo in Plate 38, fig. 53 (v.tnt^. The small vessel in it appears to be the remnant of the subintestinal vein.
THE GILL ON THE HYOID ARCH.
It is well known that Lepidosteus is provided with a gill on the hyoid arch, divided on each side into two parts. An excellent figure of this gill is given by Miiller (No. 13, plate 5, fig. 6), who holds from a consideration of the vascular supply that the two parts of this gill represent respectively the hyoid gill and the mandibular gill (called by MUller pseudobranch). Miiller's views on this subject have not usually been accepted, but it is the fashion to regard the whole of the gill as the hyoid gill divided into two parts. It appeared to us not improbable that embryology might throw some light on the history of this gill, and accordingly we kept a look out in our embryos for traces of gills on the hyoid and mandibular arches. The results we have arrived at are purely negative, but are not the less surprising for this fact. The hyomandibular cleft as shewn above, is never fully developed, and early undergoes a complete atrophy a fact which is, on the whole, against Muller's view ; but what astonished us most in connection with the gill in question is that we have been
1 Comparative Embryology, Vol. II. p. 514 [the original edition].
836 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
unable to find any trace of it even in the oldest larva whose head we have had (26 millims.), and at a period when the gills on the hinder arches have reached their full development.
We imagined the gill in question to be the remnant of a gill fully formed in extinct Ganoid types, and therefore expected to find it better developed in the larva than in the adult. That the contrary is the fact appears to us fairly certain, although we cannot at present offer any explanation of it.
SYSTEMATIC POSITION OF LEPIDOSTEUS.
A. Agassiz concludes his memoir on the development of Lepidosteus by pointing out that in spite of certain affinities in other directions this form is " not so far removed from the bony Fishes as has been supposed." Our own observations go far to confirm Agassiz' opinion.
Apart from the complete segmentation, the general development of Lepidosteus is strikingly Teleostean. In addition to the general Teleostean features of the embryo and larva, which can only be appreciated by those who have had an opportunity of practically working at the subject, we may point to the following developmental features 1 as indicative of Teleostean affinities :
(1) The formation of the nervous system as a solid keel of the epiblast.
(2) The division of the epiblast into a nervous and epidermic stratum.
(3) The mode of development of the gut (vide pp. 752 754).
(4) The mode of development of the pronephros ; though, as shewn on p. 822, the pronephros of Lepidosteus has primitive characters not retained by Teleostei.
(5) The early stages in the development of the vertebral column (vide p. 779).
In addition to these, so to speak, purely embryonic characters there are not a few important adult characters :
(i) The continuity of the oviducts with the genital glands.
1 The features enumerated above are not in all cases confined to Lepidosteus and Teleostei, hut are always eminently characteristic of the latter.
STRUCTURE AND DEVELOPMPINT OF LEPIDOSTEUS. 837
(2) The small size of the pancreas, and the presence of numerous so-called pancreatic caeca.
(3) The somewhat coiled small intestine.
(4) Certain characters of the brain, e.g., the large size of the cerebellum ; the presence of the so-called lobi inferiores on the infundibulum ; and of tori semicirculares in the midbrain.
In spite of the undoubtedly important list of features to which we have just called attention, a list containing not less important characters, both embryological and adult, separating Lepidosteus from the Teleostei, can be drawn up :
(1) The character of the truncus arteriosus.
(2) The fact of the genital ducts joining the ureters.
(3) The presence of vasa efferentia in the male carrying the semen from the testes to the kidney, and through the tubules of the latter into the kidney duct.
(4) The 'presence of a well-developed opercular gill.
(5) The presence of a spiral valve; though this character may possibly break down with the extension of our knowledge.
(6) The typical Ganoid characters of the thalamencephalon and the cerebral hemispheres (vide pp. 769 and 770).
(7) The chiasma of the optic nerves.
(8) The absence of a pecten, and presence of a vascular membrane between the vitreous humour and the retina.
(9) The opisthoccelous form of the vertebrae.
(10) The articulation of the ventral parts of the haemal arches of the tail with processes of the vertebral column.
(u) The absence of a division of the muscles into dorsolateral and ventro-lateral divisions.
(12) The complete segmentation of the ovum.
The list just given appears to us sufficient to demonstrate that Lepidosteus cannot be classed with the Teleostei ; and we hold that Muller's view is correct, according to which Lepidosteus is a true Ganoid.
The existence of the Ganoids as a distinct group has, however, recently been challenged by so distinguished an Ichthyologist as Glinther, and it may therefore be well to consider how far the group as defined by Mliller is a natural one for living
838 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
forms 1 , and how far recent researches enable us to improve upon Mtiller's definitions. In his classical memoir (No. 13) the characters of the Ganoids are thus shortly stated :
" These Fishes are either provided with plate-like angular or rounded cement-covered scales, or they bear osseous plates, or are quite naked. The fins are often, but not always, beset with a double or single row of spinous plates or splints. The caudal fin occasionally embraces in its upper lobe the end of the vertebral column, which may be prolonged to the end of the upper lobe. Their double nasal openings resemble those of Teleostei. The gills are free, and lie in a branchial cavity under an operculum, like those of Teleostei. Many of them have an accessory organ of respiration, in the form of an opercular gill, which is distinct from the pseudobranch, and can be present together with the latter ; many also have spiracles like Elasmobranchii. They have many valves in the stem of the aorta like the latter, also a muscular coat in the stem of the aorta. Their ova are transported from the abdominal cavity by oviducts. Their optic nerves do not cross each other. The intestine is often provided with a spiral valve, like Elasmobranchii. They have a swimming-bladder with a duct, like many Teleostei. Their pelvic fins are abdominal.
" If we include in a definition only those characters which are invariable, the Ganoids may be shortly defined as being those Fish with numerous valves to the stem of the aorta, which is also provided with a muscular coat ; with free gills and an operculum, and with abdominal pelvic fins."
To these distinctive characters, he adds in an appendix to his paper, the presence of the spiral valve, and the absence of a processus falciformis and a choroid gland.
To the distinctive set of characters given by Miiller we may probably add the following :
(1) Oviducts and urinary ducts always unite, and open by a common urinogenital aperture behind the anus.
(2) Skull hyostylic.
1 We do not profess to be able to discuss this question for extinct forms of Fish, though of course it is a necessary consequence of the theory of descent that the various groups should merge into each other as we go back in geological time.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 839
(3) Segmentation complete in the types so far investigated, though perhaps Amia may be found to resemble the Teleostei in this particular.
(4) A pronephros of the Teleostean type present in the larva.
(5) Thalamencephalon very large and well developed.
(6) The ventricle in the posterior part of the cerebrum is not divided behind into lateral halves, the roof of the undivided part being extremely thin.
(7) Abdominal pores always present.
The great number of characters just given are amply sufficient to differentiate the Ganoids as a group ; but, curiously enough, the only characters amongst the whole series which have been given, which can be regarded as peculiar to the Ganoids, are (i) the characters of the brain, and (2) the fact of the oviducts and kidney ducts uniting together and opening by a common pore to the exterior.
This absence of characters peculiar to the Ganoids is an indication of how widely separated in organization are the different members of this great group.
At the same time, the only group with which existing Ganoids have close affinities is the Teleostei. The points they have in common with the Elasmobranchii are merely such as are due to the fact that both retain numerous primitive Vertebrate characters 1 , and the gulf which really separates them is very wide.
There is again no indication of any close affinity between the Dipnoi and, at any rate, existing Ganoids.
Like the Ganoids, the Dipnoi are no doubt remnants of a very primitive stock ; but in the conversion of the air-bladder into a true lung, the highly specialized character of their limbs 2 , their peculiar autostylic skulls, the fact of their ventral nasal openings leading directly into the mouth, their multisegmented bars (interspinous bars), directly prolonged from the neural and haemal arches and supporting the fin-rays of the unpaired dorsal and ventral fins, and their well-developed cerebral hemispheres,
1 As instances of this we may cite (i) the spiral valve; (2) the frequent presence of a spiracle; (3) the frequent presence of a communication between the pericardium and the body-cavity ; (4) the heterocercal tail.
2 Vide F. M. Balfour, "On the Development of the Skeleton of the Paired Fins of Elasmobranchs," Proc. Zool. Soc., 1881 [This edition, No. XX.].
840 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
very unlike those of Ganoids and approaching the Amphibian type, they form a very well-defined group, and one very distinctly separated from the Ganoids.
No doubt the Chondrostean Ganoids are nearly as far removed from the Teleostei as from the Dipnoi, but the links uniting these Ganoids with the Teleostei have been so fully preserved in the existing fauna of the globe, that the two groups almost run into each other. If, in fact, we were anxious to make any radical change in the ordinary classification of Fishes, it would be by uniting the Teleostei and Ganoids, or rather constituting the Teleostei into one of the sub-groups of the Ganoids, equivalent to the Chondrostei. We do not recommend such an arrangement, which in view of the great preponderance of the Teleostei amongst living Fishes would be highly inconvenient, but the step from Amia to the Teleostei is certainly not so great as that from the Chondrostei to Amia, and is undoubtedly less than that from the Selachii to the Holocephali.
LIST OF MEMOIRS ON THE ANATOMY AND DEVELOPMENT OF LEPIDOSTEUS.
1. Agassiz, A. "The Development of Lepidosteus? Part I., Proc. Amer. A cad. Arts and Sciences, Vol. xiv. 1879.
2. Agassiz, L. Recherches s. I. Poissons Fossiles. Neuchatel. 1833 45 3. Boas, J. E. " Ueber Herz u. Arterienbogen bei Ceradotus u. Protopterus" Morphol. Jahrbitch, Vol. VI. 1880.
4. Davidoff, M. von. " Beitrage z. vergleich. Anat. d. hinteren Gliedmassen d. Fische," Morphol. Jahrbuch, Vol. vi. 1880.
5. Gegenbaur, C. Untersuch. z. vergleich. Anat. d. Wirbelthiere, Heft II., Schultergiirtel d. Wirbelthiere. Brnstflosse der Fische. Leipzig, 1865.
6. Gegenbaur, C. "Zur Entwick. d. Wirbelsaule d. Lepidosteus, &c." Jenaische Zeitschrift, Vol. ill. 1867.
7. Hertwig, O. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)? Morphol. Jahrbuch, Vol. V. 1879.
8. H ceven, Van der. " Ueber d. zellige Schvvimmblase d. Lepidosteus." M tiller's Archiv, 1841.
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 84!
9. Hyrtl, J. "Ueber d. Schwimmblase von Lepidosteus osseus" Sitz. d. Wiener Akad. Vol. vin. 1852.
10. Hyrtl, J. "Ueber d. Pori abdominales, d. Kiemen-Arterien, u. d. Glandula thyroidea d. Ganoiden," Sitz. d. Wiener Akad. Vol. VIII. 1852.
u. H y r 1 1, J . Ueber d, Zussammenhang d. Geschlechts u. Harnwerkzeuge bet d. Ganoiden, Wien, 1855.
12. Kolliker, A. Ueber d. Ende d. Wirbelsaitle b. Ganoiden, Leipzig, 1860.
13. M tiller, J. "Ueber d. Bau u. d, Grenzen d. Ganoiden," Berlin Akad. 1844.
14. Schneider, H. "Ueber d. Augenmuskelnerven d. Ganoiden," Jcnaische Zeitschrift, Vol. XV. 1881.
15. Wilder, Burt G. " Notes on the North American Ganoils, Amia, Lepidosteus, Acipenser, and Polyodon? Proc. Amer. Assoc.for the Advancement of Science, 1875.
LIST OF REFERENCE LETTERS.
a. Anus, a b. Air-bladder, a b'. Aperture of air-bladder into throat, ac. Anterior commissure, af. Anal fin. al. Alimentary canal, ao. Aorta, ar. Artery. ati. Auditory pit. b. Brain, be. Body-cavity, bd. Bile duct. bd'. Aperture of bile duct into duodenum, bl. Coalesced portion of segmental ducts, forming urinogenital bladder. bra. Branchial arches, brc. Branchial clefts. c. Pyloric caeca. c'. Apertures of caeca into duodenum. cb. Cerebellum. c</v. Cardinal vein. ce. Cerebrum : in figs. 47 A and B, anterior lobe of cerebrum, ce'. Posterior lobe of cerebrum, cf. Caudal fin. en. Centrum, ch. Choroidal fissure, crv. Circular vein of vascular membrane of eye. csh. Cuticular sheath of notochord. cv. Caudal vein. d. Duodenum, d c. Dorsal cartilage of neural arch. df. Dermal fin-rays. dl. Dorsal lobe of caudal fin. dlf. Dorsal fin. e. Eye. ed. Epidermis, ep. Epiblast. fb. Fore-brain, fe. Pyriform bodies surrounding the zona radiata of the ovum, probably the remains of epithelial cells, gb. Gall-bladder, gd. Genital duct. gl. Glomerulus. gr. Genital ridge. h. Heart, h a. Haemal arch. h b. Hindbrain, h c. Head-cavity, hp d. Hepatic duct, h m. Hyomandibular cleft, h op. Operculum. hy. Hypoblast ; in fig. 10, hyoid arch. hyl. Hyaloid membrane, ic. Intercalated cartilaginous elements of the neural arches, in. Infundibulum. ir. Iris. is. Interspinous cartilage or bones, iv. Sub-intestinal vein. ivr. Intervertebral ring of cartilage, k. Kidney. /. Lens. / c . Longitudinal canal, formed by union of the vasa efferentia. I in. Lobi inferiores. //. Ligamentum longitudinale superius. /;-. Liver. It. Lateral line. ly. Lymphatic body in front of kidney, m. Mouth. m b. Mid-brain. m c. Medullary cord. m el. Membrana elastica externa. vies. Mesorchium. mn. Mandible, md. and mo. Medulla oblongata. ms. Mesoblast. na. Neural arch. na'. Dorsal element of neural arch. nc. Notochord. nve. Network formed by vasa efferentia on inner face of testis. od. Oviduct, oif. Aperture of oviduct into bladder, ol. Nasal pit or aperture, olf. Olfactory lobe. op. Optic vesicle, opch. Optic chiasma. op I. Optic lobes, opth. Optic thalami. or ep.
B. 54
842 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
Oral epithelium, ov. Ovary. p. Pancreas, pc. Pericardium, pcf. Pectoral fin. / ch. Pigmented layer of choroid. pf. Peritoneal funnel of segmental tube of mesonephros. pfp- Peritoneal funnel leading into pronephric chamber, p g. Pectoral girdle, pi/. Pelvic fin. pn. Pineal gland, po. Primitive germinal cells, pr. Mesoblastic somite, prc. Pronephric chamber, prn. Pronephros. prn'. Opening of pronephros into pronephric chamber, ft. Pituitary body. py. Pyloric valve. p z. Parietal zone of blastoderm, r. Rostrum. rb. Rib. re. Rectum, s. Spleen. -s c. Seminal vessels passing from the longitudinal canal into the kidney, s d. Suctorial disc. sg. Segmental or archinephric duct, sg t. Segmental tubules, sh. Granular outer portion of the sheath of the notochord in the vertebral regions, s mx. Superior maxillary process. s nc. Sub-notochordal rod. so. Somatic mesoblast. sp. Splanchnic mesoblast. sp n. Spinal nerve, sp v. Spiral valve, st. Stomach. st. Seminal tubes of the testis. sup. Suctorial papillae, t. Testis. th. Thalamencephalon. thl. Lobes of the roof of the thalamencephalon. tr. Trabeculse. ug. Urinogenital aperture, v. Ventricle, v e. Vasa efferentia. v h. Vitreous humour. v 1. Ventral lobe of the caudal fin. v mi. Ventral mesentery, vn. Vein. vs. Bloodvessel, v sh. Vascular sheath between the hyaloid membrane and the vitreous humour, v th. Vesicle of the thalamencephalon. x. Groove in epiblast, probably formed in process of hardening, y. Yolk. z. Commissure in front of pineal gland. zr. Outer striated portion of investing membrane (zona radiata) of ovum. zr 1 . Inner non-striated portion of investing membrane of ovum. I. Olfactory nerve. II. Optic nerve. III. Oculomotor nerve. V. Trigeminal nerve. VIII. Facial and auditory nerves.
EXPLANATION OF PLATES 3442. PLATE 34.
Figs, i 4. Different stages in the segmentation of the ovum. Fig. T. Ovum with a single vertical furrow, from above. Fig. i. Ovum with two vertical furrows, from above. Fig. 3. Side view of an ovum with a completely formed blastodermic disc. Fig. 4. The same ovum as fig. 3, from below, shewing four vertical furrows nearly meeting at the vegetative pole.
Figs. 5 10. External views of embryos up to time of hatching.
Fig. 5. Embryo, 3^5 millims. long, third day after impregnation. Fig. 6. Embryo on the fifth day after impregnation. Fig. 7. Posterior part of same embryo as fig. 6, shewing tail swelling. Fig. 8. Embryo on the sixth day after impregnation. Fig. 9. Embryo on the seventh day after impregnation. Fig. 10. Embryo on the eleventh day after impregnation (shortly before hatching).
Fig. ii. Head of embryo about the same age as fig. 10, ventral aspect.
Fig. 12. Side view of a larva about n millims. in length, shortly after hatching.
Fig. 13. Head of a larva about the same age as fig. 12, ventral aspect.
EXPLANATION OF PLATES 35, 36. 843
Fig. 14. Side view of a larva about 15 millims. long, five days after hatching. Fig. 15. Head of a larva 23 millims. in length. Fig. 16. Tail of a larva n centims. in length.
Fig. 17. Transverse section through the egg-membranes of a just-laid ovum. We are indebted to Professor W. K. Parker for figs. 12, 14 and 15.
PLATE 35.
Figs. 18 22. Transverse sections of embryo on the third day after impregnation.
Fig. 18. Through head, shewing the medullary keel.
Fig. 19. Through anterior part of trunk.
Fig. 20. Through same region as fig. 19, shewing a groove (x) in the epiblast, probably artificially formed in the process of hardening.
Fig. 21. Through anterior part of tail region, shewing partial fusion of layers.
Fig. 22. Through posterior part of tail region, shewing more complete fusion of layers than fig. 2 1 .
Figs. 23 25. Transverse sections of an embryo on the fifth day after impregnation.
Fig. 23. Through fore-brain and optic vesicles.
Fig. 24. Through hind -brain and auditory pits.
Fig. 25. Through anterior part of trunk.
Figs. 26 27. Tranverse sections of the head of an embryo on the sixth day after impregnation.
Fig. 26. Through fore-brain and optic vesicles.
Fig. 27. Through hind-brain and auditory pits.
PLATE 36.
Figs. 28 29. Transverse sections of the trunk of an embryo on the sixth day after impregnation.
Fig. 28. Through anterior part of trunk (from a slightly older embryo than
the other sections of this stage).
Fig. 29. Slightly posterior to fig. 28, shewing formation of segmental duct as a fold of the somatic mesoblast.
Fig. 30. Longitudinal horizontal section of embryo on the sixth day after impregnation, passing through the mesoblastic somites, notochord, and medullary canal.
Figs. 31 34. Transverse sections through an embryo on the seventh day after impregnation.
Fig. 31. Through anterior part of trunk.
Fig. 32. Through the trunk somewhat behind fig. 31.
F'g- 33- Through tail region.
Fig- 34- Further back than fig. 33, shewing constriction of tail from the yolk.
54-2
844 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
Figs- 35 37- Transverse sections through an embryo on the eighth day after impregnation.
Fig- 35- Through fore-brain and optic vesicles.
Fig. 36. Through hind-brain, shewing closed auditory pits, &c.
Fig- 37- Through anterior part of trunk.
Fig. 38. Section through tail of an embryo on the ninth day after impregnation.
PLATE 37.
Fig- 39- Section through the olfactory involution and part of fore-brain of a larva on the ninth day after impregnation, shewing olfactory nerve.
Fig. 40. Section through the anterior part of the head of the same larva, shewing pituitary involution.
Figs. 41 43. Transverse sections through an embryo on the eleventh day after impregnation.
Fig. 41. Through fore-part of head, shewing the pituitary body still con nected with the oral epithelium. Fig. 42. Slightly further back than fig. 41, shewing the pituitary body
constricted off from the oral epithelium. Fig. 43. Slightly posterior to fig. 42, to shew olfactory involution, eye,
and hyomandibular cleft.
Fig. 44. Longitudinal section of the head of an embryo of 1 5 millims. in length, a few days after hatching, shewing the structure of the brain.
Fig. 45. Longitudinal section of the head of an embryo, about five weeks after hatching, 26 millims. in length, shewing the structure of the brain. In the front part of the brain the section passes slightly to one side of the median line.
Figs. 46 A to 46 G. Transverse sections through the brain of an embryo 25 millims. in length, about a month after hatching.
Fig. 46 A. Through anterior lobes of cerebrum.
Fig. 46 B. Through posterior lobes of cerebrum.
Fig. 46 C. Through thalamencephalon.
Fig. 46 D. Through optic thalami and optic chiasma.
Fig. 46 E. Through optic lobes and infundibulum.
Fig. 46 F. Through optic lobes and cerebellum.
Fig. 46 G. Through optic lobes and cerebellum, slightly in front of fig. 46 F.
PLATE 38.
Figs. 47 A, B, C. Figures of adult brain. Fig. 47 A. From the side. Fig. 47 B. From above. Fig. 47 C. From below.
Fig. 48. Longitudinal vertical section through the eye of an embryo, about a week after hatching, shewing the vascular membrane surrounding the vitreous humour.
EXPLANATION OF PLATES 38, 39. 845
Fig. 49. Diagram shewing the arrangement of the vessels in the vascular membrane of the vitreous humour of adult eye.
Fig. 50. Capillaries of the same vascular membrane.
Fig. 51. Transverse section through anterior part of trunk of an embryo on the ninth day after impregnation, shewing the pronephros and pronephric chamber.
Fig. 52. Transverse section through the region of the stomach of an embryo 15 millims. in length, shortly after hatching, to shew the glomerulus and peritoneal funnel of pronephros.
Fig. 53- Transverse section through posterior part of the body of an embryo, about a month after hatching, shewing the structure of the mesonephros, the spiral valve, &c.
PLATE 39.
Figs. 54, 55, 56, and 57 are a series of transverse sections through the genital ridge and mesonephros of one side from a larva of 1 1 centims.
Fig. 54. Section of the lymphatic organ which lies in front of the mesonephros.
Fig. 55. Section near the anterior end of the mesonephros, where the genital sack is completely formed.
Fig- 56. Section somewhat further back, shewing the mode of formation of the genital sack.
Fig- 57- Section posterior to the above, the formation of the genital sack not having commenced, and the genital ridge with primitive germinal cells projecting freely into the body-cavity.
Fig. 58 A. View of the testis, mesorchium, and duct of the kidney of the left side of an adult male example of Lepidosteus, 60 centims. in length, shewing the vasa efferentia and the longitudinal canal at the base of the mesorchium. The kidney ducts have been cut open posteriorly to shew the structure of the interior.
Fig. 58 B. Inner aspect of the posterior lobe of the testis from the same example, to shew the vasa efferentia forming a network on the face of the testis.
Figs- 59 A and B. Two sections shewing the structure and relations of the efferent ducts of the testis in the same example.
Fig. 59 A. Section through the inner aspect of a portion of the testis and mesorchium, to shew the network of the vasa efferentia (n v e) becoming continuous with the seminal tubes (s t). The granular matter nearly filling the vasa efferentia and the seminal tubes represent the spermatozoa.
Fig. 59 B. Section through part of the kidney and its duct and the longitudinal canal (Ic) at the base of the mesorchium. Canals (s c) are seen passing off from the latter, which enter the kidney and join the uriniferous tubuli. Some of the latter (as well as the seminal tubes) are seen to be filled with granular matter, which we believe to be the remains of spermatozoa.
846 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
Fig. 60. Diagram of the urinogenital organs of the left side of an adult female example of Lepidosteus 100 centims. in length. This figure shews the oviduct (od) continuous with the investment of the ovary, opening at oa into the dilated part of the kidney duct (segmental duct). It also shews the segmental duct and the junction of the latter with its fellow of the right side to form the so-called bladder, this part being represented as cut open. The kidney (k) and lymphatic organ (/y).in front of it are also shewn.
PLATE 40.
Fig. 61. Transverse section through the developing pancreas (/) of a larva n millims. in length.
Fig. 62. Longitudinal section through portions of the stomach, liver, and duodenum of an embryo about a month after hatching, to shew the relations of the pancreas (/) to the surrounding parts.
Fig. 63. External view of portions of the liver, stomach, duodenum, &c., of a young Fish, u centims. in length, to shew the pancreas (/).
Fig. 64. Transverse section through the anterior part of the trunk of an embryo, about a month after hatching, shewing the connection of the air-bladder with the throat (a b').
Fig. 65. Transverse section through the same embryo as fig. 64 further back, shewing the posterior part of the air-bladder (a b).
Fig. 66. Viscera of an adult female, 100 centims. in length, shewing the alimentary canal with its appended glands in natural position, and the air-bladder with its aperture into the throat (a b'). The proximal part of the duodenum and the terminal part of the intestine are represented as cut open, the former to shew the pyloric valve and the apertures of the pyloric caeca and bile duct, and the latter to shew the spiral valve.
This figure was drawn for us by Professor A. C. Haddon.
PLATE 41.
Fig. 67. Transverse section through the tail of an advanced larva, shewing the neural and haemal processes, the independently developed interneural and interhaemal elements (is), and the commencing dermal fin-rays (df).
Fig. 68. Side view of the tail of a larva, 21 millims. in length, dissected so as to shew the structure of the skeleton.
Fig. 69. Longitudinal horizontal section through the vertebral column of a larva, 5-5 centims. in length, on the level of the haemal arches, shewing the intervertebral rings of cartilage continuous with the arches, the vertebral constriction of the notochord, &c.
Figs. 70 and 71. Transverse sections through the vertebral column of a larva of 5 '5 centims. The red represents bone, and the blue cartilage.
Fig. 70. Through the vertebral region, shewing the neural and haemal
arches, the notochordal sheath, &c.
Fig. 71. Through the intervertebral region, shewing the intervertebral cartilage.
EXPLANATION OF PLATES 41, 43. 847
Figs. 72 and 73. Transverse sections through the trunk of a larva of 5-5 centims. to shew the structure of the ribs and hremal arches.
Fig. 72. Through the anterior part of the trunk. Fig. 73. Through the posterior part of the trunk.
PLATE 42.
Figs. 74 76. Transverse sections through the trunk of the same larva as figs. 72 and 73.
Fig. 74. Through the posterior part of the trunk (rather further back than
fig- 73) Fig. 75. Through the anterior part of the tail. Fig. 76. Rather further back than fig. 75.
Fig. 77. Longitudinal horizontal section through the vertebral column of a larva of ii centims., passing through the level of the haemal arches, and shewing the intervertebral constriction of the notochord, the ossification of the cartilage, &c.
Fig. 78. Transverse section through a vertebral region of the vertebral column of a larva 1 1 centims. in length.
Fig. 79. Transverse section through an intervertebral region of the same larva as % 78.
Fig. 80. Side view of two trunk vertebrae of an adult Lepidosteus.
Fig. 8 1. Front view of a trunk vertebra of adult.
In figures 80 and 81 the red does not represent bone as in the other figures, but simply the ligamentum longitudinale superius.
XXIII. ON THE NATURE OF THE ORGAN IN ADULT TE LEOSTEANS AND GANOIDS, WHICH IS USUALLY REGARDED AS THE HEAD-KIDNEY OR PRONEPHROs 1 .
WHILE working at the anatomy of Lepidosteus I was led to doubt the accuracy of the accepted accounts of the anterior part of the kidneys in this' 2 and in allied species of Fishes. In order to test my doubts I first examined the structure of the kidneys in the Sturgeon (Acipenser), of which I fortunately had a wellpreserved specimen.
The bodies usually described as the kidneys consist of two elongated bands, attached to the dorsal wall of the abdomen, and extending for the greater part of the length of the abdominal cavity. In front each of these bands first becomes considerably narrowed, and then expands and terminates in a great dilatation, which is usually called the head-kidney. Along the outer border of the hinder part of each kidney is placed a wide ureter, which ends suddenly in the narrow part of the body, some little way behind the head-kidney. To the naked eye" there is no distinction in structure between the part of the socalled kidney in front of the ureter and that in the region of the ureter. Any section through the kidney in the region of the ureter suffices to shew that in this part the kidney is really formed of uriniferous tubuli with numerous Malpighian bodies. Just in front, however, of the point where the ureter ends the true kidney substance rapidly thins out, and its place is taken by a peculiar tissue formed of a trabecular work filled with cells,
1 From the Quarterly Journal of Microscopical Science, Vol. XXII., 1882.
2 I am about to publish, in conjunction with Mr Parker, a full account of the anatomy and development of Lepidosteus [No. XXII. of this edition], and shall therefore in this paper make no further allusion to it.
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 849
which I shall in future call lymphatic tissue. Thus the wliole of that part of the apparent kidney in front of the ureter, including the whole of the so-called head-kidney, is simply a great mass of lymphatic tissue, and does not contain a single urinifcrous tubule or MalpigJdan body,
The difference in structure between the anterior and posterior parts of the so-called kidney, although not alluded to in most modern works on the kidneys, appears to have been known to Stannius, at least I so interpret a note of his in the second edition of his Comparative Anatomy, p. 263, where he describes the kidney of the Sturgeon as being composed of two separate parts, viz. a spongy vascular substance (no doubt the so-called headkidney) and a true secretory substance.
After arriving at the above results with reference to the Sturgeon I proceeded to the examination of the structure of the so-called head-kidney in Teleostei.
I have as yet only examined four forms, viz. the Pike (Esox lucius), the Smelt (Osmerus eperlanus], the Eel (Anguilla anguilld), and the Angler (Lophius piscatorius).
The external features of the apparent kidney of the Pike have been accurately described by Hyrtl 1 . He says: "The kidneys extend from the second trunk vertebra to the end of the abdominal cavity. Their anterior extremities, w r hich have the form of transversely placed coffee beans, are united together, and lie on the anterior end of the swimming bladder. The continuation of the kidney backwards forms two small bands, separated from each other by the whole breadth of the vertebral column. They gradually, however, increase in breadth, so that about the middle of the vertebral column they unite together and form a single symmetrical, keel-shaped body," &c.
The Pike I examined was a large specimen of about 58 centimetres in length, and with an apparent kidney of about 25^ centimetres. The relations of lymphatic tissue and kidney tissue were much as in the Sturgeon. The whole of the anterior swelling, forming the so-called head-kidney, together with a considerable portion of the part immediately behind, forming not far short of half the whole length of the apparent kidney,
1 "Das Uropoetische System der Knochenfische," Si'.z. d. Wieu. Akad., 1850.
850 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS.
was entirely formed of lymphatic tissue. The posterior part of the kidney was composed of true kidney substance, but even at 1 6 centimetres from the front end of the kidney the lymphatic tissue formed a large portion of the whole.
A rudiment of the duct of the kidney extended forwards for a short way into the lymphatic substance beyond the front part of the functional kidney.
In the Smelt (Osmerus eperlamis] the kidney had the typical Teleostean form, consisting of two linear bands stretching for the whole length of the body-cavity, and expanding into a great swelling in front on the level of the ductus Cuvieri, forming the so-called head-kidney. The histological examination of these bodies shewed generally the same features as in the case of the Sturgeon and Pike. The posterior part was formed of the usual uriniferous tubuli and Malpighian bodies. The anterior swollen part of these bodies, and the part immediately following, were almost wholly formed of a highly vascular lymphatic tissue ; but in a varying amount in different examples portions of uriniferous tubules were present, mainly, however, in the region behind the anterior swelling. In some cases I could find no tubules in the lymphatic tissue, and in all cases the number of them beyond the region of the well-developed part of the kidney was so slight, that there can be little doubt that they are functionless remnants of the anterior part of the larval kidney. Their continuation into the anterior swelling, when present, consisted of a single tube only.
In the Eel (Anguilla anguilla), which, however, I have not examined w r ith the same care as the Smelt, the true excretory part of the kidney appears to be confined to the posterior portion, and to the portion immediately in front of the anus, the whole of the anterior part of each apparent kidney, which is not swollen in front, being composed of lymphatic tissue.
LopJiius piscatorius is one of the forms which, according to Hyrtl 1 , is provided with a head-kidney only, i.e. with that part of the kidney which corresponds with the anterior swelling of the kidney of other types. For this reason I was particularly anxious to investigate the structure of its kidneys.
1 "Das Uropoetische System der Knochenfische," Sitz. d. Wien. Akad., 1850.
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 851
Each of these bodies forms a compact oval mass, with the ureter springing from its hinder extremity, situated in a forward position in the body-cavity. Sections through the kidneys shewed that they were throughout penetrated by uriniferous tubules, but owing to the bad state of preservation of my specimens I could not come to a decision as to the presence of Malpighian bodies. The uriniferous tubules were embedded in lymphatic tissue, similar to that which forms the anterior part of the apparent kidneys in other Teleostean types.
With reference to the structure of the Teleostean kidneys, the account given by Stannius is decidedly more correct than that of most subsequent writers. In the note already quoted he gives it as his opinion that there is a division of the kidney into the same two parts as in the Sturgeon, viz. into a spongy vascular part and a true secreting part ; and on a subsequent page he points out the absence or poverty of the uriniferous tubules in the anterior part of the kidney in many of our native Fishes.
Prior to the discovery that the larvae of Teleosteans and Ganoids were provided with two very distinct excretory organs, viz. a pronephros or head-kidney, and a mesonephros or Wolffian body, which are usually separated from each other by a more or less considerable interval, it was a matter of no very great importance to know whether the anterior part of the socalled kidney was a true excretory organ. In the present state of our knowledge the question is, however, one of considerable interest.
In the Cyclostomata and Amphibia the pronephros is a purely larval organ, which either disappears or ceases to be functionally active in the adult state.
, Rosenberg, to whom the earliest satisfactory investigations on the development of the Teleostean pronephros are due, stated that he had traced in the Pike (Esox Indus) the larval organ into the adult part of the kidney, called by Hyrtl the pronephros ; and subsequent investigators have usually assumed that the socalled head-kidney of adult Teleosteans and Ganoids is the persisting larval pronephros.
We have already seen that Rosenberg was entirely mistaken on this point, in that the so-called head-kidney of the adult is
852 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS.
not part of the true kidney. From my own studies on young Fishes I do not believe that the oldest larvae investigated by Rosenberg were sufficiently advanced to settle the point in question ; and, moreover, as Rosenberg had no reason for doubting that the so-called head-kidney of the adult was part of the excretory organ, he does not appear to have studied the histological structure of the organ which he identified with the embryonic pronephros in his oldest larva.
The facts to which I have called attention in this paper demonstrate that in the Sturgeon the larval pronephros undoubtedly undergoes atrophy before the adult stage is reached. The same is true for Lepidosteus, and may probably be stated for Ganoids generally.
My observations on Teleostei are clearly not sufficiently extensive to prove that the larval pronephros never persists in this group. They appear to me, however, to shew that in the normal types of Teleostei the organ usually held to be the pronephros is actually nothing of the kind.
A different interpretation might no doubt be placed upon my observations on Lophius piscatorius, but the position of the kidney in this species appears to me to be far from affording a conclusive proof that it is homologous with the anterior swelling of the kidney of more normal Teleostei.
When, moreover, we consider that Lophius, and the other forms mentioned by Hyrtl as being provided with a head-kidney only, are all of them peculiarly modified and specialized types of Teleostei, it appears to me far more natural to hold that their kidney is merely the ordinary Teleostean kidney, which, like many of their other organs, has become shifted in position, than to maintain that the ordinary excretory organ present in other Teleostei has been lost, and that a larval organ has been retained, which undergoes atrophy in less specialized Teleostei.
As the question at present stands, it appears to me that the probabilities are in favour of there being no functionally active remains of the pronephros in adult Teleostei, and that in any case the burden of proof rests with those who maintain that such remnants are to be foun,d.
The general result of my investigations is thus to render it probable that the pronephros, though found in the larvce or em
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 853
bryos of almost all the IchtJiyopsida, except the Elasmobranchii, is always a purely larval organ, which never constitutes an active part of the excretory system in the adult state.
This conclusion appears to me to add probability to the view of Gegenbaur that the pronephros is the primitive excretory gland of the Chordata ; and that the mesonephros or Wolffian body, by which it is replaced in existing Ichthyopsida, is phylogenetically a more recent organ.
In the preceding pages I have had frequent occasion to allude to the lymphatic tissue which has been usually mistaken for part of the excretory organ. This tissue is formed of trabecular work, like that of lymphatic glands, in the meshes of which an immense number of cells are placed, which may fairly be compared with the similarly placed cells of lymphatic glands. In the Sturgeon a considerable number of cells are found with peculiar granular nuclei, which are not found in the Teleostei. In both groups, but especially in the Teleostei, the tissue is highly vascular, and is penetrated throughout by a regular plexus of very large capillaries, which appear to have distinct walls, and which pour their blood into the posterior cardinal vein as it passes through the organ. The relation of this tissue to the lymphatic system I have not made out.
The function of the tissue is far from clear. Its great abundance, highly vascular character, and presence before the atrophy of the pronephros, appear to me to shew that it cannot be merely the non-absorbed remnant of the latter organ. From its size and vascularity it probably has an important function ; and from its structure this must either be the formation of lymph corpuscles or of blood corpuscles.
In structure it most resembles a lymphatic gland, though, till it has been shewn to have some relation to the lymphatic system, this can go for very little.
On the whole, I am provisionally inclined to regard it as a form of lymphatic gland, these bodies being not otherwise represented in fishes.
XXIV. A RENEWED STUDY OF THE GERMINAL LAYERS OF THE CHICK. BY F. M. BALFOUR AND F. DEIGHTON'.
(With Plates 43, 44, 45-)
THE formation of the germinal layers in the chick has been so often and so fully dealt with in recent years, that we consider some explanation to be required of the reasons which have induced us to add to the long list of memoirs on this subject. Our reasons are twofold. In the first place the principal results we have to record have already been briefly put forward in a Treatise on Comparative Embryology by one of us ; and it seemed desirable that the data on which the conclusions there stated rest should be recorded with greater detail than was possible in such a treatise. In the second place, our observations differ from those of most other investigators, in that they were primarily made with the object of testing a theory as to the nature of the primitive streak. As such they form a contribution to comparative embryology ; since our object has been to investigate how far the phenomena of the formation of the germinal layers in the chick admit of being compared with those of lower and less modified vertebrate types.
We do not propose to weary the reader by giving a new version of the often told history of the views of various writers on the germinal layers in the chick, but our references to other investigators will be in the main confined to a comparison of our results with those of two embryologists, who have published their memoirs since our observations were made. One of them is L. Gerlach, who published a short memoir 2 in April last, and
1 From the Quarterly Journal of Microscopical Science, Vol. xxn. N. S. 1882..
2 " Ueb. d. entodennale Entstehungsweise d. Chorda dorsal is," Biol. Ccntralblatt, Vol. I. Nos. i and i.
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 855
the other is C. Roller, who has published his memoir l still more recently. Both of them cover part of the ground of our investigations, and their results are in many, though not in all points, in harmony with our own. Both of them, moreover, lay stress on certain features in the development which have escaped our attention. We desired to work over these points again, but various circumstances have prevented our doing so, and we have accordingly thought it best to publish our observations as they stand, in spite of their incompleteness, merely indicating where the most important gaps occur.
Our observations commence at a stage a few hours after hatching, but before the appearance of the primitive streak.
The area pellucida is at this stage nearly spherical. In it there is a large oval opaque patch, which is continued to the hinder border of the area. This opaque patch has received the name of the embryonic shield a somewhat inappropriate name, since the structure in question has no very definite connection with the formation of the embryo.
Roller describes, at this stage, in addition to the so-called embryonic shield, a sickle-shaped opaque appearance at the hinder border of the area pellucida.
We have not made any fresh investigations for the purpose of testing Roller's statements on this subject.
Embryologists are in the main agreed as to the structure of the blastoderm at this stage. There is (PL 43, Ser. A, I and 2) the epiblast above, forming a continuous layer, extending over the whole of the area opaca and area pellucida. In the former its cells are arranged as a single row, and are cubical or slightly flattened. In the latter the cells are more columnar, and form, in the centre especially, more or less clearly, a double row ; many of them, however, extend through the whole thickness of the layer.
We have obtained evidence at this stage which tends to shew that at its outer border the epiblast grows not merely by the division of its own cells, but also by the addition of cells derived from the yolk below. The epiblast has been observed to extend itself over the yolk by a similar process in many invertebrate forms.
1 " Untersuch. lib. d. Blatterbildung im Hiihnerkeim," Archiv f. mikr. Atiat. Vol. xx. 1 88 1.
856 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
Below the epiblast there is placed, in the peripheral part of the area opaca, simply white yolk ; while in a ring immediately outside and concentric with the area pellucida, there is a closelypacked layer of cells, known as the germinal wall. The constituent cells of this wall are in part relatively small, of a spherical shape, with a distinct nucleus, and a granular and not very abundant protoplasm ; and in part large and spherical, filled up with highly refracting yolk particles of variable size, which usually render the nucleus (which is probably present) invisible (A, I and 2). This mass of cell rests, on its outer side, on a layer of white yolk.
The sickle-shaped structure, visible in surface veins, is stated by Koller to be due to a special thickening of the germinal wall. We have not found this to be a very distinctly marked structure in our sections.
In the region of the area pellucida there is placed below the epiblast a more or less irregular layer of cells. This layer is continuous, peripherally, with the germinal wall ; and is composed of cells, which are distinguished both by their flattened or oval shape and more granular protoplasm from the epiblastcells above, to which, moreover, they are by no means closely attached. Amongst these cells a few larger cells are usually present, similar to those we have already described as forming an important constituent of the germinal wall.
We have figured two sections of a blastoderm of this age (Ser. A, i and 2) mainly to shew the arrangement of these cells. A large portion of them, considerably more flattened than the remainder, form a continuous membrane over the whole of the area pellucida, except usually for a small area in front, where the membrane is more or less interrupted. This layer is the hypoblast (Jiy^). The remaining cells are interposed between this layer and the epiblast. In front of the embryonic shield there are either comparatively few or none of these cells present (Ser. A, i), but in the region of the embryonic shield they are very numerous (Ser. A, 2), and are, without doubt, the main cause of the opacity of this part of the area pellucida. These cells may be regarded as not yet completely differentiated segmentation spheres.
In many blastoderms, not easily distinguishable in surface
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 857
views from those which have the characters just described, the hypoblastic sheet is often much less completely differentiated, and we have met with other blastoderms, again, in which the hypoblastic sheet was completely established, except at the hinder part of the embryonic shield ; where, in place of it and of the cells between it and the epiblast, there was only to be found a thickish layer of rounded cells, continuous behind with the germinal wall.
In the next stage, of which we have examined surface views and sections, there is already a well-formed primitive streak.
The area pellucida is still nearly spherical, the embryonic shield has either disappeared or become much less obvious, but there is present a dark linear streak, extending from the posterior border of the area pellucida towards the centre, its total length being about one third, or even less, of the diameter of the area. This streak is the primitive streak. It enlarges considerably behind, where it joins the germinal wall. By Koller and Gerlach it is described as joining the sickle-shaped structure already spoken of. We have in some instances found the posterior end of the primitive streak extending laterally in the form of two wings (PL 45, fig. L). These extensions are, no doubt, the sickle ; but the figures given by Koller appear to us somewhat diagrammatic. One or two of the figures of early primitive streaks in the sparrow, given by Kupffer and Benecke 1 , correspond more closely with what we have found, except that in these figures the primitive streak does not reach the end of the area pellucida, which it certainly usually does at this early stage in the chick.
Sections through the area pellucida (PL 43, Ser. B and c) give the following results as to the structure of its constituent parts.
The epiblast cells have undergone division to a considerable extent, and in the middle part, especially, are decidedly more columnar than at an earlier stage, and distinctly divided into two rows, the nuclei of which form two more or less distinct layers.
In the region in front of the primitive streak the cells of the. lower part of the blastoderm have arranged themselves as- a 1 " Photogramme d. Ontogcnie d. Vogel." Nova Acta. K. Leop. Carol, Dattschen Akad. d. Naturfor, Bd. x. 41, 1879.
B. 55
858 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
definite layer, the cells of which are not so flat as is the case with the hypoblast cells of the posterior part of the blastoderm, and in the older specimens of this stage they are very decidedly more columnar than in the younger specimens.
The primitive streak is however the most interesting structure in the area pellucida at this stage.
The feature which most obviously strikes the observer in transverse sections through it is the fact, proved by Kolliker, that it is mainly due to a proliferation of the epiblast cells along an axial streak, which, roughly speaking, corresponds with the dark line visible in surface views. In the youngest specimens and at the front end of the primitive streak, the proliferated cells do not extend laterally beyond the region of their origin, but in the older specimens they have a considerable lateral extension.
The hypoblast can, in most instances, be traced as a distinct layer underneath the primitive streak, although it is usually less easy to follow it in that region than elsewhere, and in some cases it can hardly be distinctly separated from the superjacent cells.
The cells, undoubtedly formed by a proliferation of the epiblast, form a compact mass extending downwards towards the hypoblast ; but between this mass and the hypoblast there are almost always present along the whole length of the primitive streak a number of cells, more or less loosely arranged, and decidedly more granular than the proliferated cells. Amongst these loosely arranged cells there are to be found a certain number of large spherical cells rilled with yolk granules. Sometimes these cells are entirely confined to the region of the primitive streak, at other times they are continuous laterally with cells irregularly scattered between the hypoblast and epiblast (Ser.C,2), which are clearly the remnants of the undifferentiated cells of the embryonic shield. The junction between these cells and the cells of the primitive streak derived from the epiblast is often obscure, the two sets of cells becoming partially intermingled. The facility with which the cells we have just spoken of can be recognized varies moreover greatly in different instances. In some cases they are very obvious (Ser. C), while in other cases they can only be distinguished by a careful examination of good sections.
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 859
The cells of the primitive streak between the epiblast and the hypoblast are without doubt mesoblastic, and constitute the first portion of the mesoblast which is established. The section of these cells attached to the epiblast, in our opinion, clearly originates from the epiblast ; while the looser cells adjoining the hypoblast must, it appears to us, be admitted to have their origin in the indifferent cells of the embryonic shield, placed between the epiblast and the hypoblast, and also very probably in a distinct proliferation from the hypoblast below the primitive streak.
Posteriorly the breadth of the streak of epiblast which buds off the cells of the primitive streak widens considerably, and in the case of the blastoderm with the earliest primitive streaks extends into the region of the area opaca. The widening of the primitive streak behind is shewn in Ser. B, 3 ; Sen c, 2 ; and Ser. E, 4. Where very marked it gives rise to the sickle-shaped appearance upon which so much stress has been laid by. Roller and Gerlach. In the case of one of the youngest of our blastoderms of this stage in which we found in surface views (PI. 45, fig. L) a very well-marked sickle-shaped appearance at the hind end of the primitive streak, the appearance was caused, as is clearly brought out by our sections, by a thickening of the hypoblast of the germinal wall.
There is a short gap in our observations between the stage with a young primitive streak and the first described stage in which no such structure is present. This gap has been filled up both by Gerlach and Koller.
Gerlach states that during this period a small portion of the epiblast, within the region of the area opaca, but close to the posterior border of the area pellucida, becomes thickened by a proliferation of its cells. This portion gradually grows outwards laterally, forming in this way a sickle-shaped structure. From the middle of this sickle a process next grows forward into the area pellucida. This process is the primitive streak, and it is formed, like the sickle, of proliferating epiblast cells.
Koller 1 described the sickle and the growth forwards from it of the primitive streak in surface views somewhat before Gerlach;
1 " Beitr. z. Kenntniss d. Hiihnerkeims im Beginne cl. Bebriitung," Site. d. k. Akad. IViss. iv. Abth. 1879.
552
860 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
and in his later memoir has entered with considerable detail into the part played by the various layers in the formation of this structure.
He believes, as already mentioned, that the sickle-shaped structure, which appears according to him at an earlier stage than is admitted by Gerlach, is in the first instance due to a thickening of the hypoblast. At a later stage he finds that the epiblast in the centre of the sickle becomes thickened, and that a groove makes its appearance in this thickening which he calls the "Sichel-rinne." This groove is identical with that first described by Kupffer and Benecke 1 in the sparrow and fowl. We have never, however, found very clear indications of it in our sections.
In the next stage, Roller states that, in the region immediately in front of the "Sichel-rinne," a prominence appears which he calls the Sichelknopf, and from this a process grows forwards which constitutes the primitive streak. This structure is in main derived from a proliferation of epiblast cells, but Koller admits that some of the cells just above the hypoblast in the region of the Sichelknopf are probably derived from the hypoblast. Since these cells form part of the mesoblast it is obvious that Roller's views on the origin of the mesoblast of the primitive streak closely approach those which we have put forward.
The primitive streak starting, as we have seen, at the hinder border of the area pellucida, soon elongates till it eventually occupies at least two-thirds of the length of the area. As Roller (loc. cit.} has stated, this can only be supposed to happen in one of two ways, viz. either by a progression forward of the region of epiblast budding off mesoblast, or by an interstitial growth of the area of budding epiblast. Roller adopts the second of these alternatives, but we cannot follow him in doing so. The simplest method of testing the point is by measuring the distance between the front end of the primitive streak and the front border of the area pellucida at different stages of growth of the primitive streak. If this distance diminishes with the elongation of the primitive streak then clearly the second of the two alternatives is out of the question.
1 Die erstc Entwick. an Eier d. Reptilien, Konigsberg, 1878.
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 86l
We have made measurements to test this point, and find that the diminution of the space between the front end of the primitive streak and the anterior border of the area pellucida is very marked up to the period in which the medullary plate first becomes established. We can further point in support of our view to the fact that the extent of the growth lateralwards of the mesoblast from the sides of the primitive streak is always less in front than behind; which would seem to indicate that the front part of the streak is the part formed latest. Our view as to the elongation of the primitive streak appears to be that adopted by Gerlach.
Our next stage includes roughly the period commencing slightly before the first formation of a groove along the primitive streak, known as the primitive groove, and terminating immediately before the first trace of the notochord makes its appearance. After the close of the last stage the primitive streak gradually elongates, till it occupies fully two-thirds of the diameter of the area pellucida. The latter structure also soon changes its form from a circular to an oval, and finally becomes pyriform with the narrow end behind, while the primitive streak occupying two-thirds of its long axis becomes in most instances marked by a light linear band along the centre, which constitutes the primitive groove.
In surface views the primitive streak often appears to stop short of the hinder border of the area pellucida.
During the period in which the external changes, which we have thus briefly described, take place in the area pellucida, great modifications are effected in the characters of the germinal layers. The most important of these concern the region in front of the primitive streak; but they will be better understood if we commence our description with the changes in the primitive streak itself.
In the older embryos belonging to our last stage we pointed out that the mesoblast of the primitive streak was commencing to extend outwards from the median line in the form of two lateral sheets. This growth of the mesoblast is continued rapidly during the present stage, so that during the latter part of it any section through the primitive streak has approximately the characters of Ser. I, 5
862 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
The mesoblast is attached in the median line to the epiblast. Laterally it extends outwards to the edge of the area pellucida, and in older embryos may even form a thickening beyond the edge (fig. G). Beneath the denser part of the mesoblast, and attached to the epiblast, a portion composed of stellate cells may in the majority of instances be recognized, especially in the front part of the primitive streak. We believe these stellate cells to be in the main directly derived from the more granular cells of the previous stage. The hypoblast forms a sheet of flattened cells, which can be distinctly traced for the whole breadth of the area pellucida, though closely attached to the mesoblast above.
In sections we find that the primitive streak extends back to the border of the area pellucida, and even for some distance bayond. The attachment to the epiblast is wider behind; but the thickness of the mesoblast is not usually greater in the median line than it is laterally, and for this reason probably the posterior part of the streak fails to shew up in surface views. The thinning out of the median portion of the mesoblast of the primitive streak is shewn in a longitudinal section of a duck's blastoderm of this stage (fig. D). The same figure also shews that the hypoblastic sheet becomes somewhat thicker behind, and more independent of the parts above.
A careful study of the peripheral part of the area pellucida, in the region of the primitive streak, in older embryos of this stage, shews that the hypoblast is here thickened, and that its upjjer part, i.e. that adjoining the mesoblast, is often formed of stellate cells, many of which give the impression of being in the act of passing into the mesoblast above. At a later stage the mesoblast of the vascular area undoubtedly receives accessions of cells from the yolk below; so that we see no grounds for mistrusting the appearances just spoken of, or for doubting that they are to be interpreted in the sense suggested.
We have already stated that during the greater part of the present stage a groove, known as the primitive groove, is to be found along the dorsal median line of the primitive streak.
The extent to which this groove is developed appears to be subject to very great variation. On the average it is, perhaps, slightly deeper than it is represented in Ser. I, 5. In some cases
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 863
it is very much deeper. One of the latter is represented in fig. G. It has here the appearance of a narrow slit, and sections of it give the impression of the mesoblast originating from the lips of a fold; in fact, the whole structure appears like a linear blastopore, from the sides of which the mesoblast is growing out; and this as we conceive actually to be the true interpretation of the structure. Other cases occur in which the primitive groove is wholly deficient, or at the utmost represented by a shallow depression along the median axial line of a short posterior part of the primitive streak.
We may now pass to the consideration of the part of the area pellucida in front of the primitive streak.
We called attention to a change in the character of the hypoblast cells of this region as taking place at the end of the last stage. During the very early part of this stage the change in the character of these cells becomes very pronounced.
What we consider to be our earliest stage in this change we have only so far met with in the duck, and we have figured a longitudinal and median section to shew it (PI. 43, fig. D). The hypoblast (hy) has become a thick layer of somewhat cubical cells several rows deep. These cells, especially in front, are characterized by their numerous yolk spherules, and give the impression that part of the area pellucida has been, so to speak, reclaimed from the area opaca. Posteriorly, at the front end of the primitive streak, the thick layer of Jiypoblast, instead of being continuous with the flattened hypoblast tinder the primitive streak, falls, in the axial line, into the mesoblast of the primitive streak (PL 43, fig. D).
In a slightly later stage, of which we have specimens both of the duck and chick, but have only figured selected sections of a chick series, still further changes have been effected in the constitution of the hypoblast (PI. 44, Ser. H, I and 2).
Near the front border of the area pellucida (i) it has the general characters of the hypoblast of the duck's blastoderm just described. Slightly further back the cells of the hypoblast have become differentiated into stellate cells several rows deep, which can hardly be resolved in the axial line into hypoblast and mesoblast, though one can fancy that in places, especially laterally, they are partially differentiated into two layers. The axial
864 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
sheet of stellate cells is continuous laterally with cubical hypoblast cells.
As the primitive streak is approached an axial prolongation forwards of the rounded and closely-packed mesoblastic elements of the primitive streak is next met with ; and at the front end of the primitive streak, where this prolongation unites with the epiblast, it also becomes continuous with the stellate cells just spoken of. In fact, close to the end of the primitive streak it becomes difficult to say which mesoblast cells are directly derived from the primitive layer of hypoblast in front of the primitive streak, and which from the forward growth of the mesoblast of the primitive streak. There is, in fact, as in the earlier stage, a fusion of the layers at this point.
Sections of a slightly older chick blastoderm are represented in PI. 45, Ser.l, I, 2, 3, 4 and 5.
Nearly the whole of the hypoblast in front of the primitive streak has now undergone a differentiation into stellate cells. In the second section the products of the differentiation of this layer form a distinct mesoblast and hypoblast laterally, while in the median line they can hardly be divided into two distinct layers.
In a section slightly further back the same is true, except that we have here, in the axial line above the stellate cells, rounded elements derived from a forward prolongation of the cells of the primitive streak. In the next section figured, passing through the front end of the primitive streak, the axial cells have become continuous with the axial mesoblast of the primitive streak, while below there is an independent sheet of flattened hypoblast cells.
The general result of our observations on the part of the blastoderm in front of the primitive streak during this stage is to shew that the primitive hypoblast of this region undergoes considerable changes, including a multiplication of its cells; and that these changes result in its becoming differentiated on each side of the middle line, with more or less distinctness, into (i) a hypoblastic sheet below, formed of a single row of flattened cells, and (2) a mesoblast plate above formed of stellate cells, while in the middle line there is a strip of stellate cells in which there is no distinct differentiation into two layers.
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 865
Since the region in which these changes take place is that in which the medullary plate becomes subsequently formed, the lateral parts of the mesoblast plate are clearly the permanent lateral plates of the trunk, from which the mesoblastic somites, &c., become subsequently formed ; so that the main part of the 'mesoblast of the trunk is not directly derived from the primitive streak.
Before leaving this stage we would call attention to the presence, in one of our blastoderms of this stage, of a deep pit at the junction of the primitive streak with the region in front of it (PI. 44, Ser. F, I and 2). Such a pit is unusual, but we think it may be regarded as an exceptionally early commencement of that most variable structure in the chick, the neurenteric canal.
The next and last stage we have to deal with is that during which the first trace of the notochord and of the medullary plate make their appearance.
In surface views this stage is marked by the appearance of a faint dark line, extending forwards, from the front end of the primitive streak, to a fold, which has in the mean time made its appearance near the front end of the area pellucida, and constitutes the head fold.
PI. 45, Ser. K, represents a series of sections through a blastoderm of this stage, which have been selected to illustrate the mode of formation of the notochord.
In a section immediately behind the head fold the median part of the epiblast is thicker than the lateral parts, forming the first indication of a medullary plate (Ser. K, i). Below the median line of the epiblast is a small cord of cells, not divided into two layers, but continuous laterally, both with the hypoblast and mesoblast, which are still more distinctly separated than in the previous stage.
A section or so further back (Ser. K, 2) the axial cord, which we need scarcely say is the rudiment of the notochord, is thicker, and causes a slight projection in the epiblast above. It is, as before, continuous laterally, both with the mesoblast and with the hypoblast. The medullary plate is more distinct, and a shallow but unmistakable medullary groove has made its appearance.
866 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
As we approach the front end of the primitive streak the notochord becomes (Sen K, 3) very much more prominent, though retaining the same relation to the germinal layers as in front.
In the section immediately behind (Ser. K, 4) the convex upper surface of the notochord has become continuous with the epiblast for a very small region. The section, in fact, traverses the front end of the primitive streak.
In the next section the attachment between the epiblast and the cells below becomes considerably wider. It will be noticed that this part of the primitive streak is placed on the floor of the wide medullary groove, and there forms a prominence known as the anterior swelling of the primitive streak.
It will further be noticed that in the two sections passing through the primitive streak, the hypoblast, instead of simply becoming continuous with the axial thickening of the cells, as in front, forms a more or less imperfect layer underneath it. This layer becomes in the sections following still more definite, and forms part of the continuous layer of hypoblast present in the region of the primitive streak.
A comparison of this stage with the previous one shews very clearly that the notochord is formed out of the median plate of cells of the earlier stage, which was not divided into mesoblast and hypoblast, together with the short column of cells which grew forwards from the primitive streak;
The notochord, from its mode of origin, is necessarily contios -behind -with the axial cells of the primitive streak.
The sections immediately behind the last we have represented shew a rudiment of the neurenteric canal of the same form as that first figured by Gasser, viz. a pit perforating the epiblast with a great mass of rounded cells projecting upwards through it.
The observations just recorded practically deal with two much disputed points in the ontogeny of birds, viz. the origin of the mesoblast and the origin of the notochord.
With reference to the first of these our results are briefly as follows :
The first part of the mesoblast to be formed is that which arises in connection with the primitive streak. This part is in
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 867
the main formed by a proliferation from an axial strip of the epiblast along the line of the primitive streak, but in part also from a simultaneous differentiation of hypoblast cells also along the axial line of the primitive streak. The two parts of the mesoblast so formed become subsequently indistinguishable. The second part of the mesoblast to be formed is that which gives rise to the lateral plates of mesoblast of the head and trunk of the embryo. This part appears as two plates one on each side of the middle line which arise by direct differentiation from the hypoblast in front of the primitive streak. They are continuous behind with the lateral wings of mesoblast which grow out from the primitive streak, and on their inner side are also at first continuous with the cells which form the notochord.
In addition to the parts of mesoblast, formed as just described, the mesoblast of the vascular area is in a large measure developed by a direct formation of cells round the nuclei of the germinal wall.
The mesoblast formed in connection with the primitive streak gives rise in part to the mesoblast of the allantois, and ventral part of the tail of the embryo (?), and in part to the vascular structures found in the area pellucida.
With reference to the formation of the mesoblast of the primitive streak, our conclusions are practically in harmony with those of Koller ; except that Koller is inclined to minimise the share taken by the hypoblast in the formation of the mesoblast of the primitive streak.
Gerlach, with reference to the formation of this part of the mesoblast, adopts the now generally accepted view of Kolliker, according to which the whole of the mesoblast of the primitive streak is derived from the epiblast.
As to the derivation of the lateral plates of mesoblast of the trunk from the hypoblast of the anterior part of the primitive streak, our general result is in complete harmony with Gerlach's results, although in our accounts of the details of the process we differ in some not unimportant particulars.
As to the origin of the notochord, our main result is that this structure is formed as an actual thickening of the primitive hypoblast of the anterior part of the area pellucida. We find
868 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
that it unites posteriorly with a forward growth of the axial tissue of the primitive streak, while it is laterally continuous, at first, both with the mesoblast of the lateral plates and with the hypoblast. At a later period its connection with the mesoblast is severed, while the hypoblast becomes differentiated as a continuous layer below it.
As to the hypoblastic origin of the notochord, we are again in complete accord with Gerlach ; but we differ from him in admitting that the notochord is continuous posteriorly with the axial tissue of the primitive streak, and also at first continuous with the lateral plates of mesoblast
The account we have given of the formation of the mesoblast may appear to the reader somewhat fantastic, and on that account not very credible. We believe, however, that if the view which has been elsewhere urged by one of us, that the primitive streak is the homologue of the blastopore of the lower vertebrates is accepted, the features we have described receive an adequate explanation.
The growth outwards of part of the mesoblast from the axial line of the primitive streak is a repetition of the well-known growth from the lips of the blastopore. It might have been anticipated that all the layers would fuse along the line of the primitive streak, and that the hypoblast as well as part of the mesoblast would grow out from it. There is, however, clearly a precocious formation of the hypoblast ; but the formation of the mesoblast of the primitive streak, partly from the epiblast and partly from the hypoblast, is satisfactorily explained by regarding the whole structure as the blastopore. The two parts of the mesoblast subsequently become indistinguishable, and their difference in origin is, on the above view, to be regarded as simply due to a difference of position, and not as having a deeper significance.
The differentiation of the lateral plates of mesoblast of the trunk directly from the hypoblast is again a fundamental feature of vertebrate embryology, occurring in all types from Amphioxus upwards, the meaning of which has been fully dealt with in the Treatise on Comparative Embryology by one of us. Lastly, the formation of the notochord from the hypoblast is the typical vertebrate mode of formation of this organ, while
EXPLANATION OF PLATES. 869
the fusion of the layers at the front end of the primitive streak is the universal fusion of the layers at the dorsal lip of the blastopore, which is so well known in the lower vertebrate types.
EXPLANATION OF PLATES 4345. N. B. The series of sections are in all cases numbered from before backwards.
LIST OF REFERENCE LETTERS.
a. p. Area pellucida. ep. Epiblast. ch. Notochord. gr. Germinal wall. hy. Hypoblast. m. Mesoblast. o. p. Area opaca. pr. g. Primitive groove. pv s. Primitive streak, yk. Yolk of germinal wall.
PLATE 43.
SERIES A, i and 2. Sections through the blastoderm before the appearance of primitive streak.
I. Section through anterior part of area pellucida in front of embryonic shield. The hypoblast here forms an imperfect layer. The figure represents about half the section, i. Section through same blastoderm, in the region of the embryonic shield. Between the epiblast and hypoblast are a number of undifferentiated cells. The figure represents considerably more than half the section.
SERIES B, i, 2 and 3. Sections through a blastoderm with a very young primitive streak.
i. Section through the anterior part of the area pellucida in front of the primitive streak. 2. Section through about the middle of the primitive streak. 3. Section through the posterior part of the primitive streak.
SERIES C, i and 2. Sections through a blastoderm with a young primitive streak. r. Section through the front end of the primitive streak. 2. Section through the primitive streak, somewhat behind i. Both figures shew very clearly the difference in character between the cells of the epiblastic mesoblast of the primitive streak, and the more granular cells of the mesoblast derived from the hypoblast.
FIG. D. Longitudinal section through the axial line of the primitive streak, and the part of the blastoderm in front of it, of an embryo duck with a well-developed primitive streak.
PLATE 44.
SERIES E, i, 2, 3 and 4. Sections through blastoderm with a primitive streak, towards the end of the first stage.
i. Section through the anterior part of the area pellucida. 2. Section a little way behind i shewing a forward growth of mesoblast from the primitive streak. 3. Section through primitive streak. 4. Section through posterior part of primitive streak, shewing the great widening of primitive streak behind.
8/0 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
SERIES F, i and 2. Sections through a blastoderm with primitive groove.
i. Section shewing a deep pit in front of primitive streak, probably an early indication of the neurenteric canal. 2. Section immediately following i.
FIG. G. Section through blastoderm with well-developed primitive streak, shewing an exceptionally deep slit-like primitive groove.
SERIES H, i and 2. Sections through a blastoderm with a fully-developed primitive streak.
i. Section through the anterior part of area pellucida, shewing the cubical granular hypoblast cells in this region. 2. Section slightly behind i, shewing the primitive hypoblast cells differentiated into stellate cells, which can hardly be resolved in the middle line into hypoblast and mesoblast.
PLATE 45.
SERIES I, i, 2, 3, 4 and 5. Sections through blastoderm somewhat older than Series H.
i. Section through area pellucida well in front of primitive streak. 2. Section through area pellucida just in front of primitive streak. 3. Section through the front end of primitive streak. 4. Section slightly behind 3. 5. Section slightly behind 4.
SERIES K, 1,2, 3, 4 and 5. Sections through a blastoderm in which the first traces of notochord and medullary groove have made their appearance. Rather more than half the section is represented in each figure, but the right half is represented in i and 3, and the left in 2 and 4.
i. Section through notochord immediately behind the head-fold. 2. Section shewing medullary groove a little behind i. 3. Section just in front of the primitive streak. 4 and 5. Sections through the front end of the primitive streak.
FlG. L. Surface view of blastoderm with a very young primitive streak.
XXV. THE ANATOMY AND DEVELOPMENT OF PERIPATUS
CAPENSIS 1 . (With Plates 4653.)
INTRODUCTION.
THE late Professor Balfour was engaged just before his death in investigating the structure and embryology of Peripatus capensis, with the view of publishing a complete monograph of the genus. He left numerous drawings intended to serve as illustrations to the monograph, together with a series of notes and descriptions of a large part of the anatomy of Peripatus capensis. Of this manuscript some portions were ready for publication, others were more or less imperfect ; while of the figures many were without references, and others were provided with only a few words of explanation.
It was obviously necessary that Professor Balfour's work embodying as it did much important discovery should be published without delay; and the task of preparing his material for the press was confided to us. We have printed all his notes and descriptions without alteration 2 . Explanations which appeared to be necessary, and additions to the text in cases in which he had prepared figures without writing descriptions, together with full descriptions of all the plates, have been added by us, and are distinguished by enclosure in square brackets 3 .
We have to thank Miss Balfour, Professor Balfour's sister, for the important service which she has rendered by preparing
1 From the Quarterly Jotirnal of Microscopical Science, April, 1883.
2 Excepting in an unimportant matter of change of nomenclature used with regard to the buccal cavity.
3 The account of the external characters, generative organs, and development, has been written by the editors.
872 ANATOMY AND DEVELOPMENT
a large part of the beautiful drawings with which the monograph is illustrated. Many of these had been executed by her under Professor Balfour's personal supervision ; and the knowledge of his work which she then acquired has been of the greatest assistance to us in preparing the MSS. and drawings for publication.
Since his death she has spared no pains in studying the structure of Peripatus, so as to enable us to bring out the first part of the monograph in as complete a state as possible. It is due to her skill' that the first really serviceable and accurate representation of the legs of any species of Peripatus available for scientific purposes are issued with the present memoir 1 .
We have purposely refrained from introducing comments on the general bearing of the new and important results set forth in this memoir, and have confined ourselves to what was strictly necessary for the presentation of Mr Balfour's discoveries in a form in which they could be fully comprehended.
Mr Balfour had at his disposal numerous specimens of Peripatus nova zealandia, collected for him by Professor Jeffrey Parker, of Christchurch, New Zealand ; also specimens from the Cape of Good Hope collected by Mr Lloyd Morgan, and brought to England by Mr Roland Trimen in 1881 ; and others given to him by Mr Wood Mason, together with all the material collected by Mr Moseley during the "Challenger" voyage.
A preliminary account of the discoveries as to the embryology of Peripatus has already been communicated to the Royal Society 2 . It is intended that the present memoir shall be followed by others, comprising a complete account of all the species of the genus Peripatus.
H. M. MOSELEY. A. SEDGWICK.
1 The drawings on PI. 47, figs. 9 and 10 on PI. 48, and the drawings of the embryos (except fig. 37), have been made by Miss Balfour since Professor Balfour's death.
5 Proc. Royal Soc. 1883.
OF PERIPATUS CAPENSTS. 873
PART I. DESCRIPTION OF THE SPECIES.
Peripatus capensis (fig. i).
[The body is elongated, and slightly flattened dorso-ventrally. The dorsal surface is arched, and darkly pigmented ; while the ventral surface is nearly flat, and of a lighter colour.
The mouth is placed at the anterior end of the body, on the ventral surface.
The anus is posterior and terminal.
The generative opening is single and median, and placed in both sexes on the ventral surface, immediately in front of the anus.
There are a pair of ringed antennae projecting from the anterior end of the head, and a pair of simple eyes, placed on the dorsal surface at the roots of the antennae.
The appendages of the body behind the antennae are disposed in twenty pairs.
1. The single pair of jaws placed within the buccal cavity in front of the true mouth opening, and consisting each of a papilla, armed at its termination with two cutting blades.
2. The oral papillae placed on each side of the mouth. At their apices the ducts of the slime glands open.
3. The seventeen pairs of ambulatory appendages, each provided with a pair of chitinous claws at its extremity.
4. The anal papillae placed on each side of the generative opening.
Colour. The following statements on this head are derived from observations of spirit specimens. The colour varies in different individuals. It always consists of a groundwork of green and bluish grey, with a greater or less admixture of brown. The chief variations in the appearance of the animal, so far as colour is concerned, depend on the shade of the green. In some it is dark, as in the specimen figured (fig. i) ; in others it is of a lighter shade.
There is present in most specimens a fairly broad light band on each side of the body, immediately dorsal to the attachment B. 56
8/4 ANATOMY AND DEVELOPMENT
of the legs. This band is more prominent in the lighter coloured vaiieties than in the dark, and is especially conspicuous in large individuals. It is due to a diminution in the green pigment, and an increase in the brown.
There is a dark line running down the middle of the dorsal surface, in the middle of which is a fine whitish line.
The ventral surface is almost entirely free from the green pigment, but possesses a certain amount of light brown. This brown pigment is more conspicuous and of a darker shade on the spinous pads of the foot.
In parts of the body where the pigment is scarce, it is seen to be confined to the papillae. This is especially evident round the mouth, where the sparse green pigment is entirely confined to the papillae.
In some specimens a number of white papillae, or perhaps light brown, are scattered over the dorsal surface ; and sometimes there is a scattering of green papillae all over the ventral surface. These two peculiarities are more especially noticeable in small specimens.
Ridges and Papilla of the Skin. The skin is thrown into a number of transverse ridges, along which the primary wartlike papillae are placed.
The papillae, which are found everywhere, are specially developed on the dorsal surface, less so on the ventral. The papillae round the lips differ from the remaining papillae of the ventral surface in containing a green pigment. Each papilla bears at its extremity a well-marked spine.
The ridges of the skin are not continued across the dorsal middle line, being interrupted by the whitish line already mentioned. Those which lie in the same transverse line as the legs are not continued on to the latter, but stop at the junction of the latter with the body. All the others pass round to the ventral surface and are continued across the middle line ; they do not, however, become continuous with the ridges of the other side, but passing between them gradually thin off and vanish.
The ridges on the legs are directed transversely to their long axes, i.e. are at right angles to the ridges of the rest of the body.
OF PERIPATUS CAPENSIS. 875
The -antennae are ringed and taper slightly till near their termination, where they present a slight enlargement in spirit specimens, which in its turn tapers to its termination.
The rings consist essentially of a number of coalesced primary papillae, and are, therefore, beset by a number of spines like those of the primary papillae (described below). They are more deeply pigmented than the rest of the antenna.
The free end of the antenna is covered by a cap of tissue like that of the rings. It is followed by four or more rings placed close together on the terminal enlargement. There appears to be about thirty rings on the antennae of all adults of this species. But they are difficult to count, and a number of small rings occur between them, which are not included in the thirty.
The antennae are prolongations of the dorso-lateral parts of the anterior end of the body.
The eyes are paired and are situated at the roots of the antennae on the dorso-lateral parts of the head. Each is placed on the side of a protuberance which is continued as the antenna, and presents the appearance of a small circular crystalline ball inserted on the skin in this region.
The rings of papillae on that part of the head from which the antennae arise lose their transverse arrangement. They are arranged concentrically to the antennal rings, and have a straight course forwards between the antennae.
The oral papillae are placed at the side of the head. They are attached ventro-Iaterally on each side of the lips. The duct of the slime gland opens through their free end. They possess two main rings of projecting tissue, which are especially pigmented on the dorsal side ; and their extremities are covered by papillae irregularly arranged.
The buccal cavity, jaws, and lips are described below.
The Ambulatory Appendages. The claw-bearing legs are usually seventeen in number ; but in two cases of small females we have observed that the anal papillae bear claws, and present all the essential features of the ambulatory appendages. In one small female specimen there were twenty pairs of clawbearing appendages, the last being like the claw-bearing anal papillae last mentioned, and the generative opening being placed between them.
56 2
8/6 ANATOMY AND DEVELOPMENT
The ambulatory appendages, with the exception of the fourth and fifth pairs in both sexes, and the last pair (seventeenth) in the male, all resemble each other fairly closely. A typical appendage (figs. 2 and 3) will first be described, and the small variations found in the appendages just mentioned will then be pointed out. Each consists of two main divisions, a larger proximal portion, the leg, and a narrow distal claw-bearing portion, the foot.
The leg has the form of a truncated cone, the broad end of which is attached to the ventro-lateral body-wall, of which it appears to be, and is, a prolongation. It is marked by a number of rings of primary papillae, placed transversely to the long axis of the leg, the dorsal of which contain a green and the ventral a brown pigment. These rings of papillae, at the attachment of the leg, gradually change their direction and merge into the body rings. At the narrow end of the cone there are three ventrally placed pads, in which the brown pigment is dark, and which are covered by a number of spines precisely resembling the spines of the primary papillae. These spinous pads are continued dorsally, each into a ring of papillae.
The papillae of the ventral row next the proximal of these spinous pads are intermediate in character between the primary papillae and the spinous pads. Each of these papillae is larger than a normal papilla, and bears several spines (fig. 2). This character of the papilla of this row is even more marked in some of the anterior legs than in the one figured ; it seems probable that the pads have been formed by the coalescence of several rows of papillae on the ventral surface of the legs. On the outer and inner sides of these pads the spines are absent, and secondary papillae only are present.
In the centre of the basal part of the ventral surface of the foot there are present a group of larger papillae, which are of a slightly paler colour than the others. They are arranged so as to form a groove, directed transversely to the long axis of the body, and separated at its internal extremity by a median papilla from a deep pit which is placed at the point of junction of the body and leg. The whole structure has the appearance, when viewed with the naked eye, of a transverse slit placed at the base of the leg. The segmental organs open by the deep pit placed
OF FERIPATUS CAPENSIS. 877
at the internal end of this structure. The exact arrangement of the papillae round the outer part of the slit does not appear to be constant.
The foot is attached to the distal end of the leg. It is slightly narrower at its attached extremity than at its free end, which bears the two claws. The integument of the foot is covered with secondary papillae, but spines and primary papillae are absent, except at the points now to be described.
On each side of the middle ventral line of the proximal end of the foot is placed an elliptical elevation of the integument covered with spines. Attached to the proximal and lateral end of this is a primary papilla. At the distal end of the ventral side of the foot on each side of the middle line is a group of inconspicuous pale elevations, bearing spines.
On the front side of the distal end of the foot, close to the socket in which the claws are placed, are two primary papillae, one dorsal and the other ventral.
On the posterior side of the foot the dorsal of these only is present. The claws are sickle-shaped, and placed on papillae on the terminal portion of the foot. The part of the foot on which they are placed is especially retractile, and is generally found more or less telescoped into the proximal part (as in the figure).
The fourth and fifth pairs of legs exactly resemble the others, except in the fact that the proximal pad is broken up into three, a small central and two larger lateral. The enlarged segmental organs of these legs open on the small central division.
The last (17) leg of the male (PL 47, fig. 4) is characterized by possessing a well-marked white papilla on the ventral surface. This papilla, which presents a slit-like opening at its apex, is placed on the second row of papillae counting from the innermost pad, and slightly posterior to the axial line of the leg.
The anal papillae, or as they should be called, generative papillae, are placed one on each side of the generative aperture. They are most marked in small and least so in large specimens. That they are rudimentary ambulatory appendages is shewn by the fact that they are sometimes provided with claws, and resemble closely the anterior appendages.]
8/8 ANATOMY AND DEVELOPMENT
PART II. ALIMENTARY CANAL.
The alimentary canal of Peripatus capensis forms, in the extended condition of the animal, a nearly straight tube, slightly longer than the body, the general characters of which are shewn in figs. 6 and 7.
For the purposes of description, it may conveniently be divided into five regions, viz. (i) the buccal cavity with the tongue, jaws, and salivary glands, (2) pharynx, (3) the oesophagus, (4) the stomach, (5) the rectum.
The Buccal Cavity. The buccal cavity has the form of a fairly deep pit, of a longitudinal oval form, placed on the ventral surface of the head, and surrounded by a tumid lip.
[The buccal cavity has been shewn by Moseley to be formed in the embryo by the fusion of a series of processes surrounding the true mouth-opening, and enclosing in their fusion the jaws.]
The 'lip is covered by a soft skin, in which are numerous organs of touch, similar to those in other parts of the skin having their projecting portions enclosed in delicate spines formed by the cuticle. The skin of the lips differs, however, from the remainder of the skin, in the absence of tubercles, and in the great reduction of the thickness of the dermis. It is raised into a series of papilliform ridges, whose general form is shewn in fig. 5 ; of these there is one unpaired and median behind, and a pair, differing somewhat in character from the remainder, in front, and there are, in addition, seven on each side.
The structures within the buccal cavity are shewn as they appear in surface views in figs. 5 and 7, but their real nature is best seen in sections, and is illustrated by PL 49, figs. II and 12, representing the oral cavity in transverse section, and by PL 49, figs. 17 and 1 8, representing it in horizontal longitudinal sections. In the median line of the buccal cavity in front is placed a thick muscular protuberance, which may perhaps conveniently be called the tongue, though attached to the dorsal instead of the ventral wall of the mouth. It has the form of an elongated
OF PERIPATUS CAPENSIS. 879
ridge, which ends rather abruptly behind, becoming continuous with the dorsal wall of the pharynx. Its projecting edge is armed by a series of small teeth, which are thickenings of the chitinous covering, prolonged from the surface of the body over the buccal cavity. Where the ridge becomes flatter behind, the row of teeth divides into two, with a shallow groove between them (PI. 48, fig. 7).
The surface of the tongue is covered by the oral epithelium, in parts of which are organs of special sense, similar to those in the skin; but its interior is wholly formed of powerful muscles. The muscles form two groups, intermingled amongst each other. There are a series of fibres inserted in the free edge of the tongue, which diverge, more or less obliquely, towards the skin at the front of the head anteriorly, and towards the pharynx behind. The latter set of fibres are directly continuous with the radial fibres of the pharynx. The muscular fibres just described are clearly adapted to give a sawing motion to the tongue, whose movements may thus, to a certain extent, be compared to those of the odontophor of a mollusc.
In addition to the above set of muscles, there are also transverse muscles, forming laminae between the fibres just described. They pass from side to side across the tongue, and their action is clearly to narrow it, and so cause it to project outwards from the buccal cavity.
On each side of the tongue are placed the jaws, which are, no doubt, a pair of appendages, modified in the characteristic arthropodan manner, to subserve mastication. Their structure has never been satisfactorily described, and is very complicated. They are essentially short papillae, moved by an elaborate and powerful system of muscles, and armed at their free extremities by a pair of cutting blades or claws. The latter structures are, in all essential points, similar to the claws borne by the feet, and, like these, are formed as thickenings of the cuticle. They have therefore essentially the characters of the claws and jaws of the Arthropoda, and are wholly dissimilar to the setae of Chsetopoda. The claws are sickle-shaped and, as shewn in PL 47, fig. 5, have their convex edge directed nearly straight forwards, and their concave or cutting edge pointed backwards. Their form differs somewhat in the different species, and, as will
88O ANATOMY AND DEVELOPMENT
be shewn in the systematic part of this memoir 1 , forms a good specific character. In Peripatus capensis (PI. 48, fig. 10) the cutting surface of the outer blade is smooth and without teeth, while that of the inner blade (fig. 9), which is the larger of the two, is provided with five or six small teeth, in addition to the main point. A more important difference between the two blades than that in the character of the cutting edge just spoken of, is to be found in their relation to the muscles which move them. The anterior parts of both blades are placed on two epithelial ridges, which are moved by muscles common to both blades (PI. 49, fig. 1 1). Posteriorly, however, the behaviour of the two blades is very different. The epithelial ridge bearing the outer blade is continued back for a short distance behind the blade, but the cuticle covering it becomes very thin, and it forms a simple epithelial ridge placed parallel to the inner blade. The cuticle covering the epithelial ridge of the inner blade is, on the contrary, prolonged behind the blade itself as a thick rod, which, penetrating backwards along a deep pocket of the buccal epithelium, behind the main part of the buccal cavity for the whole length of the pharynx, forms a very powerful lever, on which a great part of the muscles connected with the jaws find their insertion. The relations of the epithelial pocket bearing this lever are somewhat peculiar.
The part of the epithelial ridge bearing the proximal part of this lever is bounded on both its outer and inner aspect by a deep groove. The wall of the outer groove is formed by the epithelial ridge of the outer blade, and that of the inner by a special epithelial ridge at the side of the tongue. Close to the hinder border of the buccal cavity (as shewn in PL 49, fig. 12, on the right hand side), the outer walls of these two grooves meet over the lever, so as completely to enclose it in an epithelial tube, and almost immediately behind this point the epithelial tube is detached from the oral epithelium, and appears in section as a tube with a chitinous rod in its interior, lying freely in the body-cavity (shewn in PI. 49, figs. 13 16 le). This apparent tube is the section of the deep pit already spoken of. It may
1 Some material for this memoir was left by Prof. Balfour, which will be published separately.
OF PERIPATUS CAPENSIS. 88 1
be traced back even beyond the end of the pharynx, and serves along its whole length for the attachment of muscles.
The greater part of the buccal cavity is filled with the tongue and jaws just described. It opens dorsally and behind by the mouth into the pharynx, there being no sharp line of demarcation between the buccal cavity and the pharynx. Behind the opening into the pharynx there is a continuation of the buccal cavity shewn in transverse section in fig. 13, and in longitudinal and horizontal section in fig. 17, into which there opens the common junction of the two salivary glands. This diverticulum is wide at first and opens by a somewhat constricted mouth into the pharynx above (PL 49, fig. 13, also shewn in longitudinal and horizontal section in fig. 17). Behind it narrows, passing insensibly into what may most conveniently be regarded as a common duct for the two salivary glands (PL 49, fig. 17).
The Salivary Glands, These two bodies were originally described by Grube, by whom their nature was not made out, and subsequently by Moseley, who regarded them as fat bodies. They are placed in the lateral compartments of the body-cavity immediately dorsal to the ventral nerve cords, and extend for a very variable distance, sometimes not more than half the length of the body, and in other instances extending for nearly its whole length. Their average length is perhaps about twothirds that of the body. Their middle portion is thickest, and they thin off very much behind and to a slight extent in front. Immediately behind the mouth and in front of the first pair of legs, they bend inwards and downwards, and fall (fig. 7) one on each side into the hind end of the narrow section of the oral diverticulum just spoken of as the common duct for the two salivary glands. The glandular part of these organs is that extending back from the point where they bend inwards. This part (fig. 1 6) is formed of very elongated cells supported by a delicate membrana propria. The section of this part is somewhat triangular, and the cells are so long as to leave a comparatively small lumen. The nuclei of the cells are placed close to the supporting membrane, and the remainder of the cells arc filled with very closely packed secretory globules, which have a high index of refraction. It was the presence of these globules which probably led Moseley to regard the salivary glands as fat
882 ANATOMY AND DEVELOPMENT
bodies. The part of each gland which bends inwards must be regarded as the duct.
The cells lining the ducts are considerably less columnar than those of the gland proper. Their nuclei (fig. 14) are situated at the free extremities instead of at the base of the cells, and they are without secretory globules. The cells lining the ducts of the salivary glands pass, without any sharp line of demarcation, into those of the oral epithelium, which are flatter and have their nuclei placed in the middle.
The Pharynx. The Pharynx is a highly muscular tube (fig. 7) with a triangular lumen (figs. 14, 15), .which extends from the mouth to about half way between the first and second pair of legs. It is lined by a flattish epithelium bounded by a cuticle continuous with that of the mouth. On the dorsal side is a ridge projecting into the lumen of the pharynx. This ridge may be traced forwards (PI. 49, figs. II 14) into the tongue, and the two grooves at the side of this ridge, forming the two upper angles of the triangular lumen, may be followed into those at the sides of the tongue. The muscles of the pharynx are very highly developed, consisting of an intrinsic and an extrinsic set. The former consists, as is best seen in longitudinal sections, of (PI. 51, fig. 23) radial fibres, arranged in somewhat wedgeshaped laminae, between which are rings of circular fibres. The latter are thicker externally than internally, and so also appear wedge-shaped in longitudinal sections. Very characteristic of the pharynx are the two sympathetic nerves placed close to the two dorsal angles of the triangular lumen (fig. 14, sy).
The pharynx of Peripatus is interesting in that it is unlike, so far as I know, the pharynx of any true Arthropod, in all of which the region corresponding with the pharynx of Peripatus is provided with relatively very thin walls.
The pharynx of Peripatus has, on the other hand, a very close and obvious resemblance to that of many of the Chaetopoda, a resemblance which is greatly increased by the characteristic course of the sympathetic nerves.
The form of the lumen, as already pointed out by Grube, resembles that of the Nematoda.
Ttte (Esophagus. Behind the pharynx there follows a narrow oesophagus (fig. 7, o e] shewn in section in fig. 16. It has some
OF PERIPATUS CAPENSIS. 883
what folded and fairly thick walls, and lies freely in the central division of the body-cavity without any mesenteric support. Its walls are formed of five layers, viz. from without inwards.
(1) A peritoneal investment.
(2) A layer of longitudinal fibres.
(3) A layer of circular fibres, amongst which are numerous nuclei.
(4) A connective-tissue layer supporting (5) a layer of fairly columnar hyaline epithelium, bounded on its inner aspect by a cuticle continued from that of the pharynx. In front it passes insensibly into the pharynx, and beyond the region where the dorsal walls of the pharynx have clearly commenced, the ventral walls still retain the characters of the cesophageal walls. The oesophagus is vertically oval in front, but more nearly circular behind. Characteristic of the cesophagus is the junction of the two sympathetic nerves on its dorsal wall (fig. 16). These nerves cannot be traced far beyond their point of junction.
The Stomach. The next section of -the alimentary tract is the stomach or rnesenteron (fig. 6). It is by far the largest part of the alimentary tract, commencing at about the second pair of legs and extending nearly to the hind end of the body. It tapers both in front and behind, and is narrowest in the middle, and is marked off sharply both from the cesophagus in front and the rectum behind, and is distinguished from both of these by its somewhat pinker hue. In the retracted condition of the animal it is, as pointed out by Moseley, folded in a single short dorsal loop, at about the junction of its first with its second third, and also, according to my observations, at its junction with the rectum ; but in the extended condition it is nearly straight, though usually the posterior fold at the junction of the rectum is not completely removed. Its walls are always marked by plications which, as both Moseley and Grube have stated, do not in any way correspond with the segmentation of the body. In its interior I have frequently found the chitinous remains of the skins of insects, so that we are not justified in considering that the diet is purely vegetable. It lies free, and is, like the remainder of the alimentary tract, without a mesentery. The structure of the walls of the stomach has not hitherto been very satisfactorily described.
884 ANATOMY AND DEVELOPMENT
The connective tissue and muscular coats are extremely thin. There is present everywhere a peritoneal covering, and in front a fairly well-marked though very thin layer of muscles formed of an external circular and an internal longitudinal layer. In the middle and posterior parts, however, I was unable to recognize these two layers in section ; although in surface view Grube found an inner layer of circular fibres and an outer layer formed of bands of longitudinal fibres, which he regards as muscular.
The layer supporting the epithelium is reduced to a basement membrane. The epithelial part of the wall of the stomach is by far the thickest (fig. 20), and is mainly composed of enormously elongated, fibre-like cells, which in the middle part of the stomach, where they are longest, are nearly half a millimetre in length, and only about '006 mm. in breadth. Their nuclei, as seen in fig. 20, are very elongated, and are placed about a quarter of the length from the base.
The cells are mainly filled with an immense number of highly refracting spherules, probably secretory globules, but held by Grube, from the fact of their dissolving in ether, to be fat. The epithelial cells are raised into numerous blunt processes projecting into the lumen of the stomach.
In addition to the cells just described there are present in the anterior part of the stomach a fair sprinkling of mucous cells. There are also everywhere present around the bases of the columnar cells short cells with spherical nuclei, which are somewhat irregularly scattered in the middle and posterior parts of the stomach, but form in the front part a definite layer. I have not been able to isolate these cells, and can give no account of their function.
The rectum extends from the end of the stomach to the anus. The region of junction between the stomach and the rectum is somewhat folded. The usual arrangement of the parts is shewn in fig. 6, where the hind end of the stomach is seen to be bent upon itself in a U-shaped fashion, and the rectum extending forwards under this bent portion and joining the front end of the dorsal limb of the U. The structure of the walls of the rectum is entirely different to that of the stomach, and the transition between the two is perfectly sudden.
OF PERIPATUS CAPENSIS. 885
Within the peritoneal investment comes a well-developed muscular layer with a somewhat unusual arrangement of its layers, there being an external circular layer and an internal layer formed of isolated longitudinal bands. The epithelium is fairly columnar, formed of granular cells with large nuclei, and is lined by a prolongation of the external cuticle. It is raised into numerous longitudinal folds, which are visible from the surface, and give a very characteristic appearance to this part of the alimentary tract. The muscular layers do not penetrate into the epithelial folds, which are supported by a connective tissue layer.
NERVOUS SYSTEM.
The central nervous system consists of a pair of supra-cesophageal ganglia united in the middle line, and of a pair of widely divaricated ventral cords, continuous in front with the supra-cesophageal ganglia.
It will be convenient in the first instance to deal with the general anatomy of the nervous system and then with the histology.
Ventral Cords. The ventral cords at first sight appear to be without ganglionic thickenings, but on more careful examination they are found to be enlarged at each pair of legs (PI. 48, fig. 8). These enlargements may be regarded as imperfect ganglia. There are, therefore, seventeen such pairs of ganglia corresponding to the seventeen pairs of legs. There is in addition a ganglionic enlargement at the commencement of the cesophageal commissures, where the nerves to the oral papillae are given off (PL 51, fig. 22 or. g.\ and the region of junction between the cesophageal commissures with the supra-cesophageal ganglia, where another pair of nerves are given off to the jaws (PI. 51, fig. 22/0), may be regarded as the anterior ganglion of the ventral cords. There are, therefore, according to the above reckoning, nineteen pairs of ganglia connected with the ventral cords.
The ventral cords are placed each in the lateral compartments of the body-cavity, immediately within the longitudinal layer of muscles.
886 ANATOMY AND DEVELOPMENT
They are connected with each other, rather like the pedal nerves of Chiton and the lower Prosobranchiata, by a number of commissures. These commissures exhibit a fairly regular arrangement from the region included between the first and the last pair of true feet. There are nine or ten of them between each pair of feet (PI. 52, fig. 26). They pass along the ventral wall of the body, perforating the ventral mass of longitudinal muscles. On their way they give off nerves which innervate the skin.
In Peripatus nova zealandicz, and probably also in P. capensis, two of these nerves, coming off from each pair of ganglia, are distinguished from the remainder by the fact that they are provided with numerous nerve-cells, instead of being composed of nerve-fibres only, like the remaining commissures (PL 52, fig. 26 g co). In correlation with the nerves given off from them to the skin the commissures are smaller in the middle than at the two ends.
Posteriorly the two nerve-cords nearly meet immediately in front of the generative aperture, and between this aperture and the last pair of feet there are about six commissures passing between them (PL 48, fig. 8). Behind the generative aperture the two cords bend upwards, and, as is shewn in fig. 8, fall into each other dorsally to the rectum. The section of the two cords placed dorsally to the rectum is solely formed of nerve-fibres; the nerve-cells, present elsewhere, being here absent.
In front of the ganglion of the first foot the commissures have a more dorsal situation than in the remainder of the body. The median longitudinal ventral muscle here gradually thins out and comes to an end, while the commissures pass immediately below the wall of the pharynx (PL 49, figs. 14, 15). The ventral cords themselves at first approach very close to each other in this region, separating again, however, to envelope between them the pharynx (PL 51, fig. 22).
There are eleven commissures in front of the first pair of legs (PL 51, fig. 22). The three foremost of these are very close together, the middle one arising in a more ventral position than the other two, and joining in the median ventral line a peculiar mass of cells placed in contact with the oral epithelium (fig. 14). It is probably an organ of special sense.
OF PERIPATUS CAPENSIS. 887
The ventral cords give off a series of nerves from their outer borders, which present throughout the trunk a fairly regular arrangement. From each ganglion two large nerves (figs. 8, 22, 26) are given off, which, diverging somewhat from each other, pass into the feet, and, giving off branches on their way, may be traced for a considerable distance within the feet along their anterior and posterior borders.
In front of each of the pair of pedal nerves a fairly large nerve may be seen passing outwards towards the side of the body (fig. 22). In addition to this nerve there are a number of smaller nerves passing off from the main trunk, which do not appear to be quite constant in number, but which are usually about seven or eight. Similar nerves to those behind are given off from the region in front of the first pair of legs, while at the point where the two ventral cords pass into the oesophageal commissures two large nerves (fig. 22), similar to the pairs of pedal nerves, take their origin. These nerves may be traced forwards into the oral papillae, and are therefore to be regarded as the nerves of these appendages. On the ventral side of the cords, where they approach most closely, between the oral papillae and the first pair of legs, a number of small nerves are given off to the skin, whose distribution appears to be to the same region of the skin as that of the branches from the commissures behind the first pair of legs.
From the cesophageal commissures, close to their junction with the supra-cesophageal ganglia, a nerve arises on each side which passes to the jaws, and a little in front of this, apparently from the supra-cesophageal ganglion itself, a second nerve to the jaws also takes its origin (PI. 51, fig. 22 j n}.. These two nerves I take to be homologous with a pair of pedal nerves.
Between the nerves to the jaws and those to the oral papillae a number of small nerves take their origin. Three of these on each side pass in a dorsal direction and one or two in a ventral one.
The Supra-cesophageal Ganglia. The supra-cesophageal ganglia (figs. 8 and 22) are large, somewhat oval masses, broader in front than behind, completely fused in the middle, but free at their extremities. Each of them is prolonged anteriorly into an antennary nerve, and is continuous behind with one of the
888 ANATOMY AND DEVELOPMENT
cesophageal commissures. On the ventral surface of each, rather behind the level of the eye, is placed a very peculiar protuberance (fig. 22 d], of which I shall say more in dealing with the histology of the nervous system.
A number of nerves arise from the supra-cesophageal ganglia, mainly from their dorsal surface.
In front are the immense antennary nerves extending along the whole length of each antenna, and giving off numerous lateral twigs to the sense organs. Near the origin of the antennary nerves, and rather on the dorsal surface, there spring a few small twigs, which pass to the skin, and are presumably sensory. The largest of them is shewn in PI. 50, fig. 19 A. About one-third of the way back the two large optic nerves take their origin, also arising laterally, but rather from the dorsal surface (PL 50, fig. 19 D and E). Each of them joins a large ganglionic mass placed immediately behind the retina. Nearly on a level with the optic nerves and slightly nearer the middle dorsal line a pair of small nerves (fig. 19 D) spring from the brain and pass upwards, while nearly in the same line with the optic nerves and a little behind them a larger pair of nerves take their origin.
Behind all these nerves there arises from the line of suture between the two supra-cesophageal ganglia a large median nerve which appears to supply the integument of the dorsal part of the head (PL 48, fig. 8 ; PL 49, figs. 11 14 d it).
Sympathetic System. In addition to the nerves just described there are two very important nerves which arise near the median ventral line, close to the hind end of the supracesophageal ganglia. The origin of these two nerves is shewn in the surface view (fig. 22 sy, and in section in fig. n). They at first tend somewhat forwards and pass into the muscles near the epithelium lining the groove on each side of the tongue. Here they suddenly bend backwards again and follow the grooves into the pharynx.
The two grooves are continuous with the two dorsal angles of the pharynx ; and embedded in the muscles of the pharynx, in juxtaposition with the epithelium, these two nerves may easily be traced in sections. They pass backwards the whole length of the pharynx till the latter joins the oesophagus.
OF PERIPATUS CAPENSIS. 889
Here they at once approach and shortly meet in the median dorsal line (fig. 16). They can only be traced for a very short distance beyond their meeting point. These nerves are, without doubt, the homologues of the sympathetic system of Chaetopods, occupying as they do the exact position which Semper has shewn to be characteristic of the sympathetic nerves in that group, and arising from an almost identical part of the brain 1 .
Histology of the Nervous System.
Ventral Cords. The histology of the ventral cords and cesophageal commissures is very simple and uniform. They consist of a cord almost wholly formed of nerve-fibres, placed dorsally, and a ventral layer of ganglion cells (figs. 16 and 20).
The fibrous portion of the cord has the usual structure, being formed mainly of longitudinal fibres, each probably being a bundle of fibres of various sizes, enveloped in a sponge-work of connective tissue. The larger bundles of fibres are placed near the inner borders of the cords. In this part of the cord there are placed a very small number of ganglion cells.
The layer of ganglion cells is somewhat crescent-shaped in section, and, as shewn in figs. 16 and 20, envelopes the whole ventral aspect of the fibrous parts of the cord, and even creeps up slightly on to the dorsal side. It is thicker on the inner than on the outer side, and increases considerably in bulk at each ganglionic enlargement. The cells of which it is composed are for the most part of a nearly uniform size, but at the border of the fibrous matter a fair sprinkling of larger cells is found.
The tracheal vessels supplying the nervous system are placed amongst the larger cells, at the boundary between the ganglionic and fibrous regions of the cords.
With reference to the peripheral nerve-stems there is not much to be said. They have for the most part a similar structure to the fibrous parts of the main cord, but are provided with a somewhat larger number of cells.
1 Vide Spengel, " Oligognathus Boncllioc." Naples Mittheilungen, Bel. III. pi. iv. fig- 52 B. 57
890 ANATOMY AND DEVELOPMENT
Sheath of tlie Ventral Cords. The ventral cords are enveloped by a double sheath, the two layers of which are often in contact, while in other cases they may be somewhat widely separated from each other. The inner layer is extremely thin and always very closely envelopes the nerve-cords. The outer layer is thick and fibrous, and contains a fair sprinkling of nuclei.
Supra-cesophageal Ganglia. In the present state of our knowledge a very detailed description of the histology of the supracesophageal ganglia would be quite superfluous, and I shall confine myself to a description of the more obvious features in the arrangement of the ganglionic and fibrous portions (PI. 50, fig. 19 A G).
The ganglion cells are in the first place confined, for the most part, to the surface. Along the under side of each ganglion there is a very thick layer of cells, continuous behind, with the layer of ganglion cells which is placed on the under surface of the cesophageal commissures. These cells have, moreover, an arrangement very similar to that in the ventral cords, so that a section through the supra-cesophageal ganglia has an obvious resemblance to what would be the appearance of a section through the united ventral cords. On the outer borders of the ganglia the cells extend upwards, but they end on about the level of the optic nerve (fig. 19 D). Immediately dorsal to this point the fibrous matter of the brain is exposed freely on the surface (fig. 19 A, B, &c., a}. I shall call the region of fibrous matter so exposed the dorso-lateral horn of white matter.
Where the two ganglia separate in front the ganglion cells spread up the inner side, and arch over so as to cover part of the dorsal side. Thus, in the anterior part, where the two ganglia are separate, there is a complete covering of ganglionic substance, except for a narrow strip, where the dorso-lateral lobe of white matter is exposed on the surface (fig. 19 A). From the point where the two ganglia meet in front the nerve-cells extend backwards as a median strip on the dorsal surface (fig. 19 D and E). This strip, becoming gradually smaller behind, reaches nearly, though not quite, the posterior limit of the junction of the ganglia. Behind it there is, however, a region where
OF PERIPATUS CAPENSIS. 891
the whole dorsal surface of the ganglia is without any covering of nerve-cells.
This tongue of ganglion cells sends in, slightly behind the level of the eyes, a transverse vertical prolongation inwards into the white matter of the brain, which is shewn in the series of transverse sections in fig. 19 E, and also in the vertical longitudinal section (PL 51, fig. 21), and in horizontal section in PL 51, fig. 22.
On the ventral aspect of each lobe of the brain there is present a very peculiar, bluntly conical protuberance of ganglion cells (PL 51, fig. 22), which was first detected by Grube (No. 10), and described by him as "a white thick body of a regular tetrahedral form, and exhibiting an oval dark spot in the middle of two of the faces." He further states that it is united by a delicate nerve to the supra-cesophageal ganglion, and regards it as an organ of hearing.
In Peripatus capensis the organ in question can hardly be described as tetrahedral. It is rather, of a flattened oval form, and consists, as shewn in sections (PL 50, fig. 19 C and D, d\ mainly of ganglion cells. In its interior is a cavity with a distinct bounding membrane : the cells of which it is composed vary somewhat in size, being smallest near the point of attachment. At its free end is placed a highly refractive, somewhat oval body, probably forming what Grube describes as a dark spot, half embedded in its substance, and kept in place by the sheath of nervous matter surrounding it. This body appears to have fallen out in my sections. The whole structure is attached to the under surface of the brain by a very short stalk formed of a bundle of cells and nervous fibres.
It is difficult to offer any interpretation of the nature of this body. It is removed considerably from the surface of the animal, and is not, therefore, so far as I can see, adapted to serve as an organ of hearing.
The distribution of the white or fibrous matter of the ganglia is not very easy to describe.
There is a central lobe of white matter (fig. 19 E), which is continuous from ganglion to ganglion, where the two are united. It is smaller behind than in front. On its ventral side it exhibits fairly well-marked transverse commissural fibres, con 572
892 ANATOMY AND DEVELOPMENT
necting the two halves of the ganglion. Laterally and somewhat ventrally it is prolonged into a horn (fig. 19 D, E, b], which I propose calling the ventro-lateral horn. In front it is placed in a distinct protuberance of the brain, which is placed ventrally to and nearly in the same vertical plane as the optic nerve. This protuberance is best shewn in the view of the brain from below given in PL 51, fig. 22. This part of the horn is characterized by the presence of large vertically-directed bundles of nerve-fibres, shewn in transverse section in fig. 190. Posteriorly the diameter of this horn is larger than in front (fig. 19, E, F, G), but does not give rise to a protuberance on the surface of the brain owing to the smaller development of the median lobe behind.
The median lobe of the brain is also prolonged into a dorsolateral lobe (fig. 19, a], which, as already mentioned, is freely exposed on the surface. On its ventral border there springs the optic nerve, and several pairs of sensory nerves already described (fig. 19 D, E), while from its dorsal border a pair of sensory nerves also spring, nearly in the same vertical plane as the optic nerves.
Posteriorly where the dorsal surface of the brain is not covered in with ganglion cells the dorso-lateral horn and median lobe of the brain become indistinguishable.
In the front part of the brain the median lobe of white matter extends dorsalwards to the dorsal strip of ganglion cells, but behind the region of the transverse prolongation of these cells, into the white matter already described (p. 890), there is a more or less distinctly defined lobe of white matter on the dorsal surface, which I propose calling the postero-dorsal lobe of white matter. It is shewn in the transverse sections (fig. 19 F and G, c). It gradually thins away and disappears behind. It is mainly characterized by the presence on the ventral border of definite transverse commissural fibres.
OF PERIPATUS CAPENSIS. 893
THE SKIN.
The skin is formed of three layers.
1. The cuticle.
2. The epidermis or hypodermis.
3. The dermis.
The cuticle is a layer of about O'CO2 mm. in thickness. Its surface is not, however, smooth, but is everywhere, with the exception of the perioral region, raised into minute secondary papillae, the base of which varies somewhat in diameter, but is usually not far from O'O2 mm. On the ventral surface of the body these papillae are for the most part somewhat blunt, but on the dorsal surface they are more or less sharply pointed. In most instances they bear at their free extremity a somewhat prominent spine. The whole surface of each of the secondary papillae just described is in its turn covered by numerous minute spinous tubercles. In the perioral region, where the cuticle is smooth, it is obviously formed of two layers which easily separate from each other, and there is I believe a similar division elsewhere, though it is not so easy to see. It is to be presumed that the cuticle is regularly shed.
The epidermis, placed immediately within the cuticle, is composed of a single row of cells, which vary, however, a good deal in size in different regions of the body. The cells excrete the cuticle, and, as shewn in fig. 32, they stand in a very remarkable relation to the secondary papillae of the cuticle just described. Each epidermis cell is in fact placed within one of these secondary papillae, so that the cuticle of each secondary papilla is the product of a single epidermis cell. This relation is easily seen in section, while it may also be beautifully shewn by taking a part of the skin which is not too much pigmented, and, after staining it, examining from the surface.
In fig. 32 a region of the epidermis is figured, in which the cells are exceptionally columnar. The cuticle has, moreover, in the process of cutting the section, been somewhat raised and carried away from the subjacent cells. The cells of the epidermis are provided with large oval nuclei, which contain a well
894 ANATOMY AND DEVELOPMENT
developed reticulum, giving with low powers a very granular appearance to the nuclei. The protoplasm of the cells is also somewhat granular, and the granules are frequently so disposed as to produce a very well-marked appearance of striation on the inner end of the cells. The pigment which gives the characteristic colour to the skin is deposited in the protoplasm of the outer ends of the cells in the form of small granules. An attempt is made to shew this in fig. 32.
At the apex of most, if not all, the primary wart-like papillae there are present oval aggregations, or masses of epidermis cells, each such mass being enclosed in a thickish capsule (fig. 31). The cells of these masses appear to form the wall of a cavity which leads into the hollow interior of a long spine. These spines when carefully examined with high objectives present a rather peculiar structure. The base of the spine is enveloped by the normal cuticle, but the spine itself, which terminates in a very fine point, appears, as shewn in fig. 31, to be continuous with the inner layer of the cuticle. In the perioral region the outer layer of the cuticle, as well as the inner, appear to be continued to the end of the spines. Within the base of the spine there is visible a finely striated substance which may often be traced into the cavity enclosed by the cells, and appears to be continuous with the cells. Attached to the inner ends of most of the capsules of these organs a delicate fibrillated cord may be observed, and although I have not in any instance succeeded in tracing this cord into one of the nervestems, yet in the antennas, where the nerve-stems are of an enormous size, I have satisfied myself that the minute nerves leaving the main nerve-stems and passing out towards the skin are histologically not to be distinguished from these fibrillated cords. I have therefore but little hesitation in regarding these cords as nerves.
In certain regions of the body the oval aggregations of cells are extremely numerous ; more especially is this the case in the antennas, lips, and oral papillae. On the ventral surface of the peripheral rings of the thicker sections of the feet they are also very thick set (fig. 20 P). They here form a kind of pad, and have a more elongated form than in other regions. In the antennae they are thickly set side by side on the rings of skin
OF PERIPATUS CAPENSIS. 895
which give such an Arthropod appearance to these organs in Peripatus.
The arrangement of the cells in the bodies just described led me at first to look upon them as glands, but a further investigation induced me to regard them as a form of tactile organ. The arguments for this view are both of a positive and a negative kind.
The positive arguments are the following :
(1) The organs are supplied with large nerves, which is distinctly in favour of their being sense organs rather than glands.
(2) The peculiar striae at the base of the spines appear to me like the imperfectly preserved remains of sense hairs.
(3) The distribution of these organs favours the view that they are tactile organs. They are most numerous on the antennas, where such organs would naturally be present, especially in a case like that of Pe'ripatus, where the nerve passing to the antennas is simply gigantic. On the other hand, the antennae would not be a natural place to look for an enormous development of dermal glands.
The lips, oral papillae, and under surface of the legs, where these bodies are also very numerous, are situations where tactile organs would be of great use.
Under the head of negative arguments must be classed those which tell against these organs being glandular. The most important of these is the fact that they have no obvious orifice. Their cavities open no doubt into the spines, but the spines terminate in such extremely fine points that the existence of an orifice at their apex is hardly credible.
Another argument, from the distribution of these organs over the body is practically the converse of that already used. The distribution being as unfavourable to the view that they are glands, as it is favourable to that of their being sense organs.
THE TRACHEAL SYSTEM.
The apertures of the tracheal system are placed in the depressions between the papillae or ridges of the skin. Each of them leads into a tube, which I shall call the tracheal pit (fig. 30), the walls of which are formed of epithelial cells bounded
896 ANATOMY AND DEVELOPMENT
towards the lumen of the pit by a very delicate cuticular membrane continuous with the cuticle covering the surface of the body. The pits vary somewhat in depth; the pit figured was about O'CX) mm. It perforates the dermis and terminates in the subjacent muscular layer. The investigation of the inner end of the pit gave me some little trouble.
Transverse sections (fig. 30) through the trunk containing a tracheal opening shew that the walls of the pit expanded internally in a mushroom-like fashion, the narrow part being, however, often excentric in relation to the centre of the expanded part.
Although it was clear that the tracheae started from the expanded region of the walls of the pit, I could not find that the lumen of the pit dilated into a large vesicle in this part, and further investigation proved that the tracheae actually started from the slightly swollen inner extremity of the narrow part of the pit, the expanded walls of the pit forming an umbrella-like covering for the diverging bundles of tracheae.
I have, in fig. 30, attempted to make clear this relation between the expanded walls of the tracheal pits and the tracheae. In longitudinal sections of the trunk the tracheal pits do not exhibit the lateral expansion which I have just described, which proves that the divergence of the bundles of tracheae only takes place laterally and not in an antero-posterior direction. Cells similar in general character to those of the walls of the tracheal pits are placed between the branches of tracheae, and somewhat similar cells, though generally with more elongated nuclei, accompany the bundles of tracheae as far as they can be followed in my sections. The structure of these parts in the adult would, in fact, lead one to suppose that the tracheae had originated at the expense of the cells of pits of the epidermis, and that the cells accompanying the bundles of tracheae were the remains of cords of cells which sprouted out from the blind ends of the epidermis pits and gave rise in the first instance to the tracheae.
The tracheae themselves are extremely minute, unbranched (so far as I could follow them) tubes. Each opening by a separate aperture into the base of the tracheal pit, and measuring about O-QO2 mm. in diameter. They exhibit a faint transverse striation, which I take to be the indication of a spiral fibre.
OF PERIPATUS CAPENSIS. 897
[Moseley (Phil. Trans., 1874, PI. 73, fig. i) states that the tracheae branch, but only exceptionally.]
Situation of the tracheal apertures. Moseley states (No. 13) that the tracheae arise from the skin all over the surface of the body, but are especially developed in certain regions. He finds "a row of minute oval openings on the ventral surface of the body," the openings being "situate with tolerable regularity in the centres of the interspaces between the pairs of members, but additional ones occurring at irregular intervals. Other similar openings occur in depressions on the inner side of the conical foot protuberance." It is difficult in preserved specimens to make out the exact distributions of the tracheal apertures, but I have been able to make out certain points about them.
There is a double row of apertures on each side of the median dorsal line, forming two sub-dorsal rows of apertures. The apertures are considerably more numerous than the legs. There is also a double row of openings, again more numerous than the legs, on each side of the median ventral line between the insertions of the legs. Moseley speaks of a median row in this position. I think this must be a mistake.
Posteriorly the two inner rows approach very close to each other in the median ventral line, but I have never seen them in my section opening quite in the middle line. Both the dorsal and ventral rows are very irregular.
I have not found openings on the ventral or dorsal side of the feet but there are openings at the anterior and posterior aspects of the feet. There are, moreover, a considerable number of openings around the base of the feet.
The dorsal rows of tracheal apertures are continued into the head and give rise in this situation to enormous bundles of tracheae.
In front of the mouth there is a very large median ventral tracheal pit, which gives off tracheae to the ventral part of the nervous system, and still more in front a large number of such pits close together. The tracheae to the central nervous system in many instances enter the nervous system bound up in the same sheath as the nerves.
898 ANATOMY AND DEVELOPMENT
THE MUSCULAR SYSTEM.
The general muscular system consists of (i) the general wall of the body; (2) the muscles connected with the mouth, pharynx, and jaws; (3) the muscles of the feet; (4) the muscles of the alimentary tract.
The muscular wall of the body is formed of (i) an external layer of circular fibres; (2) an internal layer of longitudinal muscles; (3) a layer of transverse fibres.
The layer which I have spoken of as formed of circular fibres is formed of two strata of fibres which girth the body somewhat obliquely (PI. 51, fig. 25). In the outer stratum the rings are arranged so that their ventral parts are behind, while the ventral parts of the rings of the inner stratum are most forward. Both in the median dorsal and ventral lines the layer of circular fibres become somewhat thinner, and where the legs are attached the regularity of both strata is somewhat interfered with, and they become continuous with a set of fibres inserted in the wall of the foot.
The longitudinal muscles are arranged as five bands (vide fig. 1 6), viz. two dorsal, two lateral, and three ventral. The three ventral may be spoken of as the latero-ventral and medioventral bands.
The transverse fibres consist of (i) a continuous sheet on each side inserted dorsally in the cutis, along a line opposite the space between the dorsal bands of longitudinal fibres, and ventrally between the ventro-median and ventro-lateral bands. Each sheet at its insertion slightly breaks up into separate bands. They divide the body-cavity into three regions a median, containing the alimentary tract, slime glands, &c., and two lateral, which are less well developed, and contain the nervous system, salivary glands, segmental organs, &c.
(2) Inserted a little dorsal to the transverse band just described is a second band which immediately crosses the first, and then passes on the outer side of the nervous cord and salivary gland, where such is present, and is inserted ventrally in the space between the ventro-lateral and lateral longitudinal band.
OF PERIPATUS CAPENSIS. 899
Where the feet are given off the second transverse band becomes continuous with the main retractor muscular fibres in the foot, which are inserted both on to the dorsal side and ventral side.
Muscular system of the feet. This consists of the retractors of the feet connected with the outer transverse muscle and the circular layer of muscles. In addition to these muscles there are intrinsic transverse muscles which cross the cavity of the feet in various directions (PI. 51, fig. 20). There is no special circular layer of fibres.
Histology of the muscle, The main muscles of the body are unstriated and divided into fibres, each invested by a delicate membrane. Between the membrane and muscle are scattered nuclei, which are never found inside the muscle fibres. The muscles attached to the jaws form an exception in that they are distinctly transversely striated.
THE BODY-CAVITY AND VASCULAR SYSTEM.
The body-cavity, as already indicated, is formed of three compartments one central and two lateral. The former is by far the largest, and contains the alimentary tract, the generative organs, and the mucous glands. It is lined by a delicate endothelial layer, and is not divided into compartments nor traversed by muscular fibres.
The lateral divisions are much smaller than the central, and are shut .off from it by the inner transverse band of muscles. They are almost entirely filled with the nerve-cord and salivary gland in front and with the nerve-cord alone behind, and their lumen is broken up by muscular bands. They further contain the segmental organs which open into them. They are prolonged into the feet, as is the embryonic body-cavity of most Arthropoda.
The vascular system is usually stated to consist of a dorsal heart. I find between the dorsal bands of longitudinal fibres a vessel in a space shut off from the body-cavity by a continuation of the endothelial. lining of the latter (fig. 16). The vessel has definite walls and an endothelial lining, but I could not make out whether the walls were muscular. The ventral
9OO ANATOMY AND DEVELOPMENT
part of it is surrounded by a peculiar cellular tissue, probably, as suggested by Moseley, equivalent to the fat bodies of insects. It is continued from close to the hind end of the body to the head, and is at its maximum behind. In addition to this vessel there is present a very delicate ventral vessel, by no means easy to see, situated between the cutis and the outer layer of circular muscles.
SEGMENTAL ORGANS.
A series of glandular organs are found in Peripatus which have their external openings situated on the ventral surface of a certain number of the legs, and which, to the best of my belief, end internally by opening into the lateral compartments of the body-cavity. These organs are probably of an excretory nature, and I consider them homologous with the nephridia or segmental organs of the Chaetopoda.
In Peripatus capensis they are present in all the legs. In all of them (except the first three) the following parts may be recognized :
(1) A vesicular portion opening to the exterior by a narrow passage.
(2) A coiled portion, which is again subdivided into several sections.
(3) A terminal section ending by a somewhat enlarged opening into the lateral compartment of the body-cavity.
The last twelve pairs of these organs are all constructed in a very similar manner, while the two pairs situated in the fourth and fifth pairs of legs are considerably larger than those behind, and are in some respects very differently constituted.
It will be convenient to commence with one of the hinder nephridia. Such a nephridium from the ninth pair of legs is represented in fig. 28. The external opening is placed at the outer end of a transverse groove placed at the base of one of the feet, while the main portion of the organ lies in the body-cavity in the base of the leg, and extends into the trunk to about the level of the outer edge of the nerv.e-cord of its side. The external opening (p s) leads into a narrow tube (s d\ which gradually dilates into a large sack (s).
OF PERIPATUS CAPENS1S. QOI
The narrow part is lined by small epithelial cells, which are directly continuous with and perfectly similar to those of the epidermis (fig. 20). It is provided with a superficial coating of longitudinal muscular fibres, which thins out where it passes over the sack, along which it only extends for a short distance.
The sack itself, which forms a kind of bladder or collecting vesicle for the organ, is provided with an extremely thin wall, lined with very large flattened cells. These cells are formed of granular protoplasm, and each of them is provided with a large nucleus, which causes a considerable projection into the lumen of the sack (figs. 20, 29 s). The epithelial wall of the sack is supported by a membrana propria, over which a delicate layer of the peritoneal epithelium is reflected.
The coiled tube forming the second section of the nephridium varies in length, and by the character of the epithelium lining it may be divided into four regions. It commences with a region lined by a fairly columnar epithelium with smallish nuclei (fig. 28 s c i). The boundaries of the cells of this epithelium are usually very indistinct, and the protoplasm contains numerous minute granules, which are usually arranged in such a manner as to give to optical or real sections of the wall of this part of the tube a transversely striated appearance. These granules are very probably minute balls of excretory matter.
The nuclei of the cells are placed near their free extremities, contrary to what might have been anticipated, and the inner ends of the cells project for very different lengths into the interior, so causing the inner boundary of the epithelium of this part of the tube to have a very ragged appearance. This portion of the coiled tube is continuous at its outer end with the thin-walled vesicle. At its inner end it is continuous with region No. 2 of the coiled tube (fig. 28 s c 2), which is lined by small closely-packed columnar cells. This portion is followed by region No. 3, which has a very characteristic structure (fig. 28 s c 3). The cells lining this part are very large and flat, and contain large disc-shaped nuclei, which are usually provided with large nucleoli, and often exhibit a beautiful reticulum. They may frequently be observed in a state of division. The protoplasm of this region is provided with similar granules to that in the first region, and the boundaries of the cells are usually
902 ANATOMY AND DEVELOPMENT
very indistinct. The fourth region is very short (fig. 28 s c 4), and is formed of small columnar cells. It gradually narrows till it opens suddenly into the terminal section (s o t], which ends by opening into the body-cavity, and constitutes the most distinct portion of the whole organ. Its walls are formed of columnar cells almost filled by oval nuclei, which absorb colouring matters with very great avidity, and thus renders this part extremely conspicuous. The nuclei are arranged in several rows.
The study of the internal opening of this part gave me some trouble. No specimens ever shew it as rounded off in the characteristic fashion of tubes ending in a cul-de-sac. It is usually somewhat ragged and apparently open. In the best preserved specimens it expands into a short funnel-shaped mouth, the free edge of which is turned back. Sections confirm the results of dissections. Those passing longitudinally through the opening prove its edges are turned back, forming a kind of rudimentary funnel. This is represented in fig. 29, from the last leg of a female. I have observed remains of what I consider to be cilia in this section of the organ. The fourth region of the organ is always placed close to the thin-walled collecting vesicle (figs. 28 and 29). In the whole of the coiled tube just described the epithelium is supported by a membrana propria, which in its turn is invested by a delicate layer of peritoneal epithelium.
The fourth and fifth pairs are very considerably larger than those behind, and are in other respects peculiar. The great mass of each organ is placed behind the leg, on which the external opening is placed, immediately outside one of the lateral nerve-cords. Its position is shewn in fig. 8.
The external opening, instead of being placed near the base of the leg, is placed on the ventral side of the third ring (counting from the outer end) of the thicker portion of the leg. It leads (fig. 27) into a portion which clearly corresponds with the collecting vesicle of the hinder nephridia. This part is not, however, dilated into a vesicle in the same sort of way, and the cells which form the lining epithelium have not the same characteristic structure, but are much smaller. Close to the point where the vesicle joins the coiled section of the nephridium the
OF PERIPATUS CAPENSIS. 903
former has a peculiar nick or bend in it. At this nick it is firmly attached to the ventral side of the foot by muscles and tracheae, and when cut away from its attachment the muscles and tracheae cannot easily be detached from it. The main part of the coils are formed by region No. i, and the epithelial cells lining this part present very characteristically the striated appearance which has already been spoken of. The large-celled region of the coiled tube (fig. 2 ; ") is also of considerable dimensions, and the terminal portion is wedged in between this and the commencing part of the coiled tube. The terminal portion with its internal opening is in its histological characters exactly similar to the homologous region in the hinder nephridia.
The three pairs of nephridia in the three foremost pairs of legs are very rudimentary, consisting, so far as I have been able to make out, solely of the collecting vesicle and the duct leading from them to the exterior. The external opening is placed on the ventral side of the base of the feet, in the same situation as that of the posterior nephridia, but the histological
part of the body to which it belongs), does not acquire the
normal relations of a blastopore, but presents only those
rudimentary features (deep groove connected with origin of
mesoblast) which the whole blastopore of other tracheates
presents.
We think it probable that the larval anus eventually shifts to the hind end of the body, and gives rise to the adult anus. We reserve the account of the internal structure of these embryos (Stages A E) and of the later stages for a subsequent memoir.
We may briefly summarise the more important facts of the early development of Peripatus capensis, detailed in the preceding account.
1. The greater part of the mesoblast is developed from the walls of the archenteron.
2. The embryonic mouth and anus are derived from the respective ends of the original blastopore, the middle part of the blastopore closing up.
3. The embryonic mouth almost certainly becomes the adult mouth, i.e. the aperture leading from the buccal cavity into the pharynx, the two being in the same position. The embryonic anus is in front of the position of the adult anus, but in all probability. shifts back, and persists as the adult anus.
4. The anterior pair of mesoblastic somites gives rise to the swellings of the praeoral lobes, and to the mesoblast of the head 1 .
There is no need for us to enlarge upon the importance of these facts. Their close bearing upon some of the most important problems of morphology will be apparent to all, and we may with advantage quote here some passages from Balfour's Comparative Embryology, which shew that he himself long ago had anticipated and in a sense predicted their discovery.
"Although the mesoblastic groove of insects is not a gastrula, it is quite possible that it is the rudiment of a blastopore, the gastrula corresponding to which has now vanished
1 We have seen nothing in any of our sections which we can identify as of socalled mesenchymatous origin.
OF PERIPATUS CAPENSIS. 913
from development." (Comparative Embryology, Vol. I. p. 378, the original edition 1 .)
"TRACHEATA. Insecta. It (the mesoblast) grows inwards from the lips of the germinal groove, which probably represents the remains of a blastopore." (Comparative Embryology, Vol. II. p. 291, the original edition 2 .)
"It is, therefore, highly probable that the paired ingrowths of the mesoblast from the lips of the blastopore may have been, in the first instance, derived from a pair of archenteric diverticula." (Comparative Embryology, Vol. II. p. 294, the original edition 3 .)
The facts now recorded were discovered in June last, only a short time before Balfour started for Switzerland ; we know but little of the new ideas which they called up in his mind. We can only point to passages in his published works which seem to indicate the direction which his speculations would have taken.
After speculating as to the probability of a genetic connection between the circumoral nervous system of the Ccelenterata, and the nervous system of Echinodermata, Platyelminthes, Chaetopoda, Mollusca, &c., he goes on to say :
" A circumoral nerve-ring, if longitudinally extended, might give rise to a pair of nerve-cords united in front and behind exactly such a nervous system, in fact, as is present in many Nemertines (the Enopla and Pelagonemertes), in Peripatus and in primitive molluscan types (Chiton, Fissurella, &c.). From the lateral parts of this ring it would be easy to derive the ventral cord of the Chaetopoda and Arthropoda. It is especially deserving of notice, in connection with the nervous system of the above-mentioned Nemertines and Peripatus, that the commissure connecting the two nerve-cords behind is placed on the dorsal side of the intestines. As is at once obvious, by referring to the diagram (fig. 231 B), this is the position this commissure ought, undoubtedly, to occupy if derived from part of a nervering which originally followed more or less closely the ciliated edge of the body of the supposed radiate ancestor." (Comparative Embryology, Vol. II. pp. 311, 312, the original edition 4 .)
1 This edition, Vol. n. p. 457. 2 This edition, Vol. III. p. 352.
3 This edition, Vol. m. p. 356. 4 This edition, Vol. in. pp. 378, 379.
9 14 ANATOMY AND DEVELOPMENT OF PERIPATUS CAPENSIS.
The facts of development here recorded give a strong additional support to this latter view, and seem to render possible a considerable extension of it along the same lines.]
LIST OF MEMOIRS ON PERIPATUS.
1. M. Lansdown Guilding. "An Account of a New Genus of Mollusca," Zoological Journal, Vol. II. p. 443, 1826.
2. M. Andouin and Milne-Edwards. " Classific. des Anndlides et description de celles qui habitent les cotes de France," p. 411, Ann. Scien. Nat. ser. I. Vol. xxx. 1833.
3. M. Gervais. "Etudes p. servir a 1'histoire naturelle des Myriapodes," Ann. Scien. Nat. ser. n. Vol. vn. 1837, p. 38.
4. Wiegmann. Wiegmann's Archiv, 1837.
5. H. Milne-Edwards. "Note sur le Peripate juluforme" Ann. Scien. Nat. ser. n. Vol. xvm. 1842.
6. Blanchard. "Sur Forganisation des Vers," chap. IV. pp. 137 141, Ann. Scien. Nat. ser. in. Vol. Vlll. 1847.
7. Quatrefages. " Anat. des Hermelles, note on," p. 57, Ann. Scien. Nat. ser. in. Vol. x. 1848.
8. Quatrefages. Hist. Nat. des Anneles, 1865, Appendix, pp. 675 6.
9. De Blainville. SuppL au Diet, des Sc. Nat. Vol. I.
10. Ed. Grube. " Untersuchungen lib. d. Bau von Peripatus Edwardsii? Archiv fur Anat. und Physiol. 1853.
11. Saenger. " Moskauer Naturforscher Sammlung," Abth. Zool. 1869.
12. H. N. Moseley. "On the Structure and Development of Peripatus capensis? Proc. Roy. Soc. N.O. 153, 1874.
13. H. N. Moseley. " On the Structure and Development of Peripatus capensis," Phil. Trans. Vol. CLXIV. 1874.
14. H. N. Moseley. "Remarks on Observations by Captain Hutton, Director of the Otago Museum, on Peripatus novce zealandice," Ann. and Mag. of Nat. History, Jan. 1877.
15. Captain Hutton. " Observations on Peripatus novce sealandice," Ann. and Mag. of Nat. History, Nov. 1876.
16. F. M. Balfour. "On Certain Points in the Anatomy of Peripatus capensis" Quart. Journ. of Micr. Science, Vol. xix. 1879.
17. A. Ernst. Nature, March loth, 1881.
EXPLANATION OF PLATES. 915
EXPLANATION OF PLATES 4653!.
COMPLETE LIST OF REFERENCE LETTERS.
A. Anus. a. Dorso-lateral horn of white matter in brain, a.g. Accessory gland of male (modified accessory leg gland), at. Antenna, at. n. Antennary nerve, b. Ventro-lateral horn of white matter of brain. b. c. Body-cavity. bl. Blastopore. C. Cutis. c. Postero-dorsal lobe of white matter of brain. e.g. Supracesophageal ganglia, cl. Claw. c. m. Circular layer of muscles, co. Commissures between the ventral nerve-cords, co. i. Second commissure between the ventral nerve-cords. co 1 . 2. Mass of cells developed on second commissure, cor. Cornea, c. s. d. Common duct for the two salivary glands. . cu. Cuticle, d. Ventral protuberance of brain. d. 1. m. Dorsal longitudinal muscle of pharynx. d. n. Median dorsal nerve to integument from supraoesophageal ganglia, d. o. Muscular bands passing from the ventro-lateral wall of the pharynx at the region of its opening into the buccal cavity. E. Eye. E. Central lobe of white matter of brain, e. n. Nerves passing outwards from the ventral cords, ep. Epidermis, ep.c. Epidermis cells. F. i, F. a, &c. First and second pair of feet, c. f. Small accessory glandular tubes of the male generative apparatus. F.^. Ganglionic enlargement on ventral nerve-cord, from which a pair of nerves to foot pass off. f. gl. Accessory foot-gland. F. n. Nerves to feet. g. co. Commissures between the ventral nerve-cords containing ganglion cells, g. o. Generative orifice. H. Heart, h. Cells in lateral division of body-cavity. hy. Hypoblast, i.j. Inner jaw. j. Jaw. j. n. Nerves to jaws. L. Lips. /. Lens. /. b. c. Lateral compartment of body-cavity, le. Jaw lever (cuticular prolongation of inner jaw lying in a backwardly projecting diverticulum of the buccal cavity). /. m. Bands of longitudinal muscles. M. Buccal cavity. M 1 . Median backward diverticulum of mouth or common salivary duct which receives the salivary ducts, me. Mesenteron. mes. Mesoblastic somite, m. 1. Muscles of jaw lever, m. s. Sheets of muscle passing round the side walls of pharynx to dorsal body wall. od. Oviduct, ce. OZsophagus. a's. co. OZsophageal commissures, o.f. g. Orifice of duct of foot-gland, o.j. Outer jaw. op. Optic ganglion, op. n. Optic nerve, or.g. Ganglionic enlargements for oral papillae, o r. n. Nerves to oral papillae, or. p. Oral papillas. o. s. Orifice of duct of segmental organ, ov. Ovary, p. Pads on ventral side of foot. p. Common duct into which the vasa deferentia open. p. c. Posterior lobe of brain. /. d. c. Posterior commissure passing dorsal to rectum. /./. Internal opening of nephridium into body cavity, ph. Pharynx, pi. Pigment in outer ends of epidermic cells, pi. r. Retinal pigment, p. n. Nerves to feet. p.p. Primary papilla, pr. Prostate. R. Rectum. Re. Retinal rods. R. m. Muscle of claw. s. Vesicle of nephridium. j 1 . Part of 4th or 5th nephridium which corresponds to vesicle of other nephridia.
1 The explanations of the figures printed within inverted commas are by Professor Balfour, the rest are by the Editors.
91 6 EXPLANATION OF PLATES.
s. c. i. Region No. i of coiled tube of nephridium. s. c. 2. Region No. i of ditto. s. c. 3. Region No. 3 of ditto. s. c. 4. Region No. 4 of ditto, s. d. Salivary duct. s. g. Salivary gland, si. d. Reservoir of slime gland, sl.g. Tubules of slime gland. s. o. i, 2, 3, &c. Nephridia of ist, 2nd, &c., feet. s. o.f. Terminal portion of nephridium. s.p. Secondary papilla, st. Stomach, sf. e. Epithelium of stomach, sy. Sympathetic nerve running in muscles of tongue and pharynx, sy 1 . Origin of pharyngeal sympathetic nerves. T. Tongue, t. Teeth on tongue, te. Testis. tr. Trach.e0e. tr. c. Cells found along the course of the tracheae. tr. o. Tracheal stigma, tr. p. Tracheal pit. tit. Uterus, v. c. Ventral nerve cord. v. d. Vas deferens. v. g. Imperfect ganglia of ventral cord.
PLATE 46.
Fig. i. Peripatus capensis, x 4 ; viewed from the dorsal surface. (From a drawing by Miss Balfour. )
PLATE 47.
Fig. 2. A left leg of Peripatus capensis, viewed from the ventral surface ; x 30. (From a drawing by Miss Balfour.)
P'ig. 3. A right leg of Peripatus capensis, viewed from the front side. (From a drawing by Miss Balfour.)
Fig. 4. .The last left (i7th) leg of a male Peripatus capensis, viewed from the ventral side to shew the papilla at the apex of which the accessory gland of the male, or enlarged crural gland, opens to the exterior. (From a drawing by Miss Balfour.) Prof. Balfour left a rough drawing (not reproduced) shewing the papilla, to which is appended the following note. " Figure shewing the accessory genital gland of male, which opens on the last pair of legs by a papilla on the ventral side. The papilla has got a slit-like aperture at its extremity."
Fig. 5. Ventral view of head and oral region of Peripatus capensis. (From a drawing by Miss Balfour.)
PLATE 48.
Figs. 6 and 7 are from one drawing.
Fig. 6. Peripatus capensis dissected so as to shew the alimentary canal, slime glands, and salivary glands ; x 3. (From a drawing by Miss Balfour.)
Fig. 7. The anterior end of Fig. 6 enlarged ; x 6. (From a drawing by Miss Balfour.) The dissection is viewed from the ventral side, and the lips, L., have been cut through in the middle line behind and pulled outwards, so as to expose the jaws, /., which have been turned outwards, and the tongue, T. , bearing a median row of chitinous teeth, which branches behind into two. The junction of the salivary ducts, j. d., and the opening of the median duct so formed into the buccal cavity is also shewn. The muscular pharynx, extending back into the space between the ist and 2nd pairs of legs, is followed by a short tubular oesophagus. The latter opens into the large stomach with plicated walls, extending almost to the hind end of the animal. The stomach at its point of junction with the rectum presents an S-shaped ventrodorsal curve.
A. Anus. at. Antenna. F. i, K. 2. First and second feet. /. Jaws. L. Lips.
ae. OZsophagus. or. p. Oral papilla, ph. Pharynx. R. Rectum, s. d. Salivary
duct. s. g. Salivary gland, si. d. Slime reservoir, si. g. Portion of tubules of slime
gland, st. Stomach. T. Tongue in roof of mouth.
Fig. 8. Peripatus capensis, X4; male. (From a drawing by Miss Balfour.) Dissected so as to shew the nervous system, slime glands, ducts of the latter passing into the oral papilla, accessory glands opening on the last pair of legs (enlarged crural glands), and segmental organs, viewed from dorsal surface. The first three pairs of segmental organs consist only of the vesicle and duct leading to the exterior. The fourth and fifth pairs are larger than the succeeding, and open externally to the crural glands. The ventral nerve-cords unite behind dorsal to the rectum.
A. Anus. a. g. Accessory generative gland, or enlarged crural gland of the iyth leg. at. Antenna, c. g. Supra-oesophageal ganglia with eyes. co. Commissures between the ventral nerve-cords, d. n. Large median nerve to dorsal integument from hinder part of brain. F. i, i, &c. Feet. g. o. Generative orifice, <x. (Esophagus. KS. co. QEsophageal commissures, or. p. Oral papilla, p.d.c. Posterior dorsal commissure between the ventral nerve-cords, ph. Pharynx, p. n. Nerves to feet, one pair from each ganglionic enlargement. si. d. Reservoir of slime gland. si. g. Tubules of slime gland. s. o. i, 2, 3, &c. Segmental organs. v. c. Ventral nervecords, "v. g. Imperfect ganglia of ventral cords.
Figs. 9 and 10. Left jaw of Peripatus capensis (male), shewing reserve jaws. (From a drawing by Miss Balfour.)
Fig. 9. Inner jaw. Fig. 10. Outer jaw.
PLATE 49.
Figs, ii 16. A series of six transverse sections through the head of Peripatus capensis.
Fig. n. The section is taken immediately behind the junction of the supracesophageal ganglia, c. g., and passes through the buccal cavity, M., and jaws, o.j. and i.j.
Fig. 12. The section is taken through the hinder part of the buccal cavity at the level of the opening of the mouth into the pharynx and behind the jaws. The cuticular rod-like continuation (le.) of the inner jaw lying in a backwardly directed pit of the buccal cavity is shewn; on the right hand side the section passes through the opening of this pit.
Fig. 1 3. The section passes through the front part of the pharynx, and shews the opening into the latter of the median backward diverticulum of the mouth (M 1 ), which receives the salivary ducts. It also shews the commencement of the ventral nerve-cords, and the backwardly projecting lobes of the brain.
Fig. 14. The section passes through the anterior part of the pharynx at the level of the second commissure (co. 2), between the ventral nerve-trunks, and shews the mass of cells developed on this commissure, which is in contact with the epithelium of the backward continuation of the buccal cavity (M 1 ).
QI 8 EXPLANATION OF PLATES.
Fig. 15. Section through the point of junction of the salivary ducts with the median oral diverticulum.
Fig. 1 6. Section behind the pharynx through the oesophagus.
b. c. Body-cavity. C. Cutis. c. b. c. Central compartment of body-cavity, c. g. Supra-oesophageal ganglia, c. m. Layer of circular muscles, co. Commissure between ventral nerve-cords. co. i. Second commissure between the ventral nerve-cords. co 1 . i. Mass of cells developed on second commissure (probably sensory), c. s. d. Common duct for the two salivary glands, d. /. m. Dorsal longitudinal muscles of pharynx, d. o. Muscles serving to dilate the opening of the pharynx. Ep. Epidermis, e. n. Nerve passing outwards from ventral nerve-cord. H. Heart, i.j. Inner jaw. j. p. Jaw papillae. L. Lips of buccal cavity. /. b. c. Lateral compartment of body-cavity, le. Rod-like cuticular continuation of inner jaw, lying in a pit of the buccal cavity. /. m. Bands of longitudinal muscles. M. Buccal cavity. M 1 . Median backward continuation of buccal cavity, m. 1. Muscles of jaw lever, m. s. Muscular sheets passing from side walls of pharynx to dorsal body wall. ce. CEsophagus. ces. co. CEsophageal commissures. o.j. Outer jaw. ph. Pharynx, s. d. Salivary duct. s. g. Salivary gland, si. d. Reservoir of slime gland, sy. Sympathetic nerves running in muscles of tongue or pharynx, sy 1 . Origin of sympathetic nerves to pharynx. T. Tongue, v. c. Ventral nerve-cords.
Figs. 17, 1 8. Two longitudinal horizontal sections through the head of Peripatus capensis. Fig. 17 is the most ventral. They are both taken ventral to the cerebral ganglia. In Fig. 17 dorsal tracheal pits are shewn with tracheae passing off from them. (Zeiss a a, Hartnack's camera.) C. Cutis. c. s. d. Common salivary duct. ep. Epidermis, i.j. Inner jaw. M. Buccal cavity. M 1 . Median backward diverticulum of mouth, o.j. Outer jaw. s. d. Salivary ducts. T. Tongue, t. Teeth on tongue, tr. Tracheae, tr. p. Tracheal pits.
PLATE 50.
Fig. 19. "A, B, c, D, E, F, G. Seven transverse sections illustrating the structure of the- supra- cesophageal ganglia. (Zeiss A, Hartnack's camera.) a. Dorso-lateral horn of white matter. b. Ventro-lateral horn of white matter, c. Postero-dorsal lobe of white matter, d. Ventral protuberance of brain, e. Central lobe of white matter, o.p. Optic ganglion.
" A. Section through anterior portions of ganglia close to the origin of the antennary nerve. B. Section a little in front of the point where the two ganglia unite, c. Section close to anterior junction of two ganglia. D. Section through origin of optic nerve on the right side. E. Section shewing origin of the optic nerve on the left side. F. Section through the dorso-median lobe of white matter. G. Section near the termination of the dorsal tongue of ganglion cells."
PLATE 51.
Fig. 10. Portion of a transverse section through the hinder part of Peripatus capensis (male). The section passes through a leg, and shews the opening of the segmental organ (p. s.), and of a crural gland, o.f.g., and the forward continuation of the enlarged crural gland of the i7th leg (/ g!.). (Zeiss a a, Hartnack's camera.) a-g. accessory gland of male (modified crural gland of last leg), c. Cutis. cL Claw. cu. Cuticle, ep. Epidermis, f.gl. Crural gland, h. Cells in lateral compartment of body cavity, o.f. g. Orifice of accessory foot gland, o. s. Opening of segmental organ, p. Three spinous pads on ventral surface of foot. pr. Prostate. R. M. Retractor muscle of claw. s. Vesicle of nephridium. s. c. i. Region No. i of coiled part of nephridium. si. g. Tubule of slime gland, s. o. t. Terminal portion of nephridium. st. Stomach, st. e. Epithelium of stomach, v. c. Ventral nerve-cord, v. d. Vas deferens.
Fig. 21. "Longitudinal vertical section through the supra-oesophageal ganglion and oesophageal commissures of Peripatus capensis. (Zeiss a a, Hartnack.)" at. Antenna, e. Central lobe of white matter. /. Part of jaw. s. g. Salivary gland.
Fig. 22: drawn by Miss Balfour. Brain and anterior part of the ventral nervecords of Peripatus capensis enlarged and viewed from the ventral surface. The paired appendages (d) of the ventral surface of the brain are seen, and the pair of sympathetic nerves (sy 1 ) arising from the ventral surface of the hinder part.
From the commencement of the cesophageal commissures (as. co. ) pass off on each side a pair of nerves to the jaws (/. .).
The three anterior commissures between the ventral nerve-cords are placed close together; immediately behind them the nerve-cords are swollen, to form the ganglionic enlargements from which pass off to the oral papillce a pair of large nerves on each side (or. n. )
Behind this the cords present a series of enlargements, one pair for each pair of feet, from which a pair of large nerves pass off on each side to the feet (p. n). at. n. Antennary nerves, co. Commissures between ventral cords, d. Ventral appendages of brain. E. Eye. e. n. Nerves passing outwards from ventral cord. F-g- Ganglionic enlargements from which nerves to feet pass off. j. n. Nerves to jaws. or. g. Ganglionic enlargement from which nerves to oral papillce pass off. or. n. Nerves to oral papillae, p.c. Posterior lobe of brain, p. n. Nerves to feet. s.y. Sympathetic nerves.
Fig. 23. "Longitudinal horizontal section through the head of Peripatus capensis, shewing the structure of the brain, the antennary and optic nerves, &c. (Zeiss a a, Hartnack's camera.)" at. Antenna, at. n. Antennary nerve, cor. Cornea, e. Central mass of white matter. /. Lens. op. n. Optic nerve, ph. Pharynx, p.p. Primary papilla covered with secondary papillte and terminating in a long spine, sy. Pharyngeal sympathetic nerves.
Fig. 24. "Eye of Peripatus capensis, as shewn in a longitudinal horizontal section through the head. The figure is so far diagrammatic that the lens is represented as filling up the whole space between the rods and the cornea. In the actual section there is a considerable space between the parts, but this space is probably artificial, being in part caused by the shrinkage of the lens and in part by the action of the razor. (Zeiss c, Hartnack's camera.)" (It appears that the ganglionic region of the eye is covered by a thin capsule, which is omitted in the figure.)
cor. Cornea. /. Lens. op. Optic ganglion, op-, n. Optic nerve. //'. r. Pigment. Re. rods. s. p. Secondary papillae.
Fig. 25. Longitudinal horizontal section through the dorsal skin, shewing the
peculiar arrangement of the circular muscular fibres. (Zeiss A, Hartnack's camera.)
PLATE 52.
Fig. 26. Portion of ventral cord of Peripatus capensis enlarged, shewing two ganglionic enlargements and the origin of the nerves and commissures. (From a drawing by Miss Balfour.)
co. Commissures. E. n. Nerves passing out from ventral cords. F. n. Nerves to feet. g. co. Commissures between the ventral cords containing ganglion cells, v. g. Ganglionic enlargements.
Fig. 27. Segmental organ from the 5th pair of legs of Peripatus capensis. This nephridium resembles those of the 4th legs, and differs from all the others in its large size and in the absence of any dilatation giving rise to a collecting vesicle on its external portion (enlarged). The terminal portion has the same histological characters as in the case of the hinder segmental organs. (From a drawing by Miss Balfour. )
Fig. 28. Segmental organ or nephridium from the 9th pair of legs of Peripatus capensis^ shewing the external opening, the vesicle, the coiled portion and the terminal portion with internal opening (enlarged). (From a drawing by Miss Balfour.)
o. s. External opening of segmental organ, p.f. Internal opening of nephridium into the body-cavity (lateral compartment). s. Vesicle of segmental organ, j 1 . Portion of segmental organ of 4th and 5th legs, corresponding to vesicle of the other nephridia. s. c. i. First or external portion of coiled tube of nephridium, lined by columnar epithelium with small nuclei ; the cells project for very different distances, giving the inner boundary of this region a ragged appearance, s. c. 2. Region No. 2 of coiled tube of nephridium, lined by small closely-packed columnar cells, s. c. 3. Region No. 3 of coiled tube of segmental organ, lined by large flat cells with large disc-shaped nuclei, s. c. 4. Region No. 4 of coiled tube of nephridium ; this region is very short and lined by small columnar cells, s. o. t. Terminal portion of nephridium.
Fig. 29. " Portion of nephridium of the hindermost leg of Peripatus capensis, seen in longitudinal and vertical section. The figure is given to shew the peritoneal funnel of the nephridium. Portions of the collecting sack (s.) and other parts are also represented. (Zeiss B, Hartnack's camera.)"
p.f. Peritoneal funnel, s. Vesicle, s.c.i, s.c.i, s.c.$. Portions of coiled tube.
Fig. 30. " Section through a tracheal pit and diverging bundles of tracheal tubes" taken transversely to the long axis of the body. (Zeiss E, oc. 2.) (From a rough drawing by Prof. Balfour.)
tr. Tracheae, shewing rudimentary spiral fibre, tr. c. Cells resembling those lining the tracheal pits, which occur at intervals along the course of the trachere. tr. s. Tracheal stigma, tr. p. Tracheal pit.
Fig- 31. "Sense organs and nerves attached from antenna of Peripatus capensis (Zeiss, immersion 2, oc. 2.)" (From a rough drawing by Prof. Balfour.) The figure shews the arrangement of the epidermis cells round the base of the spine. The spine is seen to be continuous with the inner layer of the cuticle.
EXPLANATION OF PLATE 53. 92 1
Fig. 32. Section through the skin of Peripatus capensis ; it shews the secondary papillae covered with minute spinous tubercles and the relation of the epidermis to them. (The cuticle in the process of cutting has been torn away from the subjacent cells.) The cells of the epidermis are provided with large oval nuclei, and there is a deposit of pigment in the outer ends of the cells. The granules in the protoplasm of the inner ends of the cells are arranged in lines, so as to give a streaked appearance. (Zeiss E, oc. 2.) (From a rough drawing by Prof. Balfour.)
c. Dermis. cu. Cuticle, ep. c. Epidermis cells, pi. Deposit of pigment in outer ends of epidermis cells, s.p. Secondary papillae.
Fig. 33. Female generative organs of Peripatus capensis, x 5. (From a rough drawing by Prof. Balfour.) The following note was appended to this drawing: "Ovary rather to dorsal side, lying in a central compartment of body-cavity and attached to one of the longitudinal septa, dividing this from the lateral compartment between the penultimate pair of legs and that next in front. The oviducts cross before opening to the exterior, the right oviduct passing under the rectum and the left over it. They meet by opening into a common vestibule, which in its turn opens below the anus. On each side of it are a pair of short papillae (aborted feet ?)."
F. 16, 17. Last two pairs of legs. od. Oviduct, ov. Ovary, ut. Uterus, v. c. Nerve-cord.
PLATE 53.
Cite this page: Hill, M.A. (2024, April 19) Embryology The Works of Francis Balfour 1-20. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/The_Works_of_Francis_Balfour_1-20
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