The Works of Francis Balfour 3-4

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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata
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This historic 1885 book edited by Foster and Sedgwick is the third of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks.



Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

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.
Modern Notes:

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)


Draft Version - Notice removed when completed.

Vol. III. A Treatise on Comparative Embryology 2 (1885)

CHAPTER IV. TELEOSTEI

THE majority of the Teleostei deposit their eggs before impregnation, but some forms are viviparous, e.g. Blennius viviparus. Not a few carry their eggs about ; but this operation is with a few exceptions performed by the male. In Syngnathus the eggs are carried in a brood-pouch of the male situated behind the anus. Amongst the Siluroids the male sometimes carries the eggs in the throat above the gill clefts. Ostegeniosus militaris, Arius falcarius, and Arius fissus have this peculiar habit.

The ovum when laid is usually invested in the zona radiata only, though a vitelline membrane is sometimes present in addition, e.g. in the Herring. It is in most cases formed of a central yolk mass, which may either be composed of a single large vitelline sphere, or of distinct yolk spherules. The yolk mass is usually invested by a granular protoplasmic layer, which is especially thickened at one pole to form the germinal disc.

In the Herring's ovum the germinal disc is formed, as in many Crustacea, at impregnation; the protoplasm which was previously diffused through the egg becoming aggregated at the germinal pole and round the periphery.

Impregnation is external, and on its occurrence a contraction of the vitellus takes place, so that a space is formed between the vitellus and the zona radiata, which becomes filled with fluid.

The peculiarities in the development of the Teleostean ovum can best be understood by regarding it as an Elasmobranch


TELEOSTEI. 69


ovum very much reduced in size. It seems in fact very probable that the Teleostei are in reality derived from a type of Fish with a much larger ovum. The occurrence of a meroblastic segmentation, in spite of the ovum being usually smaller than that of Amphibia and Acipenser, etc., in which the segmentation is complete, as well as the solid origin of many of the organs, receives its most plausible explanation on this hypothesis.

The proportion of the germinal disc to the whole ovum varies considerably. In very small eggs, such as those of the Herring, the disc may form as much as a fifth of the whole.

The segmentation, which is preceded by active movements of the germinal disc, is meroblastic. There is nothing very special to note with reference to its general features, but while in large ova like those of the Salmon the first furrows only penetrate for a certain depth through the germinal disc, in small ova like those of the Herring they extend through the whole thickness of the disc. During the segmentation a great increase in the bulk of the blastoderm takes place.

In hardened specimens a small cavity amongst the segmentation spheres may be present at any early stage ; but it is probably an artificial product, and in any case has nothing to do with the true segmentation cavity, which does not appear till near the close of segmentation. The peripheral layer of granular matter, continuous with the germinal disc, does not undergo division, but it becomes during the segmentation specially thickened and then spreads itself under the edge of the blastoderm ; and, while remaining thicker in this region, gradually grows inwards so as to form a continuous sub-blastodermic layer. In this layer nuclei appear, which are equivalent to those in the Elasmobranch ovum. A considerable number of these nuclei often become visible simultaneously (van Beneden, No. 60) and they are usually believed to arise spontaneously, though this is still doubtful 1 . Around these nuclei portions of protoplasm are segmented off, and cells are thus formed, which enter the blastoderm, and have nearly the same destination as the homologous cells of the Elasmobranch ovum.

1 Fide Vol. II. p. 108.


70 SEGMENTATION.


During the later stages of segmentation one end of the blastoderm becomes thickened and forms the embryonic swelling ; and a cavity appears between the blastoderm and the yolk which is excentrically situated near the non-embryonic part of the blastoderm. This cavity is the true segmentation cavity. Both the cavity and the embryonic swelling are seen in section in fig. 31 A and B.

In Leuciscus rutilus Bambeke describes a cavity as appearing in the middle of the blastoderm during the later stages of segmentation. From his figures it might be supposed that this cavity was equivalent to the segmentation cavity of Elasmobranchs in its earliest condition, but Bambeke states that it disappears and that it has no connection with the true segmentation cavity. Bambeke and other investigators have failed to recognize the homology of the segmentation cavity in Teleostei with that in Elasmobranchii, Amphibia, etc.

With the appearance of the segmentation cavity the portion of the blastoderm which forms its roof becomes thinned out, so that the whole blastoderm consists of (i) a thickened edge especially prominent at one point where it forms the embryonic swelling, and (2) a thinner central portion. The changes which now take place result in the differentiation of the embryonic layers, and in the rapid extension of the blastoderm round the yolk, accompanied by a diminution in its thickness.

A



FIG. 31. LONGITUDINAL SECTIONS THROUGH THE BLASTODERM OF THE

TROUT AT AN EARLY STAGE OF DEVELOPMENT.

A. at the close of the segmentation; B. after the differentiation of the germinal layers. ep' . epidermic layer of the epiblast; sc, segmentation cavity.

The first differentiation of the layers consists in a single row of cells on the surface of the blastoderm becoming distinctly


TELEOSTEI. 71


marked off as a special layer (fig. 3 1 A) ; which however does not constitute the whole epiblast but only a small part of it, which will be spoken of as the epidermic layer. The complete differentiation of the epiblast is effected by the cells of the thickened edge of the blastoderm becoming divided into two strata (fig. 31 B). The upper stratum constitutes the epiblast. It is divided into two layers, viz., the external epidermic layer already mentioned, and an internal layer known as the nervous layer, formed of several rows of vertically arranged cells. According to the unanimous testimony of investigators the roof of the segmentation cavity is formed of epiblast cells only. The lower stratum in the thickened rim of the blastoderm is several rows of cells deep, and corresponds with the lower layer cells or primitive hypoblast in Elasmobranchii. It is continuous at the edge of the blastoderm with the nervous layer of the epiblast.

In smaller Teleostean eggs there is formed, before the blastoderm becomes differentiated into epiblast and lower layer cells, a complete stratum of cells around the nuclei in the granular layer underneath the blastoderm. This layer is the hypoblast ; and in these forms the lower layer cells of the blastoderm are stated to become converted into mesoblast only. In the larger Teleostean eggs, such as those of the Salmonidae, the hypoblast, as in Elasmobranchs, appears to be only partially formed from the nuclei of the granular layer. In these forms however, as in the smaller Teleostean ova and in Elasmobranchii, the cells derived from the granular stratum give rise to a more or less complete cellular floor for the segmentation cavity. The segmentation cavity thus becomes enclosed between an hypoblastic floor and an epiblastic roof several cells deep. It becomes obliterated shortly after the appearance of the medullary plate.

At about the time when the three layers become established the embryonic swelling takes a somewhat shield-like form (fig- 33 A). Posteriorly it terminates in a caudal prominence (ts) homologous with the pair of caudal swellings in Elasmobranchs. The homologue of the medullary groove very soon appears as a shallow groove along the axial line of the shield. After these changes there takes place in the embryonic layers a series of differentiations leading to the establishment of the


72 FORMATION OF THE LAYERS.

definite organs. These changes are much more difficult to follow in the Teleostei than in the Elasmobranchii, owing partly to the similarity of the cells of the various layers, and partly to the primitive solidity of all the organs.

The first changes in the epiblast give rise to the central nervous system. The epiblast, consisting of the nervous and epidermic strata already indicated, becomes thickened along the axis of the embryo and forms a keel projecting towards the yolk below : so great is the size of this keel in the front part of the embryo that it influences the form of the whole body and causes the outline of the surface adjoining the yolk to form a strong ridge moulded on the keel of the epiblast (fig. 32 A and B). Along the dorsal line of the epiblast keel is placed the shallow medullary groove ; and according to Calberla (No. 61) the keel is formed by the folding together of the two sides of the primitively uniform epiblastic layer. The keel becomes gradually constricted off from the external epiblast and then forms a solid cord below it. Subsequently there appears in this cord a median slit-like canal, which forms the permanent central canal of the cerebrospinal cord- The peculiarity in the formation of the central nervous system of Teleostei consists in the fact that it is not formed by the folding over of the sides of the medullary groove into a canal, but by the separation, below the medullary groove, of a solid cord of epiblast in which the central canal is subsequently formed. Various views have been put forward to explain the apparently startling difference between Teleostei, with which Lepidosteus and Petromyzon agree, and other vertebrate forms. The explanations of Gotte and Calberla appear to me to contain between them the truth in this matter. The groove above in part represents the medullary groove ; but the closure of the groove is represented by the folding together of the lateral parts of the epiblast plate to form the medullary keel.

According to Gotte this is the whole explanation, but Calberla states for Syngnathus and Salmo that the epidermic layer of the epiblast is carried down into the keel as a double layer just as if it had been really folded in. This ingrowth of the epidermic layer is shewn in fig. 32 A where it is just commencing to pass into the keel ; and at a later stage in fig. 32 B where the keel has reached its greatest depth.


TELEOSTEI.


73


Gotte maintains that Calberla's statements are not to be trusted, and I have myself been unable to confirm them for Teleostei or Lepidosteus; but if they could be accepted the difference in the formation of the medullary canal in Teleostei and in other Vertebrata would become altogether unimportant and consist simply in the fact that the ordinary open medullary groove is in Teleostei obliterated in its inner part by the two sides of the groove coming together. Both layers of epiblast would thus have a share in the formation of the central nervous system ; the epidermic layer giving rise to the lining epithelial cells of the central canal, and the nervous layer to the true nervous tissue.

The separation of the solid nervous system from the epiblast takes place relatively very late ; and, before it has been completed, the first traces of the auditory pits, of the optic vesicles, and of the olfactory pits are visible. The auditory pit arises as a solid thickening of the nervous layer of the epiblast at its point of junction with the medullary keel ; and the optic vesicles spring as solid outgrowths from part of the keel itself. The olfactory pits are barely indicated as thickenings of the nervous layer of the epiblast.



FlG. 32. TWO TRANSVERSE SECTIONS OF

SYNGNATHUS. (After Calberla. )

A. Younger stage before the definite establishment of the notochord.

B. Older stage.

The epidermic layer of the epiblast is represented in black.

ep. epidermic layer of epiblast ; me. neural cord ; hy. hypoblast ; me. mesoblast ; ch. notochord.


At this early stage all the organs of special sense are attached to a layer continuous with or forming part of the central nervous system ; and

this fact has led Gotte (No. 63) to speak of a special- sense plate, belonging to the central nervous system and not to the skin, from which


74 FORMATION OF THE LAYERS.

all the organs of special sense are developed ; and to conclude that a serial homology exists between these organs in their development. A comparison between Teleostei and other forms shews that this view cannot be upheld ; even in Teleostei the auditory and olfactory rudiments arise rather from the epiblast at the sides of the brain than from the brain itself, while the optic vesicles spring from the first directly from the medullary keel, and are therefore connected with the central nervous system rather than with the external epiblast. In a slightly later stage the different connections of the two sets of sense organs is conclusively shewn by the fact that, on the separation of the central nervous system from the epiblast, the optic vesicles remain attached to the former, while the auditory and olfactory vesicles are continuous with the latter.

After its separation from the central nervous system the remainder of the epiblast gives rise to the skin, etc., and most probably the epidermic stratum develops into the outer layer of the epidermis and the nervous stratum into the mucous layer. The parts of the organs of special sense, which arise from the epiblast, are developed from the nervous layer. In the Trout (Oellacher, No. 72) both layers are continued over the yolksack; but in Clupeus and Gasterosteus only the epidermic has this extension. According to Gotte the distinction between the two layers becomes lost in the later embryonic stages.

Although it is thoroughly established that the mesoblast originates from the lower of the two layers of the thickened embryonic rim, it is nevertheless not quite certain whether it is a continuous layer between the epiblast and hypoblast, or whether it forms two lateral masses as in Elasmobranchs. The majority of observers take the former view, while Calberla is inclined to adopt the latter. In the median line of the embryo underneath the medullary groove there are undoubtedly from the first certain cells which eventually give rise to the notochord ; and it is these cells the nature of which is in doubt. They are certainly at first very indistinctly separated from the mesoblast on the two sides, and Calberla also finds that there is no sharp line separating them from the secondary hypoblast (fig. 32 A). Whatever may be the origin of the notochord the mesoblast very soon forms two lateral plates, one on each side of the body, and between them is placed the notochord (fig. 32 B). The general fate of the two mesoblast plates is the same as in Elasmobranchs. They are at first quite solid and exhibit relatively




TELEOSTEI. 75


late a division into splanchnic and somatic layers, between which is placed the primitive body cavity. The dorsal part of the plates becomes transversely segmented in the region of the trunk ; and thus gives rise to the mesoblastic somites, from which the muscle plates and the perichordal parts of the vertebral column are developed. The ventral or outer part remains unsegmented. The cavity of the ventral section becomes the permanent body cavity. It is continued forward into the head (Oellacher), and part of it becomes separated off from the remainder as the pericardial cavity.

The hypoblast forms a continuous layer below the mesoblast, and, in harmony with the generally confined character of the development of the organs in Teleostei, there is no space left between it and the yolk to represent the primitive alimentary cavity. The details of the formation of the true alimentary tube have not been made out ; it is not however formed by a folding in of the lateral parts of the hypoblast, but arises as a solid or nearly solid cord in the a'xial line, between the notochord and the yolk, in which a lumen is gradually established.

In the just hatched larva of an undetermined fresh-water fish with a very small yolk-sack I found that the yolk extended along the ventral side of the embryo from almost the mouth to the end of the gut. The gut had, except in the hinder part, the form of a solid cord resting in a concavity of the yolk. In the hinder part of the gut a lumen was present, and below this part the amount of yolk was small and the yolk nuclei numerous. Near the limit of its posterior extension the yolk broke up into a mass of cells, and the gut ended behind by falling into this mass. These incomplete observations appear to shew that the solid gut owes its origin in a large measure to nuclei derived from the yolk.

When the yolk has become completely enveloped a postanal section of gut undoubtedly becomes formed ; and although, owing to the solid condition of the central nervous system, a communication between the neural and alimentary canals cannot at first take place, yet the terminal vesicle of the postanal gut of Elasmobranchii is represented by a large vesicle, originally discovered by Kupffer (No. 68), which can easily be seen in the embryos of most Teleostei, but the relations of which have not been satisfactorily worked out (vide fig. 34, hyv). As the tail end of the embryo becomes separated off from the yolk the postanal vesicle atrophies.


7 6


GENERAL GROWTH OF THE EMBRYO.


General development of the Embryo. Attention has already been called to the fact that the embryo first appears as a thickening of the edge of the blastoderm which soon assumes a somewhat shield-like form (fig. 33 A). The hinder end of the embryo, which is placed at the edge of the blastoderm, is somewhat prominent, and forms the caudal swelling (ts). The axis of the embryo is marked by a shallow groove.

The body now rapidly elongates, and at the same time




FIG.


33. THREE STAGES IN THE DEVELOPMENT OF THE SALMON. His.)


(After


ts. tail-swelling; an.v. auditory vesicle; oc. optic vesicle; ce. cerebral rudiment; m.b. mid-brain; ^.cerebellum; md. medulla oblongata ; m.so. mesoblastic somite.

becomes considerably narrower, while the groove along the axis becomes shallower and disappears. The anterior, and at first proportionately a very large part, soon becomes distinguished as the cephalic region (fig. 33 B). The medullary cord in this region becomes very early divided into three indistinctly separated lobes, representing the fore, the mid, and the hind brains : of these the anterior is the smallest. With it are connected the optic vesicles (oc) solid at first which are pushed back into the region of the mid-brain.

The trunk grows in the usual way by the addition of fresh somites behind.

After the yolk has become completely enveloped by the blastoderm the tail becomes folded off, and the same process takes place at the front end of the embryo. The free tail end of


TELEOSTEI.


77



the embryo continues to grow, remaining however closely applied to the yolk-sack, round which it curls itself to an extent varying with the species (vide fig. 34).

The general growth of the embryo during the later stages presents a few special features of interest. The head is remarkable for the small apparent amount of the cranial flexure. This is probably due to the late development of the cerebral hemispheres. The flexure of the floor of the brain is however quite as considerable in the Teleostei as in other types. The gill clefts develop from before backwards. The first cleft is the hyomandibular, and behind this there are the hyobranchial and four branchial clefts. Simultaneously with the clefts there are developed the branchial arches. The postoral arches formed are the mandibular, hyoid and five branchial arches. In the case of the Salmon all of these appear before hatching.

The first cleft closes up very early (about the time of hatching in the Salmon) ; and about the same time there springs a membranous fold from the hyoid arch, which gradually grows backwards over the arches following, and gives rise to the operculum. There appear in the Salmon shortly before hatching double rows of papillae on the four anterior arches behind the hyoid. They are the rudiments of the branchiae. They reach a considerable length before they are covered in by the opercular membrane. In Cobitis (Gotte, No. 64) they appear in young larvae as filiform processes equivalent to the external gills of Elasmobranchs. The extremities of these processes atrophy; while the basal portions become the permanent gill lamellae. The general relation of the clefts, after the closure of the hyomandibular, is shewn in fig. 35.

The air-bladder is formed as a dorsal outgrowth of the alimentary tract very slightly in front of the liver. It grows in between the two limbs of the mesentery, in which it extends itself backwards. It appears in the Salmon,


FlG. 34. VIEW OF AN ADVANCED EMBRYO OF A HERRING IN THE

EGG. (After Kupffer.)

oc. eye ; ht. heart ; hyv, post-anal vesicle ; ch. notochord.


FORMATION OF THE TAIL.


Carp, and other types to originate rather on the right side of the median dorsal line, but whether this fact has any special significance is rather doubtful. In the Salmon and Trout it is formed considerably later than the liver, but the two are stated by Von Baer to arise in the Carp nearly at the same time. The absence of a pneumatic duct in the Physoclisti is due to a post-larval atrophy. The region of the stomach is reduced almost to nothing in the larva.

The oesophagus becomes solid, like that of Elasmobranchs, and remains so for a considerable period after hatching.

The liver, in the earliest stage in which I have met with it in the Trout (27 days after impregnation), is a solid ventral diverticulum of the intestine, which in the region of the liver is itself without a lumen.

The excretory system com


FIG. 35. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN, WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegenbaur.)

a. auricle ; v. ventricle ; abr. branchial artery ; d . carotid ; ad. aorta ; s. branchial clefts ; sv. sinus venosus ; dc. ductus Cuvieri ; n. nasal pit.


mences with the formation of a segmental duct, formed by a constriction of the parietal wall of the peritoneal cavity. The anterior end remains open to the body cavity, and forms a pronephros (head kidney). On the inner side of and opposite this opening a glomerulus is developed, and the part of the body cavity containing both the glomerulus and the opening of the pronephros becomes shut off from the remainder of the body cavity, and forms a completely closed Malpighian capsule.

The mesonephros (Wolffian body) is late in developing.

The unpaired fins arise as simple folds of the skin along the dorsal and ventral edges, continuous with each other round the end of the tail. The ventral fold ends anteriorly at the anus.

The dorsal and anal fins are developed from this fold by local hypertrophy. The caudal fin 1 , however, undergoes a more complicated metamorphosis. It is at first symmetrical or nearly so on the dorsal and ventral sides of the hinder end of the notochord. This symmetry is not long retained, but very soon the ventral part of the fin with its fin rays becomes much more developed than the dorsal part, and at the same time the posterior part of the notochord bends up towards the dorsal side.


1 In addition to the paper by Alex. Agassiz (No. 55) vide papers by Huxley, Kolliker, Vogt, etc.


TELEOSTET.


79


In some few cases, e.g. Gadus, Salmo, owing to the simultaneous appearance of a number of fin rays on the dorsal and ventral side of the notochord the external symmetry of the tail is not interfered with in the above processes. In most instances this is far from being the case.

In the Flounder, which may serve as a type, the primitive symmetry is very soon destroyed by the appearance of fin rays on the ventral side. The region where they are present soon forms a lobe; and an externally heterocercal tail is produced (fig. 36 A). The ventral lobe with its rays continues to grow more prominent and causes the tail fin to become bilobed (fig. 36 B) ; there being a dorsal embryonic lobe without fin rays (c), which contains the notochord, and a ventral lobe with fin rays, which will form the permanent caudal fin. In this condition the tail fin resembles the usual Elasmobranch form or still more that of some Ganoids, e.g. the Sturgeon. The ventral lobe continues to develop ; and soon projects beyond the dorsal, which gradually atrophies together with the notochord contained in it, and finally disappears, leaving hardly a trace on the dorsal side of the tail (fig. 36 C, c). In the meantime the fin rays of the ventral lobe gradually become parallel to the axis of



THREE STAGES IN THE DEOF THE TAIL OF THE (PLEURONECTES). (After


FIG. 36.

VELOPMENT

FLOUNDER

Agassiz.)

A. Stage in which the permanent caudal fin has commenced to be visible as an enlargement of the ventral side of the embryonic caudal fin.

B. Ganoid-like stage in which there is a tme external heterocercal tail.

C. Stage in which the embryonic caudal fin has almost completely atrophied.

c. embryonic caudal fin ; f. permanent caudal fin ; n. notochord ; it. urostyle.


the body ; and this lobe, together with a few accessory dorsal and ventral fin rays supported


80 FORMATION OF THE TAIL.

by neural and haemal processes, forms the permanent tail fin, which though internally unsymmetrical, assumes an externally symmetrical form. The upturned end of the notochord which was originally continued into the primitive dorsal lobe becomes enshcathed in a bone without a division into separate vertebrae. This bone forms the urostyle (u). The haemal processes belonging to it are represented by two cartilaginous masses, which subsequently ossify, forming the hypural bones, and supporting the primary fin rays of the tail (fig. 36 C). The ultimate changes of the notochord and urostyle vary very considerably in the different types of Teleostei. Teleostei may fairly be described as passing through an Elasmobranch stage or a stage like that of most pre-jurassic Ganoids or the Sturgeon as far as concerns their caudal fin.

The anterior paired fins arise before the posterior ; and there do not appear to be any such indications as in Elasmobranchii of the paired fins arising as parts of a continuous lateral fin.

Most osseous fishes pass through more or less considerable post-embryonic changes, the most remarkable of which are those undergone by the Pleuronectidae 1 . These fishes, which in the adult state have the eyes unsymmetrically placed on one side of the head, leave the egg like normal Teleostei. In the majority of cases as they become older the eye on the side, which in the adult is without an eye, travels a little forward and then gradually rotates over the dorsal side of the head, till finally it comes to lie on the same side as the other eye. During this process the rotating eye always remains at the surface and continues functional ; and on the two eyes coming to the same side of the head the side of the body without an organ of vision loses its pigment cells, and becomes colourless.

The dorsal fin, after the rotation of the eye, grows forward beyond the level of the eyes. In the genus Plagusia (Steenstrup, Agassiz, No. 56) the dorsal fin grows forward before the rotation of the eye (the right eye in this form), and causes some modifications in the process. The eye in travelling round gradually sinks into the tissues of the head, at the base of the fin above the frontal bone ; and in this process the original large opening of the orbit becomes much reduced. Soon a fresh opening on the opposite and left side of the dorsal fin is formed ; so that the orbit has two external openings, one on the left and one on the right side. The original one on the right soon atrophies, and the eye passes through the tissues at the base of the dorsal fin completely to the left side.

The rotating eye may be either the right or the left according to the species.

1 Vide Agassiz (No. 56) and Steenstrup, Malm.


TELEOSTEI. 8 1


The most remarkable feature in which the young of a large number of Teleostei differ from the adults is the possession of provisional spines, very often formed as osseous spinous projections the spaces between which become filled up in the adult. These processes are probably, as suggested by Gunther, secondary developments acquired, like the Zocea spines of larval Crustaceans, for purposes of defence.

The yolk-sack varies greatly in size in the different types of Teleostei.

According as it is enclosed within the body-wall, or forms a distinct ventral appendage, it is spoken of by Von Baer as an internal or external yolk-sack. By Von Baer the yolk-sack is stated to remain in communication with the intestine immediately behind the liver, while Lereboullet states that there is a vitelline pedicle opening between the stomach and the liver which persists till the absorption of the yolk-sack. My own observations do not fully confirm either of these statements for the Salmon and Trout. So far as I have been able to make out, all communication between the yolk-sack and the alimentary tract is completely obliterated very early. In the Trout the communication between the two is shut off before hatching, and in the just-hatched Salmon I can find no trace of any vitelline pedicle. The absorption of the yolk would seem therefore to be effected entirely by bloodvessels.

The yolk-sack persists long after hatching, and is gradually absorbed. There is during the stages either just before hatching or shortly subsequent to hatching (Cyprinus) a rich vascular development in the mesoblast of the yolk-sack. The blood is at first contained in lacunar spaces, but subsequently it becomes confined to definite channels. As to its exact relations to the vascular system of the embryo more observations seem to be required.

The following account is given by Rathke (No. 72*) and Lereboullet (No. 71). At first a subintestinal vein (vide chapter on Circulation) falls into the lacunae of the yolk-sack, and the blood from these is brought back direct to the heart. At a later period, when the liver is developed, the subintestinal vessel breaks up into capillaries in the liver, thence passes into the yolksack, and from this to the heart. An artery arising from the aorta penetrates the liver, and there breaks up into capillaries continuous with those of the yolk-sack. This vessel is perhaps the equivalent of the artery which supplies the yolk-sack in Elasmobranchii, but it seems possible that there is some error in the above description.

BIBLIOGRAPHY.

(55) Al. Agassiz. " On the young Stages of some Osseous Fishes. I. Development of the Tail." Proceedings of the American Academy of Arts and Sciences, Vol. xm. Presented Oct. u, 1877.

B. III. 6


82 BIBLIOGRAPHY.


(66) Al. Agassiz. "II. Development of the Flounders." Proceedings of the American Acad. of Arts and Sciences, Vol. xiv. Presented June, 1878.

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