Vertebrate Embryology - A Text-book for Students and Practitioners (1893) 4
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Marshall AM. Vertebrate Embryology: A Text-book for Students and Practitioners. (1893) Elder Smith & Co., London.
|Vertebrate Embryology 1893: 1 Introduction | 2 Amphioxus | 3 Frog | 4 Chick | 5 The Rabbit | 6 Human Embryo | Illustrations|
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- 1 Chapter IV. The Development of the Chick
- 1.1 General Account
- 1.2 The Egg
- 1.3 The Early Stages Of Development
- 1.4 The Development of the Nervous System
- 1.5 Development of the Sense Organs
- 1.6 Development of the Alimentary Canal
- 1.7 Development of the Heart and Bloodvessel
- 1.8 Development of the Urinary Organs
- 1.9 The Development of the Body Cavity and Muscular System
- 1.10 The Development of the Skeleton
- 1.11 Development Of The Feathers
- 1.12 Bibliography
Chapter IV. The Development of the Chick
1. Historical Sketch
The development of the chick has attracted great attention on account of the ease with which embryos of any desired age may be obtained, and of the shortness of the period within which the embryonic development is completed. Almost all the earlier investigations into the development of animals were made on chick embryos, and it is round the chick that the most famous embryological controversies have centred. Even at the present day, on account of their great convenience for laboratory purposes, chick embryos usually afford the material from which the student derives his first lessons in practical embryology.
Embryology as a science is barely three centuries old ; the earliest descriptions and figures of the development of the chick within the egg, that are of any real value, are contained in two treatises published by Fabricius, professor at Padua, in 1600 and 1601. Half a centuiy later, Harvey added important details in his ' Theoria Generationis ; ' and towards the close of the seventeenth century, in 1687, Malpighi published the first accounts of chick embryos based on microscopical examination.
During the eighteenth century facts accumulated rapidly, but the theories quite outpaced them ; and the current doctrine throughout the century, supported by many, and notably by Haller, was that of Preformation. according to which the chick was stated to be present in the egg at the time it is laid ; all its parts and organs being there from the beginning, but in an extremely minute and unexpanded condition ; the development of the embryo being regarded as a process comparable to the unfolding and enlargement of the several parts of a bud to form the perfect flower.
This theory of Preformation was vigorously combated by Caspar Friedrich Wolff, who in 1759, when only twenty-six years old, published as a thesis for the doctor's degree his theory of Epigenesis, which offered an entirely new explanation of the mode of development of the chick and other animals. Wolff showed conclusively that in the hen's egg, as laid, there is no trace whatever of the embryo, or of any of its parts or organs ; and that the formation of the embryo does not commence until after the egg is laid and incubation has begun. He noted further, and described accurately, the manner in which the embryo is formed by folding of the germinal layers or membranes.
Wolff was too far ahead of his age, and his conclusions, though perfectly sound, did not obtain acceptance until towards the middle of the present century, when their correctness was demonstrated, not merely for the chick, but for many other groups of animals as well, by von Baer, Remak, Bischoff, Kolliker, and others.
Although the chick has thus played a more important part in the history of embryology than any other animal, it must be borne in mind that birds are one of the most highly specialised groups of animals, and that their development is, more particularly in the early stages, very greatly modified. It is practical convenience alone that justifies the great attention they have received.
2. The Egg
The hen's egg is of large size, and ovoid in shape. It consists (Fig. 97) of a calcareous shell, lined by a fibrous shell membrane ; and inclosing a quantity of a viscid albuminous fluid, the ' white of the egg,' WA, in the centre of which lies the yolk,' Y, a spherical mass of a yellow colour, rather more than an inch in diameter, and inclosed in an elastic vitelline membrane, to which the preservation of its shape is due.
Of these parts, the yolk is the egg proper ; it corresponds to the egg of Amphioxus, or of the frog, and from it the embryo is developed directly.
The white of the egg corresponds to the investment of the egg of Amphioxus, which swells up so greatly on reaching the water, or to the jelly of the frog's spawn. The egg-shell and shell membrane are protective envelopes, which are not represented in the eggs of Amphioxus or of the frog.
The yolk or ovum is, as in other animals, a single cell ; its great size being due to the enormous quantity of food yolk accumulated within it, and distending it. As regards the quantity of food-yolk contained within it, the hen's egg is at the opposite extreme to that of Amphioxus ; the frog's egg being midway between the two.
It is in consequence of the abundance of food material present in the egg itself, that the chick embryo is enabled to complete its development within twenty-one days, while the frog requires three months or more, and Amphioxus an even longer time. The Amphioxus larva hatches in about eight hours, but in an extremely immature condition (Figs. 25, 26, p. 59) ; the frog hatches in about a fortnight, in a form utterly unlike the parent, and devoid of mouth and limbs (Figs. 72, 73, p. 157); the chick does not leave the egg until the twenty-first day, but is already a fully developed bird.
FIG. 97. The Hen's Egg at the time of laying, x g.
BA. blastoderm. SH, epr? shell. SM, shell membrane. .SV, air chamber. "WA, white or albumen. "WC, chalaza, or twisted cord of denser albumen. Y, yolk. Z, vitelline membrane.
A large amount of food-yolk is undoubtedly an advantage, inasmuch as it enables the embryo to develop rapidly and securely, and frees it from the necessity of obtaining food from without. However, it has also its disadvantages. Food-yolk is itself inert, as already noticed in the opening chapter. It is present in the egg as a number of granules of various shapes and sizes, embedded in the living protoplasm of the egg ; and the immediate effect of these inert, inactive, yolk-granules is, not to aid, but to mechanically impede the processes of development ; an effect which will necessarily be most marked in the early stages, when the food-yolk is most abundant. Hence the early stages of development of the chick, and especially the processes of segmentation, occur more slowly than those of the frog, and much more slowly than those of Amphioxus.
Moreover, the amount of food-yolk in the hen's egg is so great that serious distortion of the shape would occur, were the whole mass contained within the body of the embryo. To avoid this difficulty, the yolk, at a very early stage of development, becomes constricted into two parts, embryonic and vitelline respectively, which remain connected by a stalk. Of these (cf. Figs. 99 and 100), the embryonic portion, EM, is formed from the part of the egg comparatively free from food-yolk, and becomes converted directly into the embryo ; while the vitelline portion or yolk-sac, YS, which contains the bulk of the food-yolk, does not give rise directly to any part of the embryo, but forms a store of nutriment at the expense of which the development of the embryo is effected.
At first, the embryonic portion is very much smaller than the vitelline portion or yolk-sac (Fig. 98) ; but, inasmuch as the embryo grows by absorption of the food-yolk, the yolk-sac diminishes as the embryo increases in size (cf. Figs. 99, 100, 101). A time comes when the two are about equal in bulk, and in the later days of incubation the yolk-sac is much smaller than the embryo. By the twenty-first day of incubation the yolk-sac is almost completely absorbed, and the chick pecks its way out of the shell, and hatches.
3. The Embryo
The hen's egg is fertilised before it is laid, indeed before the egg-shell is formed, for no spermatozoon could possibly make its way through the shell. At the time the egg is laid, not only has fertilisation been effected, but the egg has already been developing for a period which varies in different cases, but amounts on an average to about eighteen hours.
When the egg is laid, development stops. To set it going again, to start development afresh, all that is necessary is that the egg should be kept at a temperature about equal to the blood-heat of the parent bird. This is normally effected by incubation, the hen sitting on the egg, and so keeping it warm ; but it may be effected equally well by artificial means. A certain amount of moisture, and free access of air, are necessary to insure normal development. The rate of development
FIG. 98. The yolk of a Hen's Egg at the thirty-sixth hour from the commencement of incubation. The structure of the embryo at this stage is shown on a larger scale in Figs. Ill and 112. x H.
AD. area pellucida of the blastoderm. AK, area opaoa. AV, area vasculosa. EM, embryo. SM, vitelline membrane. Y, yolk-sac.
varies to a slight extent according to the season of the year, autumn eggs developing more slowly than spring eggs ; or according to the temperature, if an artificial incubator is employed. The length of time the egg takes to travel down the oviduct, during the whole of which time it is developing, varies considerably, and individual variations may occur from other causes ; but, as a rule, the chick hatches on the twenty-first day from the commencement of incubation. The age of an embryo is always calculated from the commencement of incubation, or from the time of placing the egg in the incubator ; to obtain the true age there must be added to this the time during which the egg was developing, in its passage down the oividuct, a period averaging, as we have seen, about eighteen hoars.
Owing to the enormous amount of food-yolk, and the mechanical hindrance which this offers to the processes of development, the entire yolk, i.e. the egg proper, does not divide, but segmentation is restricted to a small circular patch (Fig. 97, BA), on the surface of the yolk, which is comparatively free from yolk-granules, and in which development can readily take place. This patch, the germinal disc, segments to form the blastoderm, a membrane composed of cells (Fig. 106), which lies like an inverted watch-glass on the surface of the yolk. The blastoderm rapidly increases in diameter, by growth all round its margin, and spreads so as to cover more and more of the surface of the yolk, which it ultimately incloses completely (Fig.s. 98, 99, 100, 101). Owing, apparently, to its less specific gravity, the germinal disc, and consequently the embryo, which is formed from its central part, lies at the top of the egg, and nearest to the body of the lien, however much the egg be rolled over.
Flu. yy. The yolk of a Hen's Egg at the end of the third day of incubation. The structure of the embryo at this stage is shown on a larger scale in Figs. 113 and 114. x ?.
AID. area pellucida of the blastoderm. AK, area opaca. AV. area vaseulo?a. EM. embryo. SM. vitelline membrane.
The central part of the blastoderm is thin and translucent, and is spoken of as the area pellucida (Fig. 98, AD) ; the marginal portion is thicker and less transparent, and is called the area opaca, AK ; the inner rim of the area opaca, bordering the area pellucida. is the seat of an abundant formation of bloodvessels, and is called in consequence the area vasculosa. AV.
The first trace of the embryo appears in the centre of the area pellucida, about the twentieth hour of incubation ; the formation of the embryo consisting essentially in a process of folding off, or constriction, of the central part of the area pellucida from the rest of the yolk. By the middle of the second day the embryo (Fig. 98) measures about 5 mm. in length, and has acquired definite shape ; the brain, spinal cord, heart, and other organs being already established (cf. Figs. Ill and 112).
FIG. 100. The Hen's Egg at the end of the fifth day of incubation, seen from the side. The embrj-o, which naturally lies with its left side on the yolk-sac and its right side towards the egg-shell, has been lifted up, in order to show its shape more clearly. The structure of the embryo at this stage is shown on a larger scale in Figs. 115 and 123. x |.
AN, inner or 'true' amnion. AV, outer margin of area vasculosa. AZ, outer or 'false' amnion, together with the vitelliiie membrane. EM, embryo. SH, egg-shell. SM, shell membrane. SV, air chamber. TA, allantois. YS, yolk-sac.
The embryo, at this stage, lies with its dorsal surface towards the shell, and its ventral surface towards the yolk. The axis of the body is straight, and is usually directed across the axis of the egg ; the head end of the embryo, in the majority of cases, pointing away from the observer if the egg is placed before him with the broader end to his left. There are, however, great variations in this respect, and the axis of the embryo may form almost any angle with that of the egg.
At thirty-six hours, the folding off of the embryo from the yolk-sac has only made slight progress ; the head of the embryo (Fig. 112) is lifted up above the yolk-sac by an anterior constriction or head fold, but the sides and tail end are as yet only very imperfectly denned.
By the end of the third day great advance has been made. The embryo (Fig. 99) has increased considerably in size. In the head, which has grown much faster than the body, and is now disproportionately large (Fig. 113), the nose, eye, and ear, and the several divisions of the brain, are well established. The head is no longer straight, but is strongly flexed, owing to the dorsal surface growing much more rapidly than the ventral. The heart and blood-vessels have acquired definite and characteristic arrangement. The folding off of the embryo from the yolk-sac has made considerable progress (Fig. 114) ; the head and neck are now quite free from the yolk-sac ; the hinder end of the embryo is lifted up from the yolk by a definite tail fold (Fig. 114, TL), and the side walls of the embryo are much more clearly defined. The yolk-stalk, connecting the embryo with the yolk-sac, is now a short tube, the diameter of which is about a third of the length of the embryo. The hinder end of the embryo still lies with its dorsal surface facing the egg-shell, and its ventral surface resting on the yolk-sac ; but the head and neck have rolled over, so as to lie with their left side 011 ' s the yolk-sac and their right side towards the egg-shell ; the axis of the body becoming spirally twisted in consequence (Fig. 113).
On the fourth day the folding off of the embryo makes further progress, and the yolk-stalk becomes greatly narrowed. The Avhole embryo becomes strongly flexed, the dorsal surface being convex along its entire length. The body, as well as the head, of the embryo now lies with its left side on the yolk-sac ; and the rudiments of the limbs have appeared as two pairs of small, ill-defined buds from the sides of the body.
By the end of the fifth day the embryo has acquired the shape and proportions shown in Fig. 100. In the natural condition, it lies with its left side on the yolk-sac, with which it is connected by the narrow tubular yolk-stalk. The whole embryo is strongly flexed, the convex dorsal surface being about four times the length of the concave ventral surface. The head is of relatively enormous size, chiefly owing to the great development of the brain vesicles and of the eyes. The limbs are still small, but have increased considerably in size as compared with the earlier stages, and already show indications of their division into segments (Fig. 115). From the under surface of the tail of the embryo a saccular diverticulum, with thin but very vascular walls, arises as an outgrowth from the alimentary canal : this is the allantois (Fig. 115, TA), a structure which grows very rapidly during the succeeding days, and forms the respiratory organ of the embryo.
FIG. 101. The Hen's Egg at the end of the ninth day of incubation, seen in vertical section. The embryo naturally lies with its left side on the yolksac, but has been lifted up in order to show its shape more clearly, x f.
AN", inner or 'true' auniion. HM, liyoinaiiilibular cleft. SV air chamber. TA ullantois. "WA, white or albumen. YS, yolk-.sac.
By the end of the ninth day the embryo has grown considerably, and has attained the shape and proportions shown in Fig. 101. The body walls are now definitely formed, and rudiments of the feathers are already present. The head is still disproportionately large, and the eyes are of enormous size. The beak, which was absent in the earlier stages, has now grown out from the front of the face, and at once gives the head a characteristic avian appearance. The neck is long and slender. The body is much more bulky than before, largely owing to the great size of the heart and the liver. The limbs have greatly increased in length ; their several segments are well established, and the division of the distal ends into fingers and toes is very evident. The white of the egg has almost disappeared, a thick and very viscid mass, WA, alone remaining at the lower surface of the egg. The yolk-sac, YS, is still large, but its walls are flabby, owing to the absorption of a large part of its contents as food by the embryo. The allantois, TA, has grown enormously, and has spread over the back of the embryo, and quite half way round the interior of the egg-shell. It lies close to the shell, so that respiratory interchanges can readily take place, by diffusion through the porous shell, between the gases of the blood, in the vessels of the allantois, and the air outside the egg. In this way the respiration of the embryo is effected.
During the second half of the period of incubation the changes are of less interest. The young chick steadily increases in size at the expense of the yolk-sac, and gradually acquires the proportions and characters which it has on hatching. About the fourteenth day it shifts its position so as to lie lengthways in the egg, rather than across it. On the twentieth or twentyfirst day the yolk-sac is nearly absorbed, and what remains of it is drawn into the body of the chick, the body walls closing over it at the umbilicus. The chick thrusts its beak through the shell membrane into the air chamber at the broader end of the egg, and for the first time draws air directly into its lungs. Invigorated in this way, it breaks through the shell, by means of a hard knob on the tip of its beak, and steps out into the world.
1 . Formation of the Egg
In the embryo fowl there are two ovaries, but in the course of development the right ovary disappears, and in the adult hen the left ovary is alone present. This is a large, irregularlyshaped body, suspended in a fold of peritoneum from the dorsal body- wall, opposite the anterior part of the left kidney. Numerous ova in different stages of development project from its surface, varying in size from dust shot up to spherical bodies an inch or more in diameter.
Of the two oviducts, the right one is rudimentary ; the left one, which alone is functional, forms in the adult hen a wide convoluted tube, which commences in front with a long, oblique, funnel-like mouth, bordered by a fimbriated edge, and lying in close contact with the ovary. Behind this mouth comes a long, convoluted, but thin-walled part of the oviduct, and then a short terminal part with very thick walls, which opens into the cloaca, and through this to the exterior.
The ovum at the time of its discharge from the ovary consists of the yolk alone, inclosed in the vitelline membrane. The albuminous investment, or ' white of the egg,' is formed around the yolk by the walls of the first, or thin-walled, part of the oviduct ; and the shell membrane and egg-shell are added while the egg is in the thick- walled terminal part of the oviduct, just before being laid.
The ovaries can be recognised in chick embryos during the third day of incubation, as a pair of slightly modified tracts of the peritoneal epithelium which clothes the dorsal wall of the body cavity, close to the root of the mesentery. This germinal epithelium is at first merely a longitudinal strip of peritoneum, of which the component cells are columnar instead of squamous in shape. By multiplication of the cells, to form a layer several cells thick, the strip becomes a prominent ridge. Vascular connective tissue soon grows in along the axis of this genital ridge, and renders it still more conspicuous.
Almost from the first, certain of the epithelial cells of the genital ridge differ from their fellows in their greater size and more spherical shape, and in possessing nuclei of unusual dimensions ; these larger cells are the primitive ova or gonoblasts. The primitive ova rapidly increase in size, and move from the surface, where they all take their origin, into the deeper parts of the genital ridge ; the smaller, indifferent epithelial cells at the same time becoming arranged so as to form follicles around them.
The follicular epithelial cells serve to nourish the ova, drawing nutriment from the blood-vessels of the genital ridge, and passing it on, probably after elaborating it, into the ovum. Within the ovum the food matter undergoes further changes, and is deposited in the form of granules, from which the definite yolkgranules of the fully-formed egg are finally derived.
During these changes the nucleus of the primitive ovum increases greatly in size, and acquires a distinctly vesicular structure, with one or more nncleoli : it is now spoken of as the germinal vesicle ; and the establishment of the germinal vesicle, together with the marked increase in size of the ovum, owing to the accumulation of yolk-rgranules within it, mark the conversion of the primitive ovum, which occurs in both sexes alike, into the permanent ovum characteristic of the ovary of the hen bird.
As the egg increases in size it forms a swelling on the surface of the ovary, which rapidly becomes more prominent. A vitelline membrane is formed round the egg, between it and the follicular epithelium, and apparently derived from the egg itself. The follicular epithelium, with the outer wall of the ovary, form a vascular capsule, investing the egg.
The accumulation of yolk-granules within the egg continues until this has reached its full size. From an early stage, a difference may be noticed between white yolk-spheres, and yellow yolk-spheres ; the former consisting of minute vesicles, each containing a highly refracting body ; while the latter, which are apparently derived from the white yolk-spheres, are much larger bodies, filled with numerous bright, highly refracting granules.
In the fully-formed egg the white and yellow yolk-spheres are arranged in a very definite manner. The yellow yolk make s up the greater part of the bulk of what we call the yolk of the egg ; the white yolk-spheres forming, (i) a somewhat flask-shaped plug in the centre of the yolk, with a neck reaching to the surface at the germinal disc ; (ii) a thin superficial layer investing the whole exterior of the yolk, immediately below the vitelline membrane ; i(iii) a series of thin concentric shells between the surface and the central plug, the spaces between the successive shells being occupied by the yellow yolk.
2. Maturation of the Egg
The ripening of the egg is accompanied by changes in the nucleus, which are as yet only imperfectly known.
The nucleus, or germinal vesicle, during the growth of the egg, is large and vesicular, and occupies a position at or close to the centre of the egg. As the egg ripens, the nucleus moves towards the surface, where it lies just beneath the vitelline membrane, in a small lenticular patch, the germinal disc, which is comparatively free from yolk-granules. The nuclear membrane disappears ; the chromatin elements form a reticular network, which then becomes distributed through the whole nuclear substance in the form of very fine granules ; and finally, these granules run together to form six chromatin rods. The further changes have not been followed with certainty in the hen's egg ; and neither the formation nor the extrusion of polar bodies has as yet been seen.
The egg, which is now ripe, is discharged from the ovary by rupture of the capsule at its most prominent part. The egg is received at once into the open mouth of the oviduct, which is closely applied to the ovary at the time, and then begins its passage down the oviduct to the exterior.
As it travels along the first or thin-walled part of the oviduct, the albumen, or ' white of the egg,' is poured out around it from the walls of the oviduct. The albumen is deposited as a continuous sheet, which is wrapped spirally round the yolk, owing to the egg being caused to rotate, in its downward passage, by spirally arranged folds on the inner wall of the oviduct.
This rotation of the egg causes the spiral twisting of the cords of denser albumen, at the ends of the egg, which are spoken of as the chalaza? (Fig. 97, we).
On reaching the lower part of the oviduct, or ' uterus,' the shell membrane, and finally the shell, are deposited on the outside of the egg, which is then passed into the cloaca, and laid.
The egg takes about three hours to travel along the thinwalled part of the oviduct ; in the uterus it remains for a variable time, estimated by different authorities as usually lasting from twelve to eighteen hours.
3. Fertilisation of the Egg
The details of the fertilisation of the hen's egg have not yet been determined. The large size of the egg offers great difficulties to the investigation of minute changes in connection with the nucleus, and these difficulties have not yet been surmounted.
All that is known with certainty is that fertilisation is effected, either in the upper part of the oviduct, or possibly, as stated by Coste, before the egg leaves the ovary ; so that during the whole time of the passage down the oviduct development is taking place. The spermatozoa are received by the hen some time before the laying of the eggs, and retain their vitality and functional activity for about a fortnight.
The Early Stages Of Development
1. Segmentation of the Egg
Segmentation commences about the time the egg arrives in the lower, thick-walled part of the oviduct, or uterus ; it is continued actively during the stay of the egg in the uterus, and is completed about the time the egg is laid.
Segmentation, as already noticed, does not concern the whole egg, but is confined to the germinal disc ; and the hen's egg is therefore spoken of as meroblastic, inasmuch as only a portion of it takes part in the process of segmentation, in contradistinction to the holoblastic eggs of Amphioxus and the frog, in which the entire egg is divided by the first cleft into two equal parts.
In the hen's egg, segmentation commences with the formation of a vertical groove or furrow, which runs across the middle of the germinal disc, but does not quite reach its edge at either end. This is very shortly followed by a second furrow, crossing the first one almost at right angles. Four radial furrows soon appear, about midway between the two first ones ; and then by cross furrows each segment becomes divided into a central and a peripheral portion. Additional furrows soon appear, both radial and concentric, and by these the germinal disc becomes cut up into a mosaic of segments of irregular shape and size, separated from one another by the furrows or grooves (Figs. 102, 103).
The segmentation is slightly excentric almost from its first commencement, the furrows extending nearer to the edge of the disc, and the segments being smaller, at one side (the lower side of Fig. 103), which corresponds to the future posterior end of the embryo ; while at the opposite, or anterior part, of the germinal disc the furrows stop short further from the edge, and the segments are of larger size.
Sections of the germinal disc at the stage represented in Fig. 103 show that, in addition to the vertical furrows by which the mosaic pattern is produced on the surface, horizontal clefts are also forming, by which the segments become completely isolated from one another, and from the underlying yolk (Fig. 104, ZA). These horizontal clefts, like the vertical ones, appear first in the centre of the germinal disc, and do not reach its margin until a later stage.
In each segment, or cell as it may now be termed, a nucleus is present from the first. The precise mode of origin of these nuclei has not been determined with certainty, but the history of the segmentation of the egg in Amphioxus, the frog, and other animals, leaves little doubt that the nuclei of all the cells are derived, by division, from the single segmentation nucleus of the fertilised egg.
FIG. 102. An early stage in the segmentation of the germinal disc of the Hen's Egg. (After Coste, and Duval.) x 10. FlG. 103. A later stage, in which the germinal disc has increased in size, and the segments have, by further division, become smaller and more numerous. (After Coste, and Duval). x 10.
Both these figures are from eggs taken from the lower part of the oviduct of the hen.
The result of the process of segmentation, up to the point shown in Figs. 103 and 104, is the formation of a cap, occupying the centre of the germinal disc, and consisting of a single layer of nucleated cells : of these, the central ones, ZA, are small, and completely isolated from their neighbours, and from the underlying yolk ; while the marginal ones, ZB, are larger, and are only imperfectly marked off from the yolk, the horizontal clefts having not yet appeared.
The process of segmentation soon extends into the deeper part of the germinal disc ; and by a further series of clefts, in different planes, this deeper part of the disc becomes cut up into cells, which from the first are nucleated, and are arranged in a layer two or three cells deep (Fig. 105, ZL).
In this way, shortly before the time of laying of the egg, the germinal disc becomes converted into a cap of cells, spoken of as the blastoderm (Fig. 105). Of these cells the uppermost or most superficial layer (Fig. 105, E), which was the first to be definitely established, constitutes the epiblast; it consists of a single layer of cells, and is separated by a very shallow space, the blastocoel or segmentation cavity, B, which appears in section
FIG. 104. Section through the germinal disc and adjacent parts of the yolk of a Hen's Egg about the middle of its stay in the uterus. The plane of section corresponds to a vertical line drawn through the centre of Fig. 103 ; the right-hand end of Fig. 104, which is the future anterior end, corresponding to the upper border of Fig. 103 ; and the left-hand end of Fig. 104 to the lower or posterior border of Fig. 103. (After Duval.) x 25.
N, nucleus of completed segment. N', nucleus of segment not yet completely separated from the yolk. VL, vacuolc. Y, yolk. ZA, completed blastomere. ZB, incompletely so para t ei 1 1 >la stouiere.
FIG. 105. Vertical section of the blastoderm and adjacent part of the yolk of a Hen's Egg towards the close of segmentation. The anterior edge is to the right, the posterior edge to the left hand. (After Duval.) x 25.
B, blast! ica-1 or segmentation cavity. E, epiblast. UP, nucleus of blastomere, which as yet is only incompletely separated from the yolk. VL, vacuole. Y .yolk. ZL, one of the lower-layer cells or blastomeres.
as a mere chink or split, from the deeper mass of cells which may be spoken of collectively as lower layer cells, ZL.
During the rest of the time that the egg stays in the uterus, while the egg-shell is forming, the process of segmentation continues actively. The clefts extend to the edge of the germinal disc ; which becomes sharply marked off from the yolk beyond it ; and, by rapid division, the cells become of nearly uniform size in all parts of the blastoderm.
The lower-layer cells become more sharply separated from the yolk, a space, filled with fluid, appearing beneath them, between the blastoderm and the yolk. This space, the subgerminal cavity (Fig. 106, BV), is sometimes spoken of as the segmentation cavity ; a name, however, which ought to be restricted to the narrow chink between the epiblast and the lower-layer cells (Fig. 105, B), which is clearly visible in the early stages, but becomes practically obliterated before the egg is laid.
Round its margin, new cells are still being cut out of the yolk, and added on to the blastoderm. Some of the cells which arise in this way, and lie between the edge of the blastoderm and the yolk, are markedly larger than any of the others, and are spoken of as formative cells (Fig. 106, ZF).
FIG. 100. Vertical section of the blastoderm and adjacent parts of the yolk of a Hen's Egg at the time of laying, but before the commencement of incubation. The anterior edge of the blastoderm is to the right, the posterior edge to the left side of the figure. (After Duval.) x 25.
BV, subgermiual cavity. E, epiblast. TT. hypoblast. N', nucleus iu yolk, round which a cell will be formed later. Y. yolk. ZF, formative cell. ZL, lower-layer cells.
2. The Blastoderm
a. The condition of the blastoderm at the time of laying- of the egg. The actual stage of development reached when the egg is laid depends on the length of time the egg remains in the uterus ; and this we have seen is subject to considerable variation. The following description will apply to an average case.
Naked-eye examination shows the blastoderm (Fig. 97, BA) to be a small circular patch, about 3'5 mm. in diameter, on the surface of the yolk : owing to its less specific gravity, the blastoderm is always uppermost, however much the egg be rolled over. The blastoderm consists of a marginal white rim, the area opaca. thickest at the posterior edge of the blastoderm (Fig. 106, ZF) ; and a central, circular, and more translucent portion, the area pellucida. Beyond the edge of the blastoderm (Fig. 97) the 3 T olk shows one or more broad concentric bands, alternately darker and lighter in appearance.
Sections of the blastoderm at this stage (Fig. 106) show that it consists of two distinct layers of cells, (i) The upper layer, or epiblast, E, is a continuous membrane, formed of small, short columnar cells, varying very little in size, and packed closely together side by side.
(ii) The lower layer consists of cells which are more loosely arranged, and which vary a good deal in shape and size in different parts. In the area pellucida, or middle portion of the blastoderm, they form a thin layer of somewhat flattened cells, H, only one, or at most two cells in thickness. At the margin of the blastoderm, or area opaca, the cells became more numerous and more spherical in shape, forming a thickened rim which rests on the underlying yolk, and in which the large formative cells, ZF, are found, especially near the posterior margin. In the yolk, on which the edge of the blastoderm rests, nuclei (Fig. 106, N^) are present, round which cells are formed at a later stage, and added on to the margin of the blastoderm.
Beneath the area pellucida, and separating it from the bed of yolk, Y, is the subgerminal cavity, BY ; a well-marked space, filled with fluid.
b. The growth of the blastoderm. Round the margin of the blastoderm the epiblast and the lower-layer cells are at first continuous with each other, but shortly before the laying of the egg this continuity is lost, except at the posterior border, where, as shown on the left-hand side of Fig. 106, the two layers are still continuous with each other at the time the egg is laid.
After incubation has commenced, the blastoderm spreads rapidly, retaining its circular shape. By the end of the first day of incubation it is about the size of a sixpence ; and by the end of the second day it has extended nearly half way round the egg; after this it proceeds more slowly, the complete inclosure of the yolk not being effected until about the seventeenth day.
In this spreading of the blastoderm (cf. Figs. 98 and 99) the peripheral part, or area opaca, grows much more rapidly than the central area pellucida ; the area opaca retains its circular outline, but the area pellucida (Figs. 98 and 99, AD) very early becomes oval, and then pyriform in shape, the broader end corresponding to the anterior end of the embryo.
The two layers of the blastoderm grow independently. The epiblast, after it has become free from the lower layer, extends slightly beyond this, so that its margin rests directly on the yolk ; its further spreading is effected mainly by division of the already formed cells, stimulated, no doubt, by absorption of nutriment from the yolk on which they are lying. The lower-layer cells, after separation from the epiblast, become directly continuous at their margin with the yolk, forming a thickened rim, spoken of as the germinal wall : the extension of the lowerlayer cells is effected principally by the addition of new cells cut out from the yolk, but partly also by division of the already formed cells, as in the epiblast.
3. The Hypoblast
A few hours after the commencement of incubation, the lower-layer cells undergo important changes, by which the hypoblast and mesoblast become established.
In the area pellucida, the majority of the lower-layer cells become flattened horizontally, and unite at their edges so as to form a continuous cellular membrane, the hypoblast ; a few isolated lower-layer cells are left between the epiblast and the hypoblast, which take part, as will be noticed immediately, in the formation of the mesoblast.
In the area opaca, or marginal part of the blastoderm, the differentiation of the hypoblast as a distinct cellular membrane occurs somewhat later ; and the hypoblast cells of this region, which are large, and cubical or slightly columnar in shape, differ markedly from the thin, pavement, hypoblast cells of the area pellucida.
4. The Primitive Streak
At the posterior border of the blastoderm, as noticed above, the fusion of the epiblast and the lower-layer cells persists longer than it does round the rest of the blastodermic rim ; and in the egg, at the time of laying, a crescentic opacity is visible at the posterior edge of the blastoderm, marking this line of fusion.
As the blastoderm grows, during the earlier hours of incu bation, this opacity becomes lengthened out into a linear band, the primitive streak, which, starting from near the centre of the blastoderm, extends backwards across the area pellucida towards its margin. The increase in length of the primitive streak is effected almost entirely by growth backwards of its hinder end, the anterior end lengthening very little, if at all.
The area pellucida grows more rapidly in its posterior than in its anterior part, and from about the fifteenth hour becomes pyriform in outline. The primitive streak keeps pace with the growth of the area pellucida ; and about the twentieth hour, when the area pellucida is markedly pyriform in shape (Fig. 107, AD), the primitive streak, PS, forms a well-defined opaque band stretching about two-thirds of the way across the area pellucida. The anterior end of the primitive streak is sharply defined ; the posterior end is less distinct, is often irregularly bent, and usually dies away a short distance before reaching the edge of the area pellucida. A shallow median furrow, the primitive groove, runs along the whole length of the primitive streak.
FIG. 107. A diagrammatic figure of the blastoderm of a Hen's Egg about the twentieth hour of incubation. (In part after Duval.) x 8.
AD. area pellucida : the part left white consists of epiblast and hypoblast alone ; in the hinder part of the pyriform area, covered by the light shading, mesobla.st is present us well. AK, area opaca. M, dotted line indicating the boundary of the me*>l)la<T. NP, neural plate, the first commencement of the central nervous system. PS, primitive streak.
Transverse sections of the blastoderm (Fig. 108) show that the primitive streak is formed by proliferation of cells from the under surface of the epiblast, in the median line. The cells grow downwards as a solid keel, which spreads out right and left as a horizontal sheet of cells, rs ; these are spherical in shape, rather closely packed together, and situated between the epiblast, E, and the hypoblast, n.
The primitive streak appeal's before any trace of the nervous or other systems of the embryo has commenced to form. The meaning of the primitive streak has been much discussed, but it is now generally agreed that it corresponds, at any rate in part, to the lips of the blastopore in the frog, which have become lengthened out, and fused together ; the primitive groove marking the line of concrescence of the lips of opposite sides of the blastopore. The anterior end of the primitive streak in the chick certainly corresponds to the anterior or dorsal lip of the blastopore in the frog ; but it is not quite clear whether the entire length of the primitive streak is to be compared to an elongated and drawn out blastopore, or whether the hinder part of it is not rather due to the peculiar method of spreading of the blastoderm, imposed on the chick embryo in consequence of the distension of the egg by the enormous mass of food-yolk which it contains.
FIG. 108. Transverse section across the blastoderm of a Hen's Egg about the twentieth hour of incubation, the section passing through the primitive streak about the middle of its length (ef. Fig. 107). x 200.
E, epiblast. H, liypobliist. M, mesoblast. PQ, primitive groove. PS, primitive streak.
5. The Mesoblast
The middle germinal layer, or mesoblast, gives rise in the chick, as in Amphioxus, in the frog, and in other animals generally, to all the connective tissue, vascular, muscular, and skeletal structures, as well as to the urinary and reproductive organs.
In the chick, the mesoblast cells have a less clearly defined origin than in Amphioxus or in the frog, and are derived from three distinct sources.
(i) In the hinder part of the blastoderm, some of the cells of the original lower layer are left, lying between the epiblast and hypoblast, on the establishment of the latter as a distinct and continuous membrane ; and these cells become mesoblast cells (Fig. 108, M).
(ii) In the middle and lateral portions of the area pellucida. about the time of appearance of the primitive streak, mesoblast cells are budded off freely from the upper surface of the hypoblast, and form a layer between the epiblast and hypoblast in this region.
(iii) The horizontal sheets of cells (Fig. 108, PS), which spread out right and left as the wing-like expansions of the primitive streak, and which, it will be remembered, are of epiblastic origin, also take part in the formation of the mesoblast.
As regards the cells themselves, those of groups (i) and (ii) agree with one another in. being usually of an irregular stellate shape (Fig. 108, M), and in being very loosely arranged. The origin of these two groups is very similar, though not identical ; the cells of the first group being derived from the lower-layer cells, formed by segmentation of the germinal disc ; while those of the second group arise directly from the hypoblast, after this is established as a distinct cellular membrane. It is not possible to draw a sharp line between the two groups, nor to determine in all cases to which group a given cell belongs. Speaking generally, the mesoblast of the body of the embryo itself is derived from group (ii), the cells of group (i) lying almost entirely in the extra-embryonic parts of the blastoderm.
The cells of group (iii) are derived directly from the epiblast, and are therefore of totally different origin to those of groups (i) and (ii). They also differ from these latter in their spherical form and more compact arrangement. They are at first (Fig. 108, PS) sharply marked off from the cells of groups (i) and (ii), but as the primitive streak spreads laterally, the cells composing it come into close relation with those of the other groups, and becoming at the same time less compactly arranged, and less regular in form, can no longer be distinguished from those of groups (i) and (ii). The cells of group (iii), or primitive streak mesoblast cells, lie almost entirely behind the embryo, and take but little share in its formation.
The mesoblast cells of all three groups soon become continuous, forming a sheet of somewhat loosely arranged and usually stellate cells, which at the twentieth hour of incubation has a shape and extent indicated by the strong dotted line, M, in Fig. 107. The two halves of the sheet are continuous with each other across the median line in the region of the primitive streak, PS, and behind it ; but in front of the primitive streak, in the region where the embryo is formed, NP, the two halves are separated in the middle line by the notochord, the description of which is given on the next page.
FlG. 109. A diagrammatic figure of the blastoderm of a Hen's Egg about the twenty-fourth hour of incubation. (In part after Duval.) x 8.
AD, area pellucida : the part left white is the proamnion, and consists of epiblast and hypoblast alone ; in the hinder part of the pyriform area pellucida. covered by the light shading, mesoblast is present as well. AK!, area opaca. BF, commencing fore-brain. M, dotted line indicating the limit to which the mesoblast has spread. MS, mesoblastic somite or protovertebra. NQ-, neural groove. PS, primitive streak.
For a more exact view of an embryo of this age see Fig. 110.
In the later stages, as the embryo appears, the mesoblast sheet spreads rapidly. It does not extend directly in front of the embryo, but grows forwards as two literal horns (Fig. 109), so that for a considerable time there is, immediately in front of the embryo, a transparent area of the blastoderm, AD, which consists of epiblast and hypoblast only, without any middle layer or mesoblast. This area, the proamnion, remains twolayered until about the middle of the third day of incubation, when the' two lateral horns of mesoblast gradually grow inwards to meet each other in front of the embryo.
As shown in Figs. 107 and 109, the mesoblast very early extends outwards beyond the area pellucida so as to underlie the inner zone of the area opaca ; this three-layered zone of the area opaca, represented by the dark shading in Figs. 107 and 109, is known as the area vasculosa, because the blood-vessels which absorb the yolk and carry it to the embryo are very early developed in it (c/. Figs. 98, 99, AV).
6. The Notochord
Before the sheet of mesoblast cells, spoken of above as group (ii), separates completely from the hypoblast, a distinction may be noticed in it between a median longitudinal rod of cells, and two lateral tracts. This median rod is the notochord (Fig. 117, CH) ; and the cells of which it consists are, from the first, more closely compacted than those of the lateral tracts.
The notochord sometimes remains attached to the hypoblast after the lateral mesoblastic sheets have completely separated from this ; in other specimens the entire sheet of cells separates as one continuous layer, which then divides into the median rod, or notochord, and the two lateral mesoblastic tracts.
The notochord of the chick has, accordingly, been described by some authorities as of hypoblastic, by others as of mesoblastic origin ; the component cells are, however, in all cases derived directly from the hypoblast, and the difference is merely in the relative times of separation of the notochord from the lateral sheets of mesoblast, and from the underlying hypoblast respectively.
The notochord lies entirely in the part of the blastoderm in front of the primitive streak ; its posterior end is, however, directly continuous with the anterior end of the primitive streak. Inasmuch as the primitive streak cells are continuous with the epiblast, and 'the notochord is, at any rate at first, continuous with the hypoblast, it follows that the three germinal layers, epiblast, mesoblast, and hypoblast, are directly continuous and fused with one another at -this point, which marks the hinder end of the chick embryo, and corresponds to the anterior or dorsal lip of the blastopore in the frog (cf. Fig. 60, B).
7. The Mesoblastic Somites and the Coelom.
The mesoblast of either side forms at first a continuous sheet of loosely arranged cells, which in trans verse section is somewhat wedge-shaped, being thickest next to the notochord and gradually thinning as it passes outwards towards the margin of the blastoderm (cf. Fig. 117, M).
About the twenty-first hour of incubation, the mesoblast cells become arranged more or less clearly in two layers, upper and lower, with a slight space between them. This splitting of the mesoblast. as it is termed, first appears in the part of the mesoblast beyond the embryo, but soon spreads inwards to the embryonic region, extending almost up to the notochord.
The cavity, formed in this way, by splitting of the mesoblast, becomes the coelom or body cavity of the chick. Of the two layers into which the mesoblast is split, the upper or outer is spoken of as the somatic layer, and the lower or inner as the splanchnic layer. From a very early period the somatic layer (Fig. 129, ME) becomes closely connected with the surface epiblast, forming with this the somatopleure or body wall ; while the splanchnic layer becomes similarly related to the hypoblast, and forms with this the splanchnopleure or wall of the alimentary canal (Fig. 129, MH).
A body cavity that is formed in this way, by splitting of the mesoblast into somatic and splanchnic layers, is spoken of as a schizoecel, in contradistinction to the enteroccel of Amphioxus, which arises as a series of hollow outgrowths from the enteron or primitive alimentary canal. Inasmuch as the mesoblast of the embryo is derived almost entirely from the hypoblast, as described above, the distinction between an enteroccel and a schizoecel may be said to consist in this : in the enteroccel the mesoblast arises as hollow outgrowths from the hypoblast, which subsequently become shut off from the gut, while the cavities of the outgrowths open into one another and become the ccelom of the adult. In the schizoecel, on the other hand, the mesoblast arises as two solid sheets, budded off from the hypoblast, in which the ccelom is formed at a later stage by splitting of the sheet into two layers, with a space between them. Of these two methods of formation of the ccelom there can be little doubt that the enteroccelic is the more primitive one, the schizocoelic the more modified.
Almost immediately after the splitting of the mesoblast is effected, about the twenty-second hour, a series of clear transverse lines, really vertical clefts through the mesoblast, appear in the embryo, extending outwards a short distance each side of the notochord ; these are quickly followed by a pair of similar but longitudinal clefts, which appear one along each side of the body, a little distance from the middle line. By these clefts the mesoblast of each side of the body becomes divided into a vertebral plate, alongside the notochord ; and a lateral plate, more peripherally placed ; the vertebral plate being further cut up by the transverse clefts into a series of somewhat cubical blocks, the mesoblastic somites or proto-vertebrae (Fig. 110, MS).
The mesoblastic somites appear first in the neck region, and increase rapidly in number during the last two hours of the first day, and the following two or three days. One or perhaps two pairs are formed in front of the pair which appears first ; the remainder are added on in succession at the hinder end of the series, as the embryo increases in length. At the twenty-fourth hour of incubation there are usually five or six pairs present (Fig. 110, MS); by the thirty-sixth hour (Fig. Ill) these have increased to about fifteen pairs ; at, the end of the second day there are twenty-seven or twenty-eight pairs, after which date the further increase takes place more slowly until, during the fourth day, the full number is established. The increase takes place in a very regular manner, and the number of somites present affords a convenient basis for estimating the age, and the grade of development, of embryos during the earlier stages of their formation.
The somites extend along the whole length of the neck, trunk, and tail, but are not formed in the head, in which no segmentation of the mesoblast occurs. In the case of the first three or four somites, the splitting of the mesoblast extends up to the notochord before the somites become marked off from the lateral plates ; and consequently the cavities of these somites communicate for a time with the ccelom or cavity of the lateral plate, though this communication is lost as soon as the longitudinal cleft is formed which separates the vertebral and lateral plates from each other.
The remaining somites, behind the first three or four pairs, do not communicate at any stage with the coelom, their cavities appearing independently, and after the separation of the vertebral from the lateral plates.
The further stages in the development of the mesoblastic somites will be described in a later section (p. 322).
8. The Amnion
The amnion is a fold of the somatopleure which rises up as a wall all round the embryo, a little distance from it, and, spreading over its back, forms a thin double membrane between the embryo and the egg-shell. Though a very characteristic structure, it is of only secondary importance, and gives rise to no part of the embryo itself.
The first trace of the amnion appears about the thirty-third hour, as a small crescentic fold immediately in front of the head of the embryo. This grows rapidly, and by the thirty-sixth hour (Figs. Ill, 112, AX), has extended back over the anterior end of the head as a transparent cap, formed by a double membranous fold.
This first formed part, or head fold, of the amnion consists at first of epiblast only, inasmuch as it arises from the proamnion, or part of the blastoderm immediately in front of the embryo, into which the mesoblast has not yet spread (cf. Fig. 109). During the latter part of the second, and the third day, the mesoblast gradually grows in from the sides, forming a thin lining to the amnion, which from this time is two-layered.
The head fold of the amnion extends backwards rapidly, and before the end of the second day covers over the whole of the head and neck region of the embryo. At the hinder end of the embryo a similar tail fold is formed during the second day ; and, a little later, side folds appear, connecting the head and tail folds together. The embryo is now completely surrounded by the amnion, which forms a low wall round its sides and tail, and extends backwards over the head and neck as a thin membranous cap.
Unlike the head fold, the side and tail folds of the amnion (cf. Fig. 129, AX) consist from the first of both epiblast and mesoblast ; i.e. are folds of the somatopleure, beyond the margin of the embryo.
During the third day the amnion grows rapidly on all sides, and by the close of the day (Fig. 114, AN, AX') has covered over the whole of the embryo, except a small patch near the hinder end. During the fourth day the side folds meet each other over the back of the embryo, which thus becomes completely covered by the amnion. As the amnion folds meet, they coalesce, the inner layers of the folds forming a continuous membrane, the inner or true amnion (Fig. 100, AX), which closely invests the embryo, and is continuous with the margin of its body wall (Fig. 129). The outer layers of the amnion folds also form a continuous membrane, the outer or false amnion (Fig. 100, AZ), which lies close beneath the vitellme membrane, and soon fuses with this, while peripherally it passes into tinlayer of somatopleure investing the yolk-sac.
The space between the inner or true amnion and the embryo is called the cavity of the amnion. It is filled with fluid, and is at first very small, the true amnion on the fourth and fifth days investing the embryo very closely (Fig. 100). During the following days, owing to accumulation of fluid within it, the amnionic cavity increases very considerably, forming a waterbath in which the embryo can move freely in any direction. During the later stages of incubation, muscle fibres are developed in the mesoblast of the amnion, which by their contractions rock the embryo to and fro within the egg.
The space between the inner and outer layers of the amnion (Fig. 100 AN and AZ) is, from the mode of formation of the amnion (Figs. 114, 129), continuous with the ccelomic space which lies between the two layers of the mesoblast, both within the embryo and in the extra- embryonic region of the blastoderm. By the sixth day the splitting of the mesoblast (cf. Fig. 100) has extended about half-way round the yolk-sac. The further extension of the splitting takes place much more slowly, and does not reach the lower pole of the yolk-sac until within a few days of the time of hatching.
About the tenth day (cf. Fig. 101), when the splitting of the mesoblast has extended about three-fourths of the way round the yolk-sac, a circular fold of somatopleure arises from near its ventral edge, and grows over the dense mass of albumen, WA, at the lower surface of the egg, inclosing this in much the same way as the amnion incloses the embryo at an earlier stage, and aiding in the absorption of this mass of albumen.
The formation of an amnion is a very characteristic feature in the development of the three higher groups of Vertebrates Reptiles, Birds, and Mammals. These same three. groups are also characterised by the presence, during the later stages of development, of an allantois. which plays an important part in the respiration of the embryo, and, in mammals, in its nutrition as well. The two structures, ammoii and allantois, are associated to this extent, that the space between the two layers of the amnion. gives the allantois a ready opportunity for free and rapid growth, and enables it to obtain a position close to the inner surface of the egg-shell (Figs. 100, 101, TA), where its respiratory efficiency is greatest. \
It would, however, not be right to regard the amnion as merely a provision to insure free growth of the allantois, for this would not explain how the amnion originated in the first instance ; and it must be remembered that all the characteristic stages in the development of the amnion are completed while the allantois is still in a very rudimentary condition. The amnion has probably to be explained quite irrespectively of the allantois.
The most satisfactory explanation of the formation of the amnion is that it is due, in the first instance, not to uprising of a fold of somatopleure, but to depression of the embryo into the yolk-sac ; the sinking of the embryo being due partly to its own weight, partly to the downward growth of the front part of the head caused by cranial flexure ; and perhaps in part to the resistance of the vitelline membrane, aided by the liquefaction of the yolk as this becomes absorbed for the nourishment of the embryo. The main purpose effected by the depression of the embryo is to remove it from the danger of pressure against the egg-shell, a consideration which has more weight in the case of Reptiles, in which group the amnion was first acquired, and in which the yolk often completely fills the egg-shell, than in their descendants, the Birds.
The Development of the Nervous System
1. General Account
The development of the nervous system of the chick is effected in practically the same manner as that of the frog. About the nineteenth or twentieth hour, almost immediately after the notochord has appeared, the epiblast in front of the primitive streak becomes thickened along the median line to form the neural plate (Fig. 107, XP).
During the next four or five hours, the anterior part of the area pellucida grows rapidly (Fig. 109) : the neural plate lengthens with it, and soon becomes considerably longer, and more prominent, than the primitive streak. A longitudinal neural groove (Fig. 117, XG) forms along its dorsal surface; this is at first shallow, but rapidly deepens by uprising of its borders as a pair of longitudinal ridges, the neural folds.
At their hinder ends (Fig. 110), the two neural folds diverge from each other, and embrace between them the anterior end of the primitive streak. In front, the neural folds rapidly increase in height, the neural groove between them becoming deeper in consequence. About the end of the first day (Fig. 110, XF), the two neural folds meet, in the region of the future hind-brain, converting the open groove into the closed neural tube (cf. Fig. 118); and this closure of the tube rapidly extends both forwards and backwards.
FIG. 110. A Chick Embryo at the twenty-fourth hour of incubation ; seen from the dorsal surface. Cf. Fig. 109, which shows the relations of an embryo of this age to the blastoderm, x 20.
AD, margin of area pellucida. HD, head of embryo. MS, mesoblastic somite or protoyertebra. JsFF, neural fold. NQ-, neural groove. PS, primitive streak. W, vitelline vein.
The anterior end of the head of the embryo is lifted up above the blastoderm by the head fold (cf. Fig. 112) ; and the neural folds are continued round this uplifted head to its under surface (Fig. 110), where they become continuous with each other in the median plane. A transverse section across the extreme anterior end of an embryo at this stage (Fig. 110) will cut the projecting neural folds, but no other part of the embryo, and will consist of two completely separate halves.
By the middle of the second day (Fig. Ill) the neural folds have met and fused, so as to complete the neural tube, along the whole length of the brain region ; the last point to close being in the position afterwards occupied by the pineal body. The fusion has also extended backwards along the greater part of the i-egion of the spinal cord, but at the hinder end of the embryo the two neural folds are still a little distance apart.
2. The Spinal Cord
The spinal cord, in the earlier stages of its development, is oval in transverse section (Fig. 129, NS) : its roof and floor, in the mid-dorsal and mid-ventral planes, remain thin ; but its side wails thicken, so as to reduce the central cavity to a narrow vertical slit.
In the side walls of the spinal cord a distinction is present, almost from the first, between (i) an inner layer of columnar ciliated epithelial cells, lining the central canal ; and (ii) the cells composing the rest of the thickness of the wall. These latter apparently do not give rise to either nerve-cells or nerve fibres, but become modified to form a supporting framework to the cord. The cells of this second group are from the first radially arranged, and during the second day they branch at their outer ends, the branches anastomosing with those of adjacent cells to form a delicate reticular framework.
In the meshes of this reticulum certain other cells, the neuroblasts, appear during the third day ; these are apparently derived, by direct modification, from certain of the columnar epithelial cells lining the central canal, which migrate outwards into the reticulum. Each neuroblast is at first bipolar, having a shorter process, directed inwards towards the central canal ; and a longer process which is directed outwards, and which by further growth becomes the axis cylinder of a nerve fibre. The axis cylinders thread their way through the meshes of the reticulum and reach the surface of the spinal cord, where some leave it to form the roots of the spinal nerves, while others run longitudinally along its outer surface to form the layer of white matter of the spinal cord.
From the third, or fourth, to about the tenth day, this process of development of neuroblasts and of nerve fibres proceeds rapidly. The iieurob lasts become the nerve cells of the spinal cord, the first cells to be established being those of the ventral cornua : their inner processes disappear, and from the bodies of the cells tine branching protoplasmic outgrowths arise at a later stage, which anastomose with those of neighbouring cells. As the nerve fibres increase in number, the layer of white matter on the surface of the spinal cord necessarily gains in thickness, and the spinal cord rapidly approaches the shape characteristic of it in the adult.
The central cavity of the spinal cord is at first a narrow vertical cleft (Fig. 129). The side walls of the dorsal half of this cleft come in contact with each other and fuse, so as to obliterate the cavity ; the ventral half of the cleft persists throughout life as the central canal of the spinal cord.
Of the two longitudinal fissures of the adult spinal cord, the ventral fissure is a median groove left between the ventral columns of white matter, as these increase in thickness ; it may be recognised on the sixth or seventh da}', and by the tenth day is a conspicuous feature in transverse sections of the spinal cord.
The dorsal fissure is formed in quite different fashion. The white matter of the dorsal surface grows down into the spinal cord, about the ninth day, as a pair of vertical plates ; these are at first separated by a thin median lamina of grey matter ; and it is by absorption of this median lamina that the dorsal fissure is foi'med. The absorption is a gradual one, and for some time the fissure remains bridged across by slender fibres, derived from the grey matter.
The neurenteric passages. In the floor of the neural canal, at the hinder end of the body, two or three pit-like depressions appear in the early stages of development, which, although they are usually incomplete, and only rarely open into the mesenteron. still appear to be homologous with the neurenteric passage in Amphioxus or in the frog.
Three of these depressions have been observed in chick embryos. They appear in succession ; the first one shortly before the end of the first day; the second one (Fig. 112, >'T) about the middle of the second day ; and the third one in the course of the third day.
FIG. 111. A Chick Embryo at the thirty-sixth hour of incubation; seen from the dorsal surface, x 20.
FIG. 112. A median longitudinal, or sagittal, section of a Chick Embryo at the thirty-sixth hour of incubation, x 20.
AM", head fold of the amiiion. BF, fore-brain. BH, hind-brain. BM, mid-brain. BO, optic vesicle. CH, uotochord. CP, pericardial cavity. El. auditory pit. GrF. fore-gut, or anterior jiortion of the mesenteron. TT, hypoblast. MS, mesoblastie somite or proto vertebra. NS, spinal cord. NT, neurenteric canal. PS, primitive streak. B.V, ventricular portion of the heart. SO, soiuatopleurc. SP, splaucliuopleure. TA. allaiitois. W, vitelline veins.
They are all three blind pockets, extending somewhat obliquely from the floor of the hinder end of the neural tube into a fused mass of cells just behind the notochord : this mass is really the anterior end of the primitive streak, and therefore corresponds to the anterior lip of the blastopore in the frog (c/. Fig. GO).
3. The Brain
The general history of the development of the brain in the chick is very closely similar to that already described in the frog.
At the commencement of the second day, and before actual fusion of the neural folds has taken place at any part of their length, the neural canal becomes dilated at its anterior end to form the anterior cerebral vesicle or fore-brain (Fig. Ill, BF), from which the optic vesicles, BO, arise almost at once as lateral outgrowths. Immediately behind the fore-brain, and separated from it by a slight constriction, is a second and rather smaller dilatation, the middle cerebral vesicle or mid-brain, BM.
The part of the brain behind the mid-brain, about half its entire length, is the hind-brain, BH; this consists of a series of vesicles, separated by slight constrictions, decreasing in size from before backwards, and passing without any limiting boundary into the spinal cord posteriorly. The vesicles of the hind-brain vary considerably in different specimens ; they are usually four or five in number, of which the two anterior ones, at any rate, appear to possess considerable constancy. Their mode of development, and their relations to the nerves and other structures, strongly suggest that they are each equivalent to a single vesicle, such as the mid-brain.
By the middle of the second day (Figs. Ill and 112) the brain is closed, by fusion of the neural folds, along its entire length ; the point where the folds last meet being at the summit of the fore-brain, in the position subsequently held by the pineal body.
The walls of the brain are at first of nearly uniform thickness in all parts ; and transverse sections of the brain are approximately circular in outline at all parts of its length.
In the following account the several parts of the brain will be considered in order from behind forwards, and the leading points in their development described.
The medulla oblongata is formed from the hind-brain, the central canal of this part of the brain becoming the fourth ventricle of the adult.
FIG. 113. A Chick Embryo at the end of the third day of incubation. Owing to the twisting of the fore part of the embryo, the head and neck are seen from the right side, and the hinder part of the body from the dorsal surface. The amnion has been removed. (Cf. Fig. 99.) x 20.
A, dorsal aorta. Al, first or mandibular aortic arch. A3, third aortic arch, in the first branchial arch. AC, carotid artery. AV, vitelline artery. BIT, thalamencephalou or fore-brain. BH medulla oblongata. BL, cerebellum. BM, mid-brain. BS, cerebral hemisphere. El, auditory vesicle. HC1, first branchial cleft. HC2, second branchial cleft. HM, hyo-maudibular cleft. MS, mesoblastic somite or protovertebra. NS, spinal cord. OC. optic cup. OF, olfactory pit. OL, lens. FN, pineal body. BA, auricle of heart. RT, truncus arteriosus. B,V, ventricle of heart. V V, vitelline veins.
The two arrows and crosses indicate the plane along which the section shown in Fig. 124 is taken.
The walls of the medulla oblongata are at first of nearly equal thickness all round ; but before the end of the second day (Fig. 121, BH) the dorsal wall or roof becomes very much thinner than the sides and floor. In the later stages this difference becomes increasingly marked ; and before the end of the third day (Figs. 113 and 114) the roof, which is now very wide, becomes reduced to a single layer of epithelial cells, entirely devoid of nervous matter ; a condition in which it remains throughout life. This thin roof soon becomes thrown into folds, which appear about the seventh day. and rapidly increase in depth, hanging down into the cavity of the medulla. Between the layers of these folds a network of vessels, which early appears on the outer surface of the roof, grows in to form the choroid plexus of the fourth ventricle (Fig. 116, XB).
The division of the hind-brain into a series of vesicles, which is very noticeable about the thirt} T -sixth hour (Fig. Ill), becomes less evident as the side walls thicken, through the formation of the white nervous matter ; and from the middle of the third day onwards it is barely perceptible.
The cerebellum is developed from the roof of the anterior vesicle of the hind-brain, immediately behind the well-marked constriction which separates the hind-brain from the mid-brain.
It appears towards the end of the second day, as a slightly marked transverse thickening of the roof of the hind-brain ; it becomes more conspicuous during the third, fourth, and following days (Figs. 113, 114, and 115, BL), but remains as a simple transverse band until a comparatively late stage of development.
About the eighth day, the cerebellum (Fig. 116, BL) becomes doubled transversely on itself; and at the same time it thickens considerably, its outer surface becoming slightly folded. From this time it steadily increases in thickness, and by further folding of its surface becomes more complicated in structure ; but up to about the sixteenth day it lies completely behind the optic lobes.
During the last few days of incubation the cerebellum enlarges considerably, growing forwards over the top of the mid-brain and between the optic lobes : by the time of hatching it has almost met the cerebral hemispheres, and has acquired the shape and proportions characteristic of the cerebellum in the adult bird.
The fact that the cerebellum remains for so long a time in the condition of a mere transverse thickening of the roof of the medulla oblongata, becomes of considerable interest when it is borne in mind that this is the- condition in which it remains throughout life in the frog, and in many fish.
FlG. 114. A median longitudinal, or sagittal, section through a Chick Embryo at the end of the third day of incubation. The amnion is represented by a dotted line. ( Cf. Fig. 113.) x 20.
A, dorsal aorta. AN", head fold of the amniou. AW, tail fold of the amnion. AV, vitelline artery. BH, fourth ventricle, or cavity of medulla oblongata. BL, cerebellum. BM, cavity of mid-brain, the future Sylviau aqueduct. BS, lateral ventricle, or cavity of the cerebral hemisphere. CH, notochord. DP, proctodteal pit. G-H, hind-gut, or posterior portion of the meseuteron. IN, infumlibulum. LG. lunir. NS, central canal of spinal cord. O, mouth. PN, pineal body. PT. pituitary body. RS, sinus venosus. B.T, truncus arteriosus. B,V. ventricle. SP. splanclmopleure. TA, allautois. TH, thyroid body. TL, tail. TO, oesophagus. TP, jiharyux.
The mid-brain undergoes comparatively slight changes. Up to the end of the fourth day it is approximately spherical in shape ; and, owing to its great size and the position which, through cranial flexure, it occupies at the apex of the head, it plays a prominent part in determining the shape of the embryo (Figs. 113 and 115, BM).
On the fifth day, the optic lobes begin to grow out as a pair of rounded swellings from the roof of the mid-bi-ain, separated by a median longitudinal groove. These steadily increase in size during the following days ; up to the sixteenth day they remain in close contact with each other, but during the last few days of incubation they become pushed apart by the forward growth of the cerebellum, and take up the position at the sides of the brain characteristic of the optic lobes in the adult bird.
The floor of the mid-brain, and the sides, ventral to the optic lobes, become greatly thickened by the formation of the crura cerebri. The cavity of the mid-brain becomes greatly reduced by this thickening of its floor and sides, and forms the Sylvian aqueduct of the adult.
The thalamencephalon is formed from the original anterior cerebral vesicle, or fore-brain (Fig. Ill, BF).
The roof and floor of the thalamencephalon remain thin throughout life, but the sides thicken very greatly to form the optic thalami, reducing the central cavity to a narrow vertical cleft, the third ventricle of the adult (cf. Fig. 116, BF).
The anterior wall of the thalamencephalon forms a thin and narrow band, the lamina terminalis (Fig. 116, BT), which lies between the roots of the two cerebral hemispheres : in connection with this, the anterior commissure is developed as a narrow transverse band of nerve fibres, running across between the basal parts of the hemispheres.
The roof of the thalamencephalon, like that of the fourth ventricle, becomes early reduced, along the greater part of its length, to a single layer of epithelial cells, devoid of nervous elements. About the middle of its length, the pineal body arises, at the commencement of the third day, as a hollow, rounded, median diverticulum : this is at first directed slightly backwards, but by the end of the third day becomes inclined forwards (Figs. 113 and 114, PN), and lies close beneath the external epiblast. In the later stages, the pineal body increases in size, becomes dilated at its distal end, and gives off a number of branching tubular diverticula. Its condition on the eighth day is shown in Fig. 116, PN.
FIG. 115. A Chick Embryo at the end of the fifth day of incubation, seen from the right side. The amnion has been removed, x 20.
BL, cerebellum. BM, optic lobe, formed from the mid-brain. El, auditory vesicle. HY, hyoid arch. LA, fore-limb or wing. LP, hind-limb or leg. NE, ganglion of first spinal nerve. ]STM, commissure connecting first and second spinal ganglia. OC, eye. OH, choroidal fissure. BV, ventricle of heart. TA, allantois. YK, yolk-stalk, cut short. I, olfactory nerve. Ill, third nerve, or motor oculi. V, fifth or trigeminal nerve. V, ophthalmic branch of trigeminal nerve. VII, seventh or facial nerve. VIII, eighth or auditory nerve. IX, ninth or glossopharyngeal nerve. X, tenth or pneumogastric nerve. X', visceral branch of pneumogastric nerve. X, commissure connecting pneumogastric nerve with the ganglion of the first spinal nerve.
In front of the pineal body the roof of the thalamencephalon is very thin, and becomes thrown into folds which hang down into the ventricle : between the layers of these folds numerous blood-vessels penetrate, to form the choroid plexus of the third ventricle (Fig. 116, XA).
Immediately behind the stalk of the pineal body, the posterior commissure is developed in the roof of the thalamencephalon, as a transverse band of nerve fibres connecting the two optic thalami; it is shown, though not lettered, in Fig. 116.
FIG. 116. A median longitudinal, or sagittal, section of the head and anterior part of the neck of a Chick Embryo at the end of the eighth day of incubation, x 10.
BB. 1 lasibranchial cartilage. BF, third ventricle, or cavity of the thalamencephalon. BL, cerebellum. BM, Sylvian aqueduct, or cavity of the luid-brain. BS, lateral ventricle, or cavity of the cerebral hemisphere. BT, lamina termiualis. BY, olfactory lobe of the cerebral hemisphere. CH, notochord. ES, aperture of Eu.stachiau tube. ET, ruesethmoid cartilage. FE, rudimentary feather. HB, basihyal cartilage. HR, ceratohyal. IN", infundibuluui. K, epithelial knob on beak. LB, trachea. LT, glottis. MC, Meckel's cartilage. NS, spinal cord. PN, pineal body. PT, pituitary body. PT', stalk of pituitary body. KG, parachordal cartilage. T^N", tongue. TO, oesophagus. VI, neural arch of first or atlas vertebra. V2, centrum of second or axis vertebra. XA, choroid plexus of third ventricle. XB, choroid plexus of fourth ventricle. I, notch in niesethruoid cartilage for olfactory nerve. II, optic chiasma.
The floor of the thalamencephalon is depressed ventralwards to form the infundibulum, which lies very close to the anterior end of the notochord, and early acquires intimate relations with the pituitary body. The infundibulum is already present on the second clay (Fig. 112) : during the third and following days it becomes much more clearly defined (Figs. 114, 123, IN); and about the eighth day (Fig. 116. IN), a pocket-like diverticulum arises from its floor, which is directed backwards, and becomes wedged in between the anterior end of the notochord and the pituitary body.
In front of the infundibulum the floor of the thalamencephalon becomes greatly thickened, in the later stages, by the development of the optic chiasma (Fig. 11G, n).
The pituitary body, though not really a part of the brain, is so intimately connected with this that it may conveniently be described here.
The pituitary body appears, towards the end of the second day, as a pocket-like diverticulum of the anterior angle of the stomatodaeum, or mouth invagination (cf. Fig. 114, PT) ; it lies wedged in between the anterior end of the mesenteron and the floor of the infundibulum, and its blind extremity is in close contact with the anterior end of the notochord.
On the formation of the mouth perforation, which places the stomatodseum in communication with the mesenteron, the pituitary body (Figs. 114, 123, PT) persists as a diverticulum from the roof of the mouth, with the same relations as before to the infundibulum and to the notochord.
During the succeeding days, while the face is being established and the beak is growing forwards prominently, the pituitary body, retaining its relations with the brain and the notochord, becomes left further and further back in the roof of the mouth.
At the eighth day its position and relations are shown in Fig. 116. The upper blind end, PT, has given off a number of branching tubular diverticula, which together form a rounded vascular mass, lying immediately below the infundibulum, IN, and in the pituitary foramen at the base of the skull, between the trabeculge cranii. The stalk of the pituitary body is still present as a narrow tube, PT', which opens into the roof of the mouth in the median plane, opposite the glottis, LT, and just in front of the opening of the Eustachian tubes, ES. By the twelfth day the stalk has become a solid rod of cells, and the communication between the pituitary body and the mouth is finally cut off.
The optic vesicles arise, early on the second day, as a pair of lateral outgrowths from the fore-brain (Fig. Ill, BO). They give rise, as in the frog, to the retina and the retinal pigment of the eye, and their developmental changes will be described in the section dealing with the formation of the eye (p. 275).
The cerebral hemispheres. About the middle of the second day, the fore-brain (Fig. Ill, BF) begins to grow forwards, in front of the optic vesicles, as an anterior, median outgrowth, the vesicle of the hemispheres. At the same time cranial flexure becomes pronounced (Fig. 112), owing to the dorsal surface of the head growing faster than the ventral surface ; the axis of the brain becoming a curved instead of a straight line, and the fore-brain being carried round to the ventral surface of the head. The curvature of the brain progresses rapidly ; the fore-brain (Fig. 113) becoming placed at right angles to the rest of the brain, and the mid-brain growing forwards so as to lie at the extreme anterior end of the head.
The vesicle of the hemispheres grows rapidly, both in length and width : during the third day the paired cerebral hemispheres arise from its anterior end as thin-walled outgrowths, separated by a median furrow. The hemispheres (Figs. 113, 115, and 123, BS) enlarge rapidly, growing upwards and forwards, and forming a pair of prominent rounded swellings at the anterior end of the head, very conspicuous in embryos of the third to the seventh or eighth day. From the ventral surface of their anterior ends the olfactory nerves arise at a very early stage.
From the eighth day onwards the hemispheres, though still increasing in size, become less conspicuous from the surface, owing to the forward growth of the face, and especially of the beak, which elongates rapidly and completely alters the shape of the head (Fig. 116). As the beak extends forwards, the anterior ends of the hemispheres, from which the olfactory nerves arise, grow out as a pair of small hollow buds, the olfactory lobes (Fig. 116, BY), from the ends of which the olfactory nerves run forwards to the nose. The walls of the hemispheres are at first thin ; in the later stages they thicken considerably, especially on the outer side of their hinder ends, where they form the corpora striata. The cavities of the hemispheres persist throughout life as the lateral ventricles of the brain, which retain their communication with the third ventricle, or cavity of the fore-brain, through a pair of narrow apertures, the foramina of Monro.
4. The Peripheral Nervous System
a. General Account. The nerves, both cranial and spinal, which compose the peripheral nervous system are entirely of epiblastic origin, and develop in a manner closely similar to that already described in the frog.
The nerves fall under two categories :
(i) The ganglionated nerves. These arise directly from the inner surface of the epiblast, as a pair of longitudinal neural ridges, along the margins of the neural plate. They appear before the neural tube is closed (Fig. .117, MA), and by the folding of its walls to complete the tube they get carried on to its dorsal surface, where they form a pair of bands (Fig. 118, NA), projecting outwards from the angles between the external epiblast and the walls of the neural tube. On the completion of the neural tube by fusion of its lips, the neural ridges separate from the surface epiblast, but remain in close contact with the dorsal surface of the tube (Fig. 119, >'B).
The neural ridges are at first continuous structures, from which the nerve ganglia arise as paired outgrowths ; these grow rapidly, extending outwards and downwards, and acquire their permanent roots of attachment by outgrowth of nerve fibres from the ganglion cells into the brain or spinal cord.
To this category belong the fifth, the seventh and eighth, the sensory roots of the ninth and tenth, with perhaps one or two of the other cranial nerves; and the dorsal or sensory roots of the spinal nerves.
FIG. 117. Transverse section across the body of a Chick Embryo at the twentyfourth hour of incubation. (Of. Fig. 110.) x 200.
CH, notochonl. E, epiblast. TT T liypoblast. M, niesoblasc. MA, commencing neural ridge. TfQ, neural groove. M"P, neural plate.
(ii) The non-ganglionated nerves. These arise as direct outgrowths from the nerve cells of the brain or spinal cord. The nerves of this category develop at a rather later period than those of the former one ; they are all motor in function, and to them belong the sixth, and perhaps some of the other cranial nerves, and the ventral or motor roots of the spinal nerves.
Certain of the cranial nerves cannot at present be referred with certainty to either category ; but in their cases our knowledge of the developmental history is incomplete, and further research is necessary before any definite statement can be made concerning their real nature.
With regard to the nerves definitely included in the first category, a distinction must be made between the cranial and the spinal nerves, similar to that already described in the frog. The cranial nerves, in their growth outwards, lie at first very superficially, just beneath the external epiblast. Near their distal ends they early acquire connection with localised thickenings of the external epiblast, situated about the horizontal level of the notochord, and just above the dorsal borders of the gill-clefts. From these thickened patches of epiblast, which are probably to be regarded as sense organs, cells are budded off into the nerves, which appear to take a direct part in their further development.
The spinal nerves, on the other hand, are from the first more deeply situated. They lie between the spinal cord and the muscle plates (Fig. 124, NE), and do not acquire the connections with the external epiblast which are characteristic of the cranial nerves.
b. The Cranial Nerves. The first trace of the cranial nerves appears, in the chick, in the region of the mid-brain, about the twenty-second hour. At this stage, slightly younger than that shown in Figs. 110 and 11 8, the neural folds have nearly met, in the region of the head and neck, but have not yet coalesced at any part of their length ; while in the body region the central nervous system is still a widely open groove. Only one or two pairs of mesoblastic somites are as yet present.
At the lips of the neural groove there is on either side a ridge-like outgrowth of epiblast cells, from the angle between the external epiblast and the wall of the neural canal. This outgrowth (Fig. 118, XA), which consists of cells more spherical in shape than those of the surface epiblast, or of the brainwall, appears first in the region of the mid-brain, but rapidly extends both forwards and backwards : forwards as far as the anterior part of the fore-brain ; backwards along the whole length of the hind-brain, and a certain distance down the spinal covd.
These outgrowths (Fig. 118, NA) are the neural ridges. As they arise before the lips of the neural canal have met, the neural ridges of the two sides are at first completely independent of each other. A few hours later, when closure of the neural canal is effected, the neural ridges separate completely from the external epiblast, but remain closely attached to the brain ; the ridges of the two sides at the same time coalescing with each other to form a continuous longitudinal band, the neural crest (Fig. 119, NB), extending along the dorsal surface of the brain.
FIG. 118. Transverse section across the head of a Chick Embryo at he twentyfourth hour of incubation, passing through the region of the mid-brain. (Cf. Fig. 110.) x 100.
B, cavity of the mid-brain. CH, notoclionl, not yet separated from the hypoblastic wall of the pharynx. E, eniblast. H, hypoblast. N A, neural rii Ige. RT, commencing heart. TP, pharynx.
Almost from its first appearance, and before the neural tube is closed, the neural crest becomes more prominent at certain places. These more prominent parts form paired outgrowths of the crest, and are situated opposite the widest parts of the cerebral vesicles. They are the rudiments of the cranial nerves (Fig. 119, NB), while the intervening narrower parts of the crest (Fig. 120, NB), opposite the constrictions between the cerebral vesicles, form commissural bands, which for a time connect together the successive pairs of nerve outgrowths.
In a typical cranial nerve, such as the facial or glosso-pharyngeal, the further changes are as follows. The nerve rudiment rapidly extends outwards, lying close beneath the external epiblast, but independent of this. Opposite the nerve, but at some little distance beyond the brain, and about the horizontal level of the notochord, a proliferation of the cells of the external epiblast takes place, forming a small, inwardly projecting knob.
FlG. 119. Transverse section across the head of a Chick Embryo at the fortythird hour of incubation. The section is taken immediately behind the auditory pits and the heart, and passes through the rudiments of the glossopharyngeal nerves, x 100.
A, aorta. BH, cavity of hind-brain. CH. notochord. E, epiblast. H, liypoblast. ME, soinatoplt'uric layer of mesoblast. MH. splanchnopleuric layer of niesobl.i.-t. NB, neural crest : the put shoivn in the figure gives rise later on to the jranjdm of the glosso-pharyngeal nerves. W, vitelline vein.
The nerve soon comes in contact with this knob, and fuses with it, close to its distal end. Cells are budded off from the knob into the nerve, which thus becomes reinforced from the epiblast. The exact fate of these cells is uncertain, but it is probable that they take part in the formation of the ganglionic thickening on the nerve.
The inner or proximal end of the nerve thins rapidly, and loses its connection with the dorsal surface of the brain, a connection which from the first has been one rather of close contact than of actual continuity. A little way beyond this point, however, the nerve acquires its permanent attachment to the brain, about half way down its side ; this attachment being effected by the outgrowth of processes from the cells of the nerve, into the substance of the brain.
This attachment is acquired by the seventh nerve about the end of the second, or early in the third day (Fig. 121, vn). Owing to the part of the brain dorsal to the nerve growing more rapidly than its ventral part, the root of attachment of the nerve becomes apparently shifted further downwards, towards the ventral surface of the brain, and by the end of the third day has acquired the position characteristic of the nerve-root in the adult. At the same time, changes occur in the trunk of the nerve. Owing to intrusion of mesoblast between the surface epiblast and the nerve, the latter becomes more deeply placed than in the early stages. The connection with the sensory patch of the surface epiblast persists, but becomes drawn out, as the nerve recedes from the surface, into a cutaneous branch of greater or less length. Beyond the origin of this cutaneous branch the nerve continues its growth, and by the end of the third day, or early on the fourth day, its main branches of distribution become definitely established.
FIG. 120. Transverse section across the head of a Chick Embryo at the fortythird hour of incubation. The section passes through the commencing auditory pits, and through the heart. x 100.
A, aorta. BH, cavity of hind-brain. CH, notocliord. El, commencing auditory pit. H, hypoblast. NB, neural crest ; the section passes through the narrow comniissural part of the crest, which connects the rudiments of the facial and auditory nerves with those of the glosso-pharyngeal nerves. HE, endothelial lining of heart. B.M, muscular wall o f heart. TP, pharynx.
These branches, in the case of the seventh or facial nerve, are closely connected with the hyo-mandibular cleft. They consist of a large hyoidean or post-branchial branch (Fig. 115, vn), which runs along the hyoicl arch ; and a smaller mandibular or prebranchial branch, which runs forwards over the dorsal end of the hyo-mandibular cleft, and then downwards a short distance along the mandibular arch.
The above account will apply to any one of what may be termed the typical cranial nerves. It will now be convenient to take the several cranial nerves one by one, and note the chief points in their individual development.
I. The olfactory, or first cranial nerve. Our knowledge of the development of the olfactory nerve in the chick is still incomplete in some respects. At the twenty-ninth hour the neural ridges extend forwards along the brain as far as the anterior end of the fore-brain, i.e. in front of the optic vesicles (cf. Fig. 111). There are reasons for thinking that it is from the anterior ends of the neural ridges that the olfactory nerves are, at any rate in part, developed ; but the point has not been proved by actual observation.
At the fiftieth hour, before the paired cerebral hemispheres have commenced to appear, the olfactory nerves may be recognised as a pair of short outgrowths, arising from the dorsal surface of the unpaired vesicle of the hemispheres, and running downwards and outwards towards a pair of slightly thickened patches of epiblast, on the under surface of the head, which form the earliest rudiments of the olfactory pits.
During the third day the cerebral hemispheres arise. These are, from the first, situated dorsally to the roots of the olfactory nerves; and, growing rapidly forwards and upwards (Fig. 113), they appear to drive the olfactory nerves down to the base or ventral surface of the brain. By the further growth of the cerebral hemispheres the original unpaired vesicle of the hemispheres becomes obliterated, or rather absorbed into the hemispheres, and the olfactory nerves from this time arise directly from the hemispheres. During the third day the olfactory pits deepen rapidly, and the distal ends of the olfactory nerves become continuous with the olfactory epithelium. The mode in which this connection is acquired is closely similar to that in which the typical cranial nerve acquires connection with the sensory patch of the surface epiblast, and it has been suggested, with much reason, that the olfactory epithelium may be homologous with one of these sensory patches.
The condition of the olfactory nerve at the end of the fifth day is shown in Fig. 115, I. The nerve, which is still very short, runs downwards and backwards from the under surface of the hemisphere to the olfactory pit.
On the seventh day, as already noticed, the beak begins to form ; and during this and the following days it grows forwards with great rapidity. The olfactory sacs become imbedded in the sides of the beak (Fig. 131, OK), and are carried forwards with the beak as it lengthens. This causes a change in the direction and in the relations of the olfactory nerves, which, previously quiescent and inactive, have now to elongate rapidly, in order to maintain the connection between the olfactory organs and the brain. This elongation is effected mainly by growth of the nerves themselves, but partly, as already explained, by pulling out of the anterior ends of the hemispheres, from which the olfactory nerves arise, to form the olfactory lobes (Fig. 116, BY).
It is very possible, therefore, though not yet proved, that the olfactory nerve is really comparable to a typical cranial nerve, such as the facial, in which the sensory cutaneous branch is the only one developed.
II. The optic, or second cranial nerve. The optic nerves in the chick are very generally described as being formed directly from the constricted necks, or stalks, of the optic vesicles, which connect these with the brain. If this be correct, the optic nerve is in no way comparable with the other nerves, cranial or spinal, but must be contrasted with all of these as being formed by direct modification of part of the brain walls.
There are, however, strong grounds for suspecting that, as in the frog (p. 139), the fibres of the optic nerve really arise in the retina, and grow inwards to the brain; the optic stalk affording the path along which they grow, but not itself taking any direct part in their formation.
The neural ridges, as already described, extend forwards along the whole length of the fore-brain, but they do not appear to take any part in the development of the optic nerves.
III. The motor oculi, or third cranial nerve. The third nerve is the only one which, in the adult bird, arises from the mid-braiu. The neural ridges appear first of.all on the top of the mid-brain, and early attain a great size in that position (Fig. 118, NA), but it is not yet clear what happens to these ridges in the later stages. It is possible that they take part in the formation of the third nerve, but this has not been proved to be the case.
The actual date of the first appearance of the third nerve has not been determined. About the middle of the third day it is clearly visible as a nerve of rather large size (Fig. 124, in), arising from the base of the mid-brain, not far from the middle line, and running backwards and downwards towards the hinder border of the eye.
By the fifth day (Fig. 115, in), the third nerve lias the characteristic course of the adult nerve, arising from the floor of the mid-brain and running downwards and backwards immediately behind the eye.
There are strong reasons for regarding the third nerve as corresponding to at any rate a part of a typical cranial nerve, but until its early development is more clearly ascertained it is impossible to speak definitely with regard to it. Its origin from the base of the brain, close to the median plane, its distribution to muscles, and the fact that its root in the early stages (Fig. 115, in) is multiple, have led most investigators to compare it with the ventral root of a spinal nerve rather than with the dorsal root.
The ciliary ganglion is stated to be formed in the chick in connection with a knob-like thickening of the surface epiblast, similar to the sensory patch of a typical cranial nerve.
IV. The fourth cranial nerve. The fourth nerve in the adult is peculiar, inasmuch as it is the only nerve which arises from the dorsal surface of the brain, and also, so far as is known, the only nerve which arises from a constriction between two brain vesicles instead of from the middle of a vesicle.
In a chick embryo of the fifth clay the fourth nerve is easily recognised. It is very slender, but has already the course and relations characteristic of the nerve in the adult bird. Its development in the chick is unknown.
V. The trigeminal, or fifth cranial nerve. The trigeminal nerve arises from the neural ridge on the first or most anterior of the vesicles of the hind-brain, and its development accords exactly with that of a typical cranial nerve as described above. The ganglion of the trigeminal nerve, or Gasserian ganglion, is formed mainly from a portion of the neural ridge, reinforced from an independently arising knob of the surface epiblast. The permanent attachment of the nerve to the side of the hind-brain is acquired at the commencement of the third day ; and about the same time the nerve divides distally into ophthalmic and mandibular branches, of which the former (cf. Fig. 115, v') runs forwards along the inner side of the eyeball to the front of the head, while the latter, v, runs downwards and backwards in the mandibular arch. From, the mandibular nerve, the maxillary nerve arises on the third day as a branch (cf. Fig. 115), which runs forwards in the maxillary arch or upper jaw.
The development of the motor root of the trigeminal nerve 'in the chick has not been determined satisfactorily, and it is not yet certain whether this is a part of the original nerve, or whether, as seems more probable, it arises independently as an outgrowth from the brain itself.
VI. The sixth cranial nerve. The sixth nerve is of a very different nature to the trigeminal or facial nerves, and in its mode of origin and relations agrees more closely than any of the other cranial nerves with the ventral or motor root of a spinal nerve.
It appears during the fourth day, arising from the base of the hind-brain, near the median plane, by a number of very slender rootlets, the most anterior of which is on a level with the hinder part of the root of the trigeminal nerve, and the most posterior one opposite the root of the facial nerve. The rootlets unite together to form a slender nerve, which runs forwards below the base of the brain to the external rectus muscle of the eyeball, in which it ends.
VII. The facial, or seventh cranial nerve arises from the neural crest on the top of the second vesicle of the hind-brain ; its development has already been described as that of a typical cranial nerve.
VIII. The auditory, or eighth cranial nerve (Fig. 115, vin) is, in the chick, continuous with the facial nerve from its first appearance. It is a short stout nerve, which at a very early period, about the fiftieth hour, comes in contact with the auditory epithelium, and fuses with this. The subsequent development of the nerve consists mainly in its division, distally, into branches supplying the several special patches of the auditory epithelium, and will be described more fully in the section dealing with the development of the ear.
So far as the chick is concerned, there appears to be no reason for separating the facial and auditory nerves from each other. The two together make up a typical cranial nerve, of which the auditory nerve represents the cutaneous branch, greatly hypertrophied in consequence of the large size and importance of the sensory patch, i.e. the internal ear, -which it supplies.
IX. The glosso-pharyngeal, or ninth cranial nerve (Figs. 115, IX, and 119, NB), is at first continuous with the pneumogastric or tenth nerve, a single elongated strip of the neural ridge on the roof of the hind-brain, immediately behind the ear, giving origin to both these nerves. The strip divides, before the end of the second day, into an anterior or glosso-pharyngeal portion, and a posterior or pneumogastric portion.
The glosso-pharyngeal develops as a typical cranial nerve ; it early acquires connection with a sensory patch of the surface epiblast, and its main stem, beyond this point, runs downwards along the first branchial arch (Fig. 115). The root of attachment of the nerve to the brain early becomes multiple, consisting of four or five small rootlets, which spread out in a fan-like manner on entering the brain. The multiple character of the roots of the glosso-pharyngeal nerve is of interest, as showing that the similarly multiple nature of the roots of the third nerve is not incompatible with a possible origin of this latter from the neural ridge.
X. The pneumogastric, or tenth cranial nerve (Fig. 115, x) arises from the posterior part of the outgrowth from the neural ridge, common to it and the glosso-pharyngeal nerve. At first the pneumogastric is, if anything, the smaller of the two nerves, but it soon becomes distinctly the larger. Like the glossopharyngeal nerve, it early acquires multiple roots, the most anterior of which is directly continuous with the hindmost of the roots of the glossopharyngeal nerve, without entering the brain.
Beyond the roots of origin, the main stem of the pneumogastric nerve runs downwards and backwards, parallel to the glosso-pharyngeal nerve; it expands into a large fusiform ganglion, from which branches are given off to the second and third branchial arches, as well as large branches to the heart, lungs, and intestines.
From the hindmost root of origin of the pneumogastric nerve from the brain, a long commissural branch (Fig. 115, x") runs backwards along the side of the medulla oblongata, and is continuous posteriorly with the ganglion of the first spinal nerve. This commissural branch is derived from the part of the neural ridge between the pneumogastric and first spinal nerves.
The mode of development of the spinal accessory or eleventh cranial nerve, and of the hypoglossal or twelfth cranial nerve, has not been satisfactorily determined in the chick. The hypoglossal nerve has, from the first, the relations characteristic of the ventral roots of the spinal nerves ; though whether it corresponds to one, or to more than one, of such roots is not determined with certainty.
It is interesting to note that the definite relations of the fifth, seventh, ninth, and tenth cranial nerves to. the visceral arches are as characteristically shown in an embryo chick of the fifth day (Fig. 115) as they are throughout life in a typical waterbreathing Vertebrate such as a dogfish.
c. The Spinal Nerves
The dorsal roots of the spinal nerves develop, as already noticed, in a manner practically identical with the typical cranial nerves. Their first appearance is almost simultaneous with that of the cranial nerves ; they may be recognised in embryos in which the first two or three pairs of mesoblastic somites are alone present (cf. Fig. 110), and sometimes even prior to the definite formation of any of the somites.
In the anterior part of the spinal cord the neural ridges appear, just as in the brain, as cellular proliferations from the reentering angles between the external epiblast and the lips of the neural plate, which latter have already grown in towards each other a certain distance. The neural ridges of the spinal cord are directly continuous with those of the brain, and from them the ganglia of the spinal nerves are derived as paired outgrowths. The spinal ganglia differ, however, from the cranial ganglia in not acquiring any distal connection with sensory patches of the epiblast, and in being from the first much more deeply situated, growing downwards close alongside the spinal cord, between this and the muscle-plates (Fig. 124, IS T E).
During the third day, the spinal ganglia acquire their definite attachments to the sides of the spinal cord, these being effected by the outgrowth of nerve fibres from the inner sides of the ganglia into the cord. The parts of the ganglia above, or dorsal to, the points of attachment persist for some time as small pointed processes, but soon become inconspicuous, and are finally absorbed into the ganglia.
The spinal ganglia are of considerable width, more than half the width of the somites to which they belong (Fig. 115, XE), so that the intervals between successive ganglia are distinctly less than the width of the ganglia themselves. The ganglia lie, from the first, opposite the anterior parts of the somites to which they belong.
The ganglia of the anterior part of the body are connected together by short commissural bands (Fig. 115, NM), situated at the same level as the attachments of the ganglia to the spinal cord ; and the most anterior spinal ganglion, as already noticed, is connected with the hindmost root of the pneumogastric nerve by a similar but much longer commissure (Fig. 115, x"). These commissures appear to be formed from the parts of the originally continuous neural ridge which are left between the successive ganglion outgrowths ; they are well developed and conspicuous structures on the fourth and fifth days, but after the latter date are difficult to detect.
The spinal nerves of the hinder part of the cord develop in slightly different fashion to those of the anterior part. They appear at a slightly later date, but at a relatively earlier stage in the formation of the spinal cord. At a time when the neural canal has hardly commenced to form, and the neural plate is only very slightly folded on itself in the middle line, the nerve rudiments may be recognised in transverse sections (Fig. 117, MA), as small conical masses of cells, cut out from the deeper part of the epiblast, at the edges of the neural plate. As the neural folds rise up, and grow in towards each other, the nerve rudiments are carried up with the folds to the dorsal surface of the spinal cord, arid then complete their development in the manner already described.
In the posterior half, or so, of the body there appears to be no continuous neural ridge developed, the nerve rudiments being, from the first, independent outgrowths. There are consequently no longitudinal commissures connecting these hinder nerves, similar to those in the anterior part of the body (Fig. 115). These commissures disappear in the anterior part of the body shortly after the fifth day, and it is possible that their absence in the hinder part of the body is to be explained as due to abbreviation of the developmental history, by omission of this stage.
The ventral roots of the spinal nerves arise later than the dorsal roots, during the latter part of the third day. They appear as small outgrowths from the lower part of the sides of the spinal cord, and from the first occupy the position held by them in the adult. This position is indicated, before the actual appearance of the root, by a slight convergence of the cells at the side of the cord ; and the nerve root is apparently formed by the direct outgrowth of processes from these cells, which, passing out from the side of the spinal cord, become the axis cylinders of the nerve fibres.
Each ventral root arises by a number of separate rootlets, which leave the spinal cord in a longitudinal series, the total length of a root being about equal to half that of a somite ; the root lies opposite the anterior half of the somite, and vertically below the corresponding dorsal root.
Towards the end of the third day (Fig. 124), the ventral roots, growing downwards and outwards, meet the dorsal roots, and with these form the trunks of the spinal nerves. Beyond the place of union of the roots the nerves continue their growth outwards and downwards, lying along the' inner surfaces of the muscle plates. By the end of the fourth day the nerves have doubled in length, and the primary dorsal and ventral divisions are already established, each division including fibres from both the dorsal and ventral roots.
In the part of the body between the fore and hind limbs (cf. Fig. 115), the main branches of the nerves run in the body wall or somatopleure. In the segments opposite the limbs, the nerves enter the limbs and divide into dorsal and ventral branches, which unite with the corresponding branches of the nerves in front of, or behind, them to form broad plates of nerve fibres, from which the individual nerves of the adult limb arise.
d. The Sympathetic Nervous System
The origin of the sympathetic nervous system in the chick has been much debated, and is not yet satisfactorily determined. The most trustworthy observations are to the effect that the sympathetic nervous system arises at an early stage, the third or fourth day, as a series of outgrowths from the spinal nerves, apparently derived directly from the spinal ganglia. These grow inwards, at a level immediately above the cardinal veins, and close to the dorsal aorta : at their ends are ganglionic enlargements, the nervecells of which are apparently derived, by direct migration, from the spinal ganglia. These ganglionic enlargements soon become connected, along each side of the body, by longitudinal commissures, apparently formed by outgrowths of nerve-fibres from tfye ganglia themselves.
If this account is correct, the sympathetic nervous system of the chick is to be regarded merely as a specialised part of the spinal nervous system.
Development of the Sense Organs
The general history of the development of the sense organs in the chick is very similar to that already described in the frog. In all cases the essential part of the organ, the actual sensitive surface itself, is derived directly or indirectly from the epiblast or epidermis.
1. The Nose
The olfactory organs appear, about the fiftieth hour, as a pair of thickened patches of the external epiblast on the under surface of the fore part of the head ; these soon become depressed, forming pits (Fig. 113, OF), with the bottoms of which the olfactory nerves very early become connected (p. 266).
The mouths of the olfactory pits narrow, and become slitlike, but remain open throughout life as the external nostrils (Figs. 125, 126, OK).
The epithelial lining of each olfactory pit becomes thrown into folds, to increase its surface ; and gives rise directly to the olfactory epithelium, or Schneiclerian membrane, of the adult nose.
The posterior narial passage is a secondary formation ; it appears at first as a groove on the under surface of the head, leading from the edge of the olfactory pit to the anterior and outer angle of the stomatoda3um. This groove is well marked on the fourth day, its inner lip being formed by the fronto-nasal process (Fig. 1 25, FP) , or median part of the face, between the two olfactory pits ; and its outer lip being formed by the maxillary arch (Fig. 125, MX), or rudiment of the upper jaw.
On the fifth day (Fig. 125) the olfactory groove deepens, and its inner and outer lips, formed by the fronto-nasal process and maxillary arch respectively, meet and coalesce, so as to convert the groove into a tube, leading from the olfactory pit to the mouth. This tube is the posterior narial passage ; it at first opens into the anterior end of the mouth cavity, immediately behind the upper lip ; but as the mouth elongates, by growth forwards of the beak, a horizontal shelf-like partition is formed at the anterior end of the upper jaw on either side. By fusion in the median plane, the two horizontal partitions form the palatal septum, which stretches across the anterior part of the mouth, separating the olfactory or nasal region above from the buccal cavity below, and shifting backwards the communication between the posterior nostrils and the mouth.
2. The Eye
As in the frog, and in Vertebrates generally, the retina or essential part of the eye is formed from the optic vesicle, while the lens is an independent invagination of the surface epiblast.
The optic vesicles arise, at the commencement of the second day, as a pair of hollow lateral outgrowths from the fore-brain ; they grow rapidly, and attain some size before the lips of the neural folds fuse to complete the neural tube. The optic vesicles at first stand out at right angles to the head, but they soon become constricted at their bases, and directed somewhat downwards and backwards (Fig. 1 1 1 , BO). These constrictions rapidly deepen, so that by the end of the second day the optic vesicles are connected with the floor of the fore-brain by narrow tubular stalks (Fig. 121, os).
FIG. 121. Transverse section across the head of a Chick Embryo at the fort)'eighth hour of incubation. The section is taken along a line corresponding to one joining the reference letters EI and OL in the three-day embryo shown in Fig. 113. Owing to the cranial flexure, both fore-brain and mid-brain are cut by the section. The right side of the section is slightly anterior in position to the left side, x 60.
A, aorta. AC, carotid artery. BF. cavity of fore-brain. BH, cavity of hind-brain. , notochord. EI, auditory pit. HM, hyo-maiulibular cleft. Itif, mandibular arch. OC, cavity of optic cup. OL. invagination of epiblast to form the lens. OS. optic stalk. PT, pituitary body. TP. pharynx. VII, facial nerve.
Towards the end of the second day a circular patch of the external epiblast, opposite the outer wall of each optic vesicle, becomes thickened, and shortly afterwards pitted in to form the vesicle of the lens (Fig. 121, OL). The formation of this pit is accompanied by an infolding of the outer wall of the optic vesicle, which thus becomes doubled on itself to form the optic cup (Fig. 121, oc).
The lens. The pitting-in of the epiblast, to form the lens, rapidly deepens ; the lips of the pit close in, and unite, so as to convert the pit into a closed sac, the lens vesicle, which separates completely from the external epiblast during the third day. After this separation, the outer wall of the lens vesicle remains thin, and is formed of a single layer of flattened epithelial cells ; the inner wall thickens rapidly, by elongation of its component cells (Fig. 122, OL) ; and by the fourth day it comes in contact with the outer wall, so as to obliterate the cavity of the vesicle entirely. From the epithelial cells of this thickened inner wall the whole of the substance of the adult lens is derived. The outer, thin wall of the lens vesicle becomes the epithelial lining of the lens capsule ; while the lens capsule itself is apparently a cuticular membrane excreted by the epithelial cells of the lens vesicle.
FIG. 122. Transverse section across the fore-brain and eye of a Chick Embryo at the sixtieth hour of incubation. On the right side the section passes through the optic stalk ; on the left side it passes just behind the stalk. x45.
BF, cavity of fore-brain. BS, cavity of commencing cerebral hemisphere. MXmaxillary arch. OC, inner wall of optic cup. OD, outer wall of optic cup. OL, lens. OS, optic stalk.
The optic cup. In the optic cup important changes occur. The two layers of the cup soon come in contact with each other (Fig. 122, OC, OD), and by the end of the third day the original cavity of the optic vesicle is practically obliterated. The whole cup grows rapidly ; its lip remains in contact with the margin of the lens the whole way round, except at one point on the under surface of the cup, below the reference line, OL, in Fig. 122, where a small chink is left between the lens and the lip of the cup. As the optic cup increases in size, this chink becomes lengthened out into a slit, the choroidal fissure (Figs. 113, 115, and 125, OH), through which the mesoblast of the head gains admittance into the cavity of the cup.
From the wall of the optic cup the retina is developed, while the mesoblast which grows into the cavity of the cup, through the choroidal fissure, gives rise to the vitreous body. The choroid and sclerotic coats of the eye are formed from the mesoblast outside the optic cup, and the cornea from mesoblast which grows in between the lens and the surface epiblast.
The exact mode of formation of the choroidal fissure is difficult to determine. The first step in the doubling up of the optic vesicle to form the optic cup (Fig. 121) is intimately associated with the ingrowth of the surface epiblast to form the lens vesicle, and is perhaps due, in part, to mechanical in-pushing by this latter : the later stages of the doubling up, however, concern the optic cup alone, and must be regarded as due to unequal rates of growth of different parts of the wall of the cup. This unequal rate of growth in different directions probably plays an important, or even predominant, part in the formation of the choroidal fissure. The formation of the choroidal fissure has been recently shown to be closely associated with the growth of the fibres of the optic nerves ; these fibres passing through the choroidal fissure on their way from the retina towards the brain.
The choroidal fissure only remains open for a short time. About the sixth day its lips come in contact, and very shortly afterwards they fuse together, so as to complete the closure of the optic cup ; by the ninth day all trace of the fissure has disappeared.
The retina is formed directly from the wall of the optic cup. Of the two layers of which the doubled-up wall of the cup consists, the inner (Fig. 122, oc) is from the first much the thicker. It consists, on the third day, of elongated nucleated cells arranged side by side, and vertically to the surface. From the fourth day onwards it increases rapidly in thickness, and by a series of histological changes which have not yet been determined very accurately, it becomes converted into the several layers of the retina. The inner layers of the retina are the first to be established, and the last elements formed are the rods and cones ; these latter growing outwards as processes from the outer nuclear layer, which until the appearance of the rods and cones is the outermost layer of the retina.
The outer wall of the optic cup (Fig. 122, OD) is, from the first, much thinner than the inner wall. By the middle of the fourth day it is reduced to a single layer of flattened cells, which soon become pigmented, and ultimately give rise to the layer of special pigmented cells which lie in close contact with the outer ends of the rods and cones.
It appears, therefore, that the whole of the sensory part of the retina is derived from the inner layer of the optic cup. It is worthy of notice that the rods and cones, the only elements of the retina directly sensitive to light, are the last parts to be formed.
The optic nerve is very commonly said to be formed from the optic stalk ; but it is more probable that it arises in the chick, as it is known to do in the frog (p. 138), by the formation of processes from cells in the retina, which grow inwards along the optic stalk to the brain, and become the fibres of the optic nerve.
The iris. The marginal part of the optic cup, nearest to the lens, does not become converted into the retina, but undergoes changes of a different character, the boundary between the retinal and non-retinal parts being indicated by the ora serrata. The inner and outer walls of this marginal, or non-retinal, part of the cup coalesce completely, and become pigmented throughout their whole thickness. They become closely connected with the choroid coat, on their outer surface ; and the combined choroid and retina grow forwards, in front of the lens, to form the iris, which reduces the mouth of the optic cup to a comparatively narrow aperture, the pupil.
The pecten arises on the fifth day as a lamellar process of mesoblast, which grows into the cavity of the optic cup through the choroidal fissure, close to the optic nerve. It early becomes very vascular ; about the tenth day it becomes folded in the fan-like manner characteristic of the adult ; and toward the close of incubation it becomes densely pigmented.
The cornea is formed from mesoblast, which grows in between the lens and the surface epiblast, at first as a ring, but soon becoming a continuous layer across the front of the eye. It is at first structureless, but cells from the mesoblast round its edge soon grow inwards into its substance to form the corneal corpuscles. These corpuscles are confined to the middle layer of the thickness of the cornea, the outer and inner surfaces remaining structureless as the anterior and posterior elastic membranes of the cornea respectively. The surface layer of epiblast persists as the conjunctival epithelium.
The anterior chamber of the eye forms as a space between the cornea and the lens ; and in it a watery fluid, the aqueous humour, soon collects.
The accessory organs of the eye. The eyelids are folds of the integument round the eye : there are three of them, an upper and a lower eyelid, and the third eyelid or nictitating membrane (Fig. 126, 'CD), which arises on the inner or nasal side of the eye. The lacrymal glands are solid ingrowths of the conjunctival epithelium, which appear on the eighth day. The lacrymal duct is also at first solid ; it appears as a ridge of epidermis, along the line of the lacrymal groove, extending from the eye to the olfactory pit (Fig. 125). This ridge sinks into the mesoblast, and soon splits off from the epiblast along the greater part of its length, but remains attached at its ends to the lower eyelid and to the wall of the olfactory pit respectively. About the twelfth day it acquires a central lumen, and becomes the tubular duct.
3. The Ear
The ears appear, about the middle of the second day, as a pair of shallow depressions of the external epiblast at the sides of the hind-brain, just in front of the first pair of mesoblastic somites (Figs. Ill and 120, EI). The pits rapidly deepen (Fig. 121, EI) ; their mouths narrow, and by the end of the third day become completely closed, the pits thus becoming vesicles imbedded in the mesoblast at the sides of the head (Fig. 113. EI). By a series of changes very similar to those already described in the frog, the vesicle gives rise to the various parts of the membranous labyrinth of the ear ; the epiblastic wall forming the epithelial lining of the labyrinth, and becoming specially developed at certain places, to form the auditory epithelium. The auditory nerve, as noticed above, very early comes in contact with the anterior and inner wall of the auditory vesicle, fusing completely with this by the fiftieth hour. This fused patch, by division and subsequent separation of the several portions, gives rise to all the special patches of auditory epithelium present in the adult labyrinth.
The accessory organs of hearing. The development of the Eustachian tube, tympanic cavity, and tympanic membrane will be described in the section dealing with the development of the pharynx and gill-clefts (p. 283). The development of the columella, or auditory ossicle, will be described with the skeleton (p. 330).
Development of the Alimentary Canal
1. General Account
The alimentary canal of the chick, like that of the frog, is developed in three portions, of independent origin, and of very unequal length.
(i) The stomatodaeum, or mouth invaginatiou, is formed by a pitting-in of the epiblast at the anterior end of the alimentary tract ; from it the anterior part of the buccal cavity, and the pituitary body are developed.
(ii) The mesenteron gives rise to almost the entire length of the alimentary canal, from the hinder part of the buccal cavity to the cloaca ; and from it the lungs, liver, pancreas, and other important structures arise as outgrowths. The mesenteron is the tubular cavity formed within the embryo, as the result of the process of folding or constriction by which the embryo becomes pinched off from the yolk-sac (Figs. 112,1 14, and 1 23). It is lined by hypoblast along its whole length. Owing to the mode of its formation, it communicates freely, through the yolk-stalk, with the yolk-sac ; and, so long as the yolk-stalk remains tubular, the mesenteron may be described as consisting of three lengths : the fore-gut (Figs. 112, GF, and 114, TP, TO), which is the part included in the head-fold, and has complete roof, sides, and floor : the midgut (Figs. 114 and 123, YS), which opens into the yolk-stalk, and which therefore has roof and sides, but no floor ; and the hind-gut (Figs. 114 and 1 23, GH), which is the part included in the tail-fold, and has, like the fore-gut, complete roof, sides, and floor. As the constriction of the embryo from the yolk-sac proceeds, the foregut and hind-gut lengthen at the expense of the mid-gut ; and after about the seventh day. when the yolk-stalk is reduced to very narrow tube, and the walls of the mesenteron are complete along its whole length, the mid-gut, as a distinct portion of the alimentary tract, ceases to exist.
(iii) The proctodaeum is a barely perceptible pitting-in of the epiblast, at the hinder end of the alimentary tract, which forms the anal or cloacal aperture.
FIG. 123. A median longitudinal, or sagittal, section through a Chick Embryo at the end of the fifth day of incubation ; the section is taken strictly in the median plane, except as regards the Wolffian body and kidney, which are introduced in the figure in order that their relations to the alimentary canal maybe shown. The optic lobe, cerebral hemisphere, and optic stalk of the left side are shown in perspective. The amnion has been removed, and the allantois and yolk-stalk cut short close to the embryo. (Compare Figs. 100 and 115 for surface views of embryos of the same age.) x 12.
A, dorsal aorta. BF, third ventricle, or cavity of thalamencephalon. BH, fourth ventricle, or cavity of medulla obloiifrata. BL, cerebellum. BM, cavity of miil-brain. BS. cavity of the vesicle of the hemispheres. CH, notochord. GH, liinrt-grut. GT, mid-crm. IN", infuiidibulum. KG, Wolffian duct. KD, ureter. KM, Wolffian body. KT, kidney. LG, lung. MET, mandibular arch. ITS, cavity of spinal cord. PN", pineal body. PT, pituitary body. B.T, truncus arteriosus. B/V, ventricle of heart. TA. -talk of allantois, cut short. TO, cloaca, TH, thyroid body. TP. pharynx. TS, stomach. W, liver. "WD, bile duct. YS, yolk-stalk, cut short.
Up to the end of the fourth day the alimentary canal is nearly straight ; but from this time it grows more rapidly than the part of the body in which it lies, and soon becomes markedly convoluted ; it retains its connection with the mid-dorsal wall of the body cavity by means of the mesentery.
The several regions of the alimentary canal, and the various organs formed in connection with it, will now be taken in order ; and the more important points in their developmental history described.
2. The Pharynx
Almost from the first, there is a great difference between the anterior or pharyngeal portion of the mesenteron, which is shallow dorso-ventrally but very wide from side to side (Fig. 118, TP) ; and the hinder part, from the oesophagus to the cloaca, which is narrow and cylindrical.
Towards the end of the second day, pouch-like folds of hypoblast grow out in pairs from the sides of the pharynx, towards the surface. These correspond exactly, in their relations and their mode of formation, to the gill-pouches of the tadpole ; and like these, they grow outwards until they meet the external epiblast, with which they fuse. At a slightly later stage, the fused patches of epiblast and hypoblast become perforated to form the gill-clefts, which place the gill pouches, and therefore the pharynx, in direct communication with the exterior.
Of these gill-pouches, four are formed on each side of the neck, and are developed in. order from before backwards. The most anterior one is the hyomandibular gill-pouch (Fig. 124, HM) ; and the succeeding three are the first, second, and third branchial pouches respectively.
The parts of the side walls of the pharynx between the successive gill-pouches are spoken of as the visceral arches ; their boundaries are indicated on the surface of the neck by grooves, marking the lines along which the hypoblastic walls of the gillpouches meet and fuse with the external epiblast, as shown on the right-hand side of Fig. 124. The first or most anterior of these visceral arches is the mandibular arch (Figs. 124 and 125, MN), which forms the basis of the lower jaw. The second and widest arch is the hyoid arch, HY ; and behind this come the first, second, and third branchial arches, the hindmost or third branchial arch being immediately behind the last or third branchial cleft. These visceral arches, and the gill-pouches separating them from one another, correspond exactly with the similarly named structures in the tadpole ; the sole difference of importance being that in the chick no gills are, at any period, developed in connection with them. The fact that these structures, which are only intelligible through their association with aquatic respiration, are present in the early developmental stages of the chick, must be held to prove the descent of birds from aquatic, gillbreathing ancestors.
FIG. 124. A section through the head of a Chick Embryo at the end of the third day of incubation, the section being taken along a plane indicated by the two arrows and crosses in Fig. 113, p. 253. The right side of the section is at a level slightly dorsal to that of the left side, x 30.
A, dorsal aorta. Al, first aortic arch, in the mandibular arch. A2, second aortic arch, in the hyoid arch. A3, third aortic arch, in the first branchial arch. AC, carotid artery. AI, internal carotid artery. BM, cavity of mid-brain. BR1, first branchial arch. CH, notochord. HM, hyomandibular cleft. HY, hyoid arch. MN, mandibular arch. MP. muscle plate. M"E, ganglion of spinal nerve. NS, spinal cord. TP, pharynx. VB, anterior cardinal vein.
The hyomandibular cleft opens to the exterior in the latter part of the third day (Figs. 113 and 124, HM) ; it remains open until about the end of the fourth day, when its walls come in contact, and the cleft becomes closed. The first branchial cleft, between the hyoid and first branchial arches, opens a little later, early on the fourth day ; and closes again during the fifth day. The second branchial cleft, between the first and second branchial arches, is open only for a short time during the fifth day : and the third branchial cleft does not open to the exterior at any time.
It is stated by some observers that none of the visceral clefts in the chick open to the exterior at any stage, but the real condition appears to be as described above ; it is possible that individual variations occur in respect to the dates of opening of the clefts, and the times during which they remain open.
The tympano-Eustachian passage. The branchial clefts close up and disappear completely at an early stage ; but the most anterior, or hyomandibular, cleft appears to persist, and to give rise directly to the tympano-Eustachian passage of the adult bird. The cleft becomes closed at its outer end, about the end of the fourth day, by a fold of skin, which becomes directly the tympanic membrane. From the gill-pouch, on the inner side of the tympanic membrane, the tympanic cavity and Eustachian passage are formed ; while the external auditory meatus is built up as a short tubular passage on the outer side of the tympanic membrane (Fig. 126, HM). The Eustachian passages of the two sides unite at their inner ends, and open into the mouth by a median aperture (Fig. 116, ES), nearly opposite the glottis.
According to some observers, the hyomandibular pouch does not open to the exterior at any period in the chick ; and the tympanic membrane is formed directly from the thin double layer, consisting of both epiblast and hypoblast, which closes the pouch at its outer end ; a layer of mesoblast growing in between the epiblast and hypoblast, which persist as the epithelial layers of the outer and inner surfaces of the tympanic membrane respectively. The whole history of the development of these parts stands in need of renewed and thorough investigation.
The thyroid body arises, towards the end of the second day, as a median longitudinal groove in the floor of the pharynx opposite the first pair of branchial arches. The hinder end of the groove deepens during the third day to form a pit (Fig. 114, TH). The walls of this pit soon join together, obliterating the cavity and giving rise to a solid plug of hypoblastic epithelium (Fig. 123, TH). About the end of the fifth, or early part of the sixth day this plug separates from the floor of the throat as a solid body, composed of epithelial cells, which lies embedded in the mesoblast, immediately in front of the truncus arteriosus.
The thyroid body soon becomes bilobed, and the lobes branch out as solid strings of cells, which later on become tubular. A sheath of vascular connective tissue early forms around the lobes, which, as development proceeds, gradually shift backwards along the neck to their adult position.
A pair of solid bodies, formed of epithelial cells, which separate from the hypoblast immediately behind the third branchial pouches, and take up a position at the sides of the larynx, are sometimes spoken of as accessory thyroid bodies.
The thymus arises, on each side, as a couple of epithelial buds from the walls of the second and third branchial pouches. The buds soon separate from the surface, and, the two buds of each side fusing together, give rise to a pair of elongated rod-like bodies, lying along the sides of the neck close to the carotid arteries.
The tongue is formed as an outgrowth from the floor of the pharynx, opposite the hyoid and first branchial arches. It first becomes conspicuous about the sixth day, and by the eighth or ninth day (Fig. 116, TN) has attained a definite shape. It is formed behind the boundary line between the pharynx and stomatodgeum, and its epithelium is therefore of hypoblastic origin.
3. The Stomatodaeum
The stomatodaeum, or mouth imagination, is formed by pitting-in of the ventral wall of the pharynx from the exterior.
From the time of its first formation the ventral wall of the pharynx, in front of the heart, is very thin (c/. Fig. 112). On the appearance of the visceral arches, as thickenings of the side walls of the pharynx, this thin-walled area on its ventral surface becomes more clearly defined, as a slightly depressed, transversely elongated patch, bordered by a thickened rim, which is formed partly by the ventral ends of the anterior visceral arches, and partly by the under surface of the head itself.
By further thickening of this rim, the depression which it surrounds becomes deepened ; and the pit formed in this way, rather by building up of its walls than by lowering of its floor, becomes the stomatodaeum.
Towards the end of the third day the floor of the stomatodaeal pit thins away and becomes perforated, placing the pharynx for the first time in direct communication with the exterior, and forming the permanent mouth opening (Fig. 114, o).
FIG. 125. The head of an Embryo Chick at the end of the fifth day of incubation ; seen from below. Compare Fig. 115 for a view of an embryo of the same age from the side, x 8.
BR', 'first branchial arch. BS, cerebral hemisphere. CH, notochord. DS. mouth. !PP, fronto-nasal process. MM, hyomandibular cleft. HY, hyoid arch. MM", mandibular arch. MX, maxillary arch. NS, spinal cord, seen in section where the neck has been cut across. OC, eye. OH, choroidal fissure. OK, olfactory pit. OL, lens.
The Face. After the definite formation of the mouth opening, the borders of the stomatodasal pit continue to develop, and gradually give rise to the beak and the anterior part of the face of the bird. At the end of the fifth day the mouth opening (Fig. 125, DS) is oblong in shape. Its anterior border is formed by the fronto-nasal process, FP, a broad plate, notched in the median line, and forming, at this stage, the under surface of the head. The posterior border of the mouth opening is formed by the ventral ends of the mandibular arches, MN, which meet each other iu the median plane at the chin ; and the sides of the opening are bounded by the maxillary arches, MX, which grow forwards from the mandibular arches to meet the outer angles of the fronto-nasal process.
The olfactory pits, OK, lie just beyond the anterior and outer angles of the mouth : the inner border of each pit is formed by the side of the fronto-nasal process, or inner nasal process ; the outer border is formed by a strip of the side of the head between the olfactory pit and the eye, which is spoken of as the outer nasal process.
FIG. 126. The head of an Embryo Chick at the. end of the seventh day of incubation ; seen from below, x 8.
BS, cerebral hemisphere. CD, third eyelid, or nictitating membrane. CH, notochord, seen in section where the neck has been cut across. DS, mouth. TTTVr. external auditory nieatus. MN, mandibular arch. MX, maxillary arch. M"S, spinal cord, seen in section. OC, eyeball. O J, epithelial knob on tip of beak. OK, external nostril. OL, lens.
Between the outer nasal process and the maxillary arch there is a slight depression, the lacrymal groove, which runs from the under surface of the eye to the outer border of the olfactory pit. Between the inner nasal process, or wing of the fronto-nasal process, and the anterior end of the maxillary arch there is a more conspicuous depression, the nasal groove, which becomes converted by fusion of its lips, as described on p. 275, into the posterior narial passage.
By the seventh day (Fig. 126) the parts of the face begin to assume more definite form. The mouth opening, DS, is more slit-like, and its boundaries are more clearly defined. The fronto-nasal process is narrower, and has begun to grow forwards as the upper beak, on the tip of which the small epithelial knob, OJ, which is used for breaking the egg-shell at the time of hatching, is already present. The maxillary arches have fused with the sides of the fronto-nasal process ; the nasal grooves are converted into the narial passages, and the lacrymal grooves have disappeared.
The two mandibular arches, MN, have fused in the median plane to complete the lower jaw, which is already beginning to grow forwards as the lower beak. Finally, the external nostrils, OK, have narrowed very considerably, and have acquired the slitlike form characteristic of them in the adult.
The pituitary body (Fig. 114, FT) is a pocket-like diverticulum from the anterior angle of the stomatodasum, which appears towards the end of the second day, and which early acquires its characteristic relations with the infundibulum, and with the anterior end of the notochord. Its development has already been described in the section dealing with the brain (p. 259).
4. The Oesophagus
Immediately behind the pharynx the alimentary canal suddenly narrows, becoming a very slender tube, the oesophagus, which runs back in a perfectly straight course through the neck (Figs. 114, TO, and 123).
The oesophagus is at first very short ; but, as the neck lengthens, the oesophagus grows rapidly, to keep pace with thi?. A curious point with regard to the oesophagus is that for a time, commencing about the middle of the sixth day, and lasting for two or three days, the lumen is completely lost, the oesophagus becoming solid along the greater part of its length. A little later, about the ninth day, the lumen is gradually re-established, from below upwards.
This temporary obliteration of the cavity of the oesophagus in the chick is perhaps to be associated with the rapid lengthening which the neck and the oesophagus are undergoing at this period ; but the fact that a similar solidification of the oesophagus occurs in dogfish, frogs, reptiles, and mammals, as well as in birds, renders it possible that it has some further and deeper signification, not yet determined.
5. The Stomach and Intestine
Up to the end of the fifth day (Fig. 123), the alimentary canal remains almost straight, except for a slight, ventrally directed loop, GT, at the place where the yolk-stalk, YS, arises, connecting the intestine with the yolk-sac.
The stomach is recognisable as a slight, fusiform dilatation, TS, about the end of the fifth day ; during the sixth day thegizzard becomes evident, as a thick-walled dilatation of the distal end of the stomach, which grows rapidly, and by the twelfth day has attained a great size.
From the sixth day onwards, the intestine lengthens rapidly ; growth occurring most markedly at two parts of its length, and giving rise to two loops, both of which are directed ventral wards. Of these, the proximal or duodenal loop is formed from the part of the intestine immediately beyond the gizzard. The distal, or vitelline loop, which is much the longer of the two, is formed by elongation of the two limbs of the V-shaped loop which is already present on the fifth day, and from the angle of which the yolk-stalk arises (Fig. 123, YS).
Between the duodenal and vitelline loops there is a part of the intestine which undergoes hardly any elongation at all, but remains throughout life closely attached to the dorsal surface of the body cavity; it corresponds to the point in Fig. 123 immediately beyond the opening of the bile-duct, WD, where the intestine bends ventralwards to form the proximal limb of the vitelline loop.
The further development of the intestine consists chiefly in great elongation of the vitelline loop, which gives rise to the whole length of the small intestine, beyond the duodenum. Both limbs of the loop lengthen very rapidly, and become twisted somewhat spirally. Up to about the seventeenth day the vitelline loop lies almost entirely in the yolk-stalk, and therefore outside the body of the embryo ; about the eighteenth day the greater part of the loop becomes withdrawn into the body, and acquires the convolutions characteristic of the adult.
The rectum, or terminal part of the intestine, grows very slowly, and remains nearly straight throughout the whole period of development. The boundary between the small intestine and the rectum is marked by the two rectal diverticula, which appear as a pair of small pouch-like outgrowths (Fig. 123, GH) about the end of the fifth day ; these grow rapidly, and by the eighth or ninth day have attained a considerable length. The rectum itself remains short ; in the later days of incubation it dilates very greatly, and shortly before the time of hatching the bursa Fabricii arises as a dorsal outgrowth from its distal end.
The mesentery. The alimentary canal, along its whole length, is at first closely attached to the dorsal wall of the body cavity, immediately below the notochord.
The pharynx, or most anterior division of the alimentary canal, retains these relations throughout life. The oesophagus shifts ventralwards to a slight extent, owing to the intrusion of mesoblast between it and the notochord. Further back the ventral shifting is much more marked j and the whole intestinal region, from the stomach to the rectum, becomes situated some distance ventral to the notochord, remaining, however, connected with the dorsal wall of the body cavity by a vertical, laterally compressed sheet of mesoblast, the mesentery.
An exception to this statement must be made with regard to the short portion of the intestine between the duodenal and vitelline loops, which, as already noticed, remains in close connection with the dorsal body wall throughout life.
As the duodenal and vitelline loops of the intestine lengthen, the mesentery grows, keeping pace with them, and becoming still further reduced in thickness ; it ultimately forms a thin sheet, consisting of two epithelial layers, derived from the peritoneum, and inclosing between them a very thin layer of mesoblast, along which the blood-vessels run to and from the alimentary canal.
The terminal part of the alimentary canal, or rectum, like the anterior part, remains closely connected with the dorsal body wall throughout life, the mesentery in this region only attaining a comparatively slight development.
6. The Proctodaeum
The proctoda3um is a slight depression of the skin at the hinder end of the body, beneath the tail (Fig. 123). It develops very late, and does not open into the rectum until about the fifteenth day. The proctodaeum in the chick is very shallow, and gives rise only to the outermost portion of the adult cloaca, and to the actual external opening.
7. The Lungs
The lungs arise, during the third day, as a pair of small hollow outgrowths from the ventral surface of the anterior end of the oesophagus. By lateral constriction, the ventral part of the oesophagus, from which the lungs arise, becomes separated off as a median chamber (Fig. 114, LG) : this lies ventral to the oesophagus, and opens in front into the hinder end of the pharynx; while from its hinder end the lungs extend backwards as posteriorly directed outgrowths.
The lungs, after their first appearance, rapidly increase in size ; they give off secondary diverticula, which branch again and again ; and from the finest branches arborescent outgrowths arise at right angles, which become the ultimate spongy substance of the lungs.
The air sacs, which are structures very characteristic of bii-ds, appear about the eighth day as thin-walled saccular diverticula from the hinder edges of the lungs ; the abdominal air sacs are in the earlier stages the best developed.
The trachea (Fig. 116, LR) is formed by elongation of the median laryngeal tube, as the neck lengthens and the lungs gradually shift backwards into the thorax.
From the mode of development of the lungs, as outgrowths from the alimentary canal, it follows that their lining epithelium, including the minutest passages, and that of the air sacs as well, is of hypoblastic origin : the rest of the thickness of the lung walls, including all the blood-vessels, is mesoblastic.
The lungs contain no air, and are not used for breathing, until immediately before the time of hatching ; when the chick, breaking through the shell membrane into the air chamber at the larger end of the egg (Fig. 101, sv), draws air into its lungs for the first time, and, invigorated by the act, proceeds to peck its way out of the shell.
8. The Liver
The liver arises, about the middle of the third day, as a tubular diverticulum from the posterior end of the fore-gut, in the angle between the two vitelline veins, and immediately behind their point of union. A second diverticulum arises from the same spot almost directly afterwards ; it is similar to the first, but of rather smaller size. Both these diverticula have hypoblastic walls, with thin mesoblastic investments.
Towards the latter part of the third day, as the folding off of the embryo from the yolk-sac proceeds, the liver diverticula are found to arise definitely from the part of the mesenteron which will later become the duodenum. At the same time they come into very close relation with a large median vein, the meatus venosus, which is formed by the union of the right and left vitelline veins behind the heart (cf. Fig. 128, VE).
The two liver diverticula lie one at each side of the meatus venosus, and in very close contact with this. The hypoblastic cells forming the walls of the diverticula now begin to proliferate freely, growing out as solid strands of cells, which form an irregular reticulum closely surrounding the meatus venosus; the meshes of the reticulum being occupied by capillary blood-vessels, which develop in the mesoblast, and early acquire connection with the meatus venosus itself.
These processes proceed rapidly during the fourth and fifth days, and by the end of the fifth day (Figs. 123 and 128) the liver is an organ of considerable size, consisting of a network of solid rods of hypoblast cells, which branch and anastomose freely in all directions ; the meshes of the network being occupied by blood-vessels, which penetrate all parts of the liver, and are in free communication with the meatus venosus, round which the liver is formed.
The liver continues to grow rapidly, and by the tenth day is the largest organ in the abdominal cavity. The trabecular network of hypoblast cells becomes the liver parenchyma ; the tubular diverticula from the duodenum branch out freely in the substance of the liver, and become the two bile ducts of the adult bird ; while the gall bladder arises on the fifth day as a saccular outgrowth from the right or larger of the two primary diverticula.
The early formation of the liver in the chick, and its large size during the greater part of the developmental history, indicate that it must be of considerable functional importance during embryonic life. Its relation to the blood system, and especially the fact that it intercepts the blood returning from the yolk-sac to the heart, suggest that its chief purpose is connected with the elaboration of the food material which is obtained from the yolk-sac, and at the expense of which the nutrition of the embryo is effected.
9. The Pancreas
The pancreas arises, rather later than the liver, as a tubular outgrowth from the duodenum, just beyond the two liver diverticula, from which secondary outgrowths arise in much the same manner as in the liver itself. A second diverticulum arises from the duodenum about the eighth day, and gives rise to the greater part of the adult pancreas ; and at a later period a third diverticulum is formed. The three diverticula persist as the three pancreatic ducts of the adult bird, while the three lobes of the pancreas, with which they are connected, soon fuse indistinguishably with one another.
10. The Allantois
The allantois is really an appendage of the alimeutaiy canal, arising as an outgrowth of its ventral wall, in front of the cloaca ; it is therefore lined by hypoblast, like all other outgrowths of the mesenteron, while the rest of the thickness of its wall is formed by the splanchnopleuric mesoblast.
The allantois of the chick is homologous with the bladder of the frog (Fig. 89, TB). It differs mainly from this in the fact that, while arising in the same manner, it is not confined within the body of the embryo, but, growing rapidly, passes out beyond this as a thin- walled vascular sac (Figs. 100 and 101, TA), which spreads out in close contact with the inner surface of the eggshell, and acts as the respiratory organ of the embryo during the greater part of its development.
In the chick the allantois commences to form about the middle of the second day. At this time the tail fold is not yet established, so that the allantois (Fig. 112, TA) appears at first as a pocket-like fold of the splanchnopleure, lying a short way behind the embryo, and with its cavity opening ventralwards.
On the formation of the tail fold, early on the third day, the part of the splanchnopleure from which the allantois arises becomes doubled forwards under the embryo to form the ventral wall of the gut, and the allantois now appears as a saccular depression of the ventral wall of the hind-gut (Fig. 114, TA).
During the third day the allantois increases considerably in size, projecting downwards and forwards, as a hollow, thickwalled bud from the ventral surface of the hind-gut, into the body cavity, or space between the somatic and splanchnic layers of the mesoblast.
During the fourth day, 'by its further growth, the allantois passes out beyond the embryo, and turns up, along its right side, into the space between the two layers of the amnion, which, from the mode of formation of the amnion, is directly continuous with the body cavity of the embryo (cf. Fig. 114).
On the fifth and following days the allantois grows rapidly ; from the first it is very vascular, and the blood-vessels now increase greatly in size ; the arteries, which lie in its superficial layer, are derived directly from the aorta (Fig. 128, AA) ; while the veins, VA, which lie in its inner or deeper layer, join the vitelline veins from the yolk-sac, and, passing through the liver, reach the heart.
By the seventh or eighth day (Fig. 101, TA), the allantois has spread all round the upper half of the egg, covering over the embryo, and extending half way round the yolk-sac as well. It is still saccular, and its cavity contains fluid. Its outer wall lies in very close contact with the outer layer of the amnion, or false amnion, and soon fuses with this completely, so that from this time the allantois lies in close contact with the shell membrane.
In its further growth the allantois does not follow the yolksac ; but, keeping close to the egg-shell, and carrying the somatopleure before it, it extends so as gradually to inclose the mass of the white, WA, which still remains on the under surface and near the smaller end of the egg. The allantois, about the sixteenth day, completely incloses this plug of white or albumen, and from this time the absorption of the plug proceeds rapidly, the albumen being apparently carried by the allantoic vessels to the embryo, and aiding in its nutrition.
Towards the close of incubation deposits of urates occur in the cavity of the allantois, indicating that it serves as a receptacle for the excretory matters formed within the embryo itself, as well as a respiratory organ in the more restricted sense of the term.
Shortly before the time of hatching, the allantoic vessels become constricted, by the closure of the body walls at the umbilicus. The allantois itself shrivels up, and is cast off' as the chick works its wav out of the shell.
Development of the Heart and Bloodvessel
1. Preliminary Account
The general arrangement of the vascular system during embryonic life is strikingly similar to that of the tadpole. The heart is at first a straight, and later a twisted tube, lying beneath the pharynx, and driving the blood through a series of paired aortic arches (Fig. 128) to the dorsal aorta?, which distribute it to all parts of the body. From the body generally, and from the Wolffian bodies, the blood is returned by anterior and posterior cardinal veins on each side ; these unite to form the Cuvierian veins, or anterior vena3 cavse, which open into the sinus venosus or posterior end of the heart. From the alimentary canal the blood is returned by the mesenteric or hepatic portal vein, which, passing through the liver, joins the posterior vena cava, the vein through which the blood is returned to the heart from the kidneys and other organs.
The chief differences between the chick and the frog as regards the arrangement of the blood-vessels are : (i) that the chick embryo has no gills, either external or internal, and therefore possesses no vessels corresponding to the gill loops of the tadpole ; and (ii) that in the chick the vessels connected with the yolk-sac and with the allantois, both of which are structures outside the embryo itself, are enormously developed. These bloodvessels, vitelline and allantoic. are in direct connection with the vessels of the embryo : the afferent vessels, i.e. the vitelline and allantoic arteries, being branches of the dorsal aorta ; while the efferent vessels, the vitelline and allantoic veins, on entering the embryo, join the mesenteric veins and run, through the liver, to the heart.
Throughout the greater part of the period of incubation, the vitelline and the allantoic vessels are of very large size ; and inasmuch as the returning vessels, the vitelline and allantoic veins, bring to the embryo food matter from the yolk-sac, and oxygen from the allantois, it follows that the blood entering the heart by the posterior vena cava is arterial, and not venous, in character. The right understanding of the peculiarities in the circulation in the chick during embryonic life is mainly dependent on a full appreciation of this fact.
The aortic arches of the chick embryo undergo changes very similar to those which occur in the frog ; the arches disappearing in part, and in part becoming modified into the arterial system of the adult. As in the frog, the pulmonary arteries are branches from the hindmost pair of aortic arches.
Histological development of the blood-vessels. The bloodvessels appear in the vascular area before they are formed in the embryo itself, and the mode of their development is easier to determine in the former situation.
Shortly before the end of the first day, when two or three pairs of mesoblastic somites are present in the embryo, a number of outgrowths from the upper surface of the hypoblast appear round the inner margin of the area opaca. These branch freely, and unite with one another to form a network, lying between the mesoblast and the hypoblast : the strands of the network are solid ; they contain numerous nuclei, but cell outlines are difficult or impossible to determine in them. Within the strands, vacuoles soon appear at intervals : these enlarge rapidly, and, running into one another, convert the solid network into a system of anastomosing tubules with nucleated walls. These tubules are capillary blood-vessels ; they are filled with fluid, but contain no blood corpuscles until a later stage.
This vascular network spreads rapidly, extending outwards as the vascular area widens, and inwards across the area pellucida to the embryo, which it invades on the second day. From their first appearance the vessels of the embryo are continuous with those of the area pellucida ; but it is not quite clear how far they arise in situ, or how far by intrusion of vessels from the area pellucida.
This network of blood-vessels lies below the mesoblast, between this and the hypoblast ; it is connected at places with the hypoblast, from which it arises in the first instance, but it is quite independent of the mesoblast. If this appears to contradict the general rule according to which the blood-vessels are derived from niesoblast, it should be remembered that the whole of the mesoblast in the chick, with the exception of the primitive streak mesoblast, is of hypoblastic origin ; and the facts with regard to the formation of the blood-vessels might therefore be expressed by saying that the blood-vessels separate from the hypoblast at a stage later than that at which the other mesoblastic structures are formed from it. It is better, however, to accept the facts as they stand ; namely, that in the chick many of the blood-vessels are derived directly and independently from the hypoblast ; and to bear in mind that the middle germinal layer, or mesoblast, cannot be regarded as in any sense equivalent to either of the two primary germinal layers, epiblast and hypoblast ; the term ' mesoblast ' being used to include a number of very diverse structures, most if not all of which owe their ultimate origin to either hypoblast or epiblast.
The hypoblastic vascular network, formed in the way described above, gives rise directly to the capillaries and to the endothelial lining of the larger blood-vessels. The connective tissue and muscular walls of these latter are derived independently from the mesoblast, which grows round and envelopes them.
The blood-vessels within the embryo are at first, like those of the area pellucida and area vasciilosa, reticular in their arrangement. The definite arteries and veins are formed by straightening and enlargement of certain of the strands of the network, with disappearance of other portions ; the dorsal aortas, for instance, arising early in the second day, in embryos with eight pairs of mesoblastic somites, as a pair of longitudinal trunks, lying along the outer and ventral borders of the somites, between the mesoblast and hypoblast, and communicating freely along the hinder part of their course with the reticular network of the area pellucida.
2. The Heart
The heart is formed on the under surface of the fore-gut, at the commencement of the second day. It consists at first of two longitudinal vessels, which, though closely applied in the median plane, ai'e for a time quite distinct, but which soon fuse to form a single median tube.
The walls of the heart consist, as in the frog, of an outer muscular coat (Fig. 120, RM), formed by the splanchnopleuric layer of mesoblast ; and an inner endothelial lining, RE, concerning the origin of which it is difficult to speak with certainty, but which appears, like that of the other blood-vessels, to be derived from the hypoblast.
The muscular wall (Fig. 120, RM) is at first incomplete dorsally, but, after the two halves of the heart have united, the muscular walls grow in towards the median plane, above the heart, and coalesce so as to complete its w r all. The endothelial tubes of the two halves of the heart remain distinct, though closely apposed, for some time after the muscular walls have coalesced, but ultimately they, also, become continuous. Between the muscular and endothelial walls there is at first a considerable space, filled with a mucous substance (Fig. 120).
FIG. 127. The anterior end of a Chick Embryo at the thirty-sixth hour of incubation, removed from the yolk-sac, and seen from the ventral surface. (Compare Figs. Ill and 112 for other views of an embryo of the same age.) x30.
Al, first aortic arch, in the mandibular arch. BF, fore-brain. 'BO, optic vesicle. portion of heart. V V, vitelliue vein, cut short.
The heart thus forms, about the thirtieth hour, a short and straight tube, lying below the fore-gut, and closely attached to its ventral surface, and with its walls consisting of an outer muscular tube and an inner endothelial tube.
The posterior end of the heart is, from the first, continuous with, or rather is formed by the union of, two large vessels, the vitelline veins (Fig. 127, vv), which collect and return to the embryo the blood from the network of capillaries in the area pellucida and area opaca (cf. Fig. 98).
The heart is at first very short ; but as the head fold becomes deeper, constricting the embryo more and more markedly from the yolk-sac, the vitelline veins remain at the edge of the fold, and so get carried back with it, causing thereby lengthening of the heart.
The heart begins to beat very shortly after its first formation, and before any distinct histological differentiation into muscle and nerve cells can be distinguished.
On its first formation, the heart is attached along its whole length to the under surface of the fore-gut. It remains attached at its two ends, but about the thirty-third hour becomes free along the middle portion of its length (Fig. 112, RV) ; and, growing more rapidly than the parts to which it is attached, becomes thrown into a loop (Fig. 127), with the convexity towards the right, and the concavity towards the left side of the embryo.
The loop, continuing to lengthen, projects downwards and backwards, so that the whole heart, towards the end of the second day, becomes twisted obliquely, into a letter S shape. Starting from the point of union of the vitelline veins, the heart runs forwards a certain distance, then makes a sharp bend downwards, backwards, and to the right side ; then, making a second, equally sharp bend (rf. Fig. 113, RV) upwards, forwards, and to the left, reaches the median plane again, and is attached to the under surface of the pharynx opposite the first two pairs of gill-clefts.
The posterior end of the heart, into which the vitelline veins open, may be called the venous end of the heart ; and the anterior end, the arterial end. The first bend or loop (Fig. 113, RA) marks the auricular part of the heart ; and the second bend, RV, the ventricular part. The forwardly directed portion of the heart, in front of the second bend, is the truncus arteriosus, RT.
During the third day (Fig. 113), the heart increases considerably in size ; the S-like twisting becomes still more pronounced than before ; and constrictions appear, separating the several chambers of the heart from one another.
On the fourth day, the auricular portion of the heart becomes widened laterally, and marked off by a sharp constriction from the ventricular portion, which, in its turn, is separated by a distinct though less pronounced constriction from the truncus arteriosus.
The most important event, however, that happens during the fourth day, so far as the heart is concerned, is the first appearance of the partitions by which the right and left sides of the heart become separated from each other. Up to the fourth day the heart is a single and continuous, though twisted tube, without any division whatever into right and left sides. The blood enters at the posterior or venous end of the heart, and passing through the several cavities in succession passes out in front, through the truiicus arteriosus, into the aortic arches.
The internal division of the heart, into right and left sides, is effected by three septa or partitions, which appear within the cavity of the heart, and which arise perfectly independently of one another : (i) the interauricular septum, which divides the auricular chamber into the right and left auricles; (ii) the interventricular septum, which divides the ventricular chamber into the right and left ventricles ; (iii) the septum of the truncus arteriosus, which divides the truncus arteriosus, or terminal chamber of the heart, into right and left halves. Of these septa, the first two commence to form on the fourth day ; the third, or septum of the truncus arteriosus, does not arise until the fifth day.
Concerning the relative times of appearance of the interauricular and interventricular septa, there is some discrepancy in the published accounts. It is commonly stated that the interventricular septum develops the earlier of the two, but according to Masius it is the interauricular septum which is the first to be formed.
The interauricular septum appears, during the fourth day, as a septum projecting into the auricular chamber from its anterior and dorsal wall; it lies between the apertures of the sinus venosus and the pulmonary vein, and ends in a free posterior edge. Of the two cavities into which it partially divides the auricular chamber, the left auricle is for a time much the larger of the two.
The interventricular septum also appears during the fourth day, as a crescentic partition which arises from the ventral wall of the apex of the ventricular chamber, and gradually extends across towards the dorsal wall. It divides the ventricular chamber somewhat obliquely, and as yet imperfectly, into a left and more dorsally placed cavity, and a right and more ventrally placed one. The position of the septum is indicated by a slight groove on the surface of the heart.
On the fifth day the interventricular septum is completed, but the interauricular septum remains imperfect throughout the whole period of development, up to the time of hatching.
The septum of the truncus arteriosus appears on this day as a longitudinal fold, corresponding exactly to the similar one in the frog. The fold commences near the distal end of the truncus arteriosus, between the fourth and fifth pairs of aortic arches, and grows backwards with a somewhat spiral course, dividing the cavity of the truncus arteriosus into right and left halves. By the end of the fifth day this longitudinal septum has grown back to the base of the truncus arteriosus, and now meets with the upper edge of the interventricular septum and fuses with this. The effect of this fusion is that the right ventricle now communicates with the right division of the truncus arteriosus, and through this with the hindmost or fifth pair of aortic arches alone ; while the left ventricle communicates, through the left division of the truncus arteriosus, with the anterior pairs of aortic arches, but no longer with the fifth or hindmost pair.
Before the completion of the septum of the truncus arteriosus three semilunar valves are formed at the base of each of the divisions of the truncus arteriosus, between these and the ventricles.
During the sixth day the shape of the heart as a whole approaches much more closely to that of the adult ; the apex of the heart becomes more pointed, and the auricular appendices more prominent.
Up to about the twelfth day the interauricular septum remains very imperfect, and there is free communication between the two auricles through a large aperture, the foramen ovale. About the twelfth day this communication narrows considerably.
On the sixteenth day the Eustachian valve is formed as a fold projecting into the right auricle, between the openings of the posterior vena cava and the right anterior vena cava. Up to this time the blood from both these vessels has passed from the right auricle, through the foramen ovale, into the left auricle and so to the left ventricle. The effect of the Eustachian valve is to direct the blood from the right anterior vena cava into the right ventricle, while still allowing the blood from the posterior vena cava to pass through the foramen ovale into the left auricle. From this time the two auricles are about equal in size.
Shortly before hatching, the foramen ovale becomes partially blocked up by a membranous, valve-like fold ; the completion of this stoppage is effected shortly after the time of hatching, from which time the structure of the heart is practically that of the adult bird.
The thickening of the ventricular wall, which is a marked feature of the later stages of development, is effected by inwardly projecting ridges of the muscular wall, which ultimately form a system of anastomosing muscular trabeculae, from which, by further thickening, the columna3 carneee and musculi papillares are developed.
The thickening of the wall of the auricles is effected in similar fashion, but is not carried to so great an extent.
The wall of the truncus arteriosus thickens by simple increase in the thickness of the muscular and other layers composing it.
3. The Arteries
a. The Aortic Arches. The truncus arteriosus divides right and left, as in the frog, into the aortic arches, which run round the sides of the pharynx to its dorsal surface ; here they open into the dorsal aortas, by which the blood is carried all over the body of the embryo, as well as to the yolk-sac and the allantois (c/. Figs. 113 and 128).
The aortic arches of the chick are developed in order, from before backwards. The first, or most anterior pair (Fig. 127, Ai),. is formed early 011 the second day, and lies opposite the anterior end of the fore-gut. The remaining arches are formed in succession behind the first pair.
By the end of the second day a second pair is present, and by the commencement of the third day a third pair appears still farther back (Fig. 113 AS).
FIG. 128. A diagrammatic figure showing the arrangement of the bloodvessels in a Chick Embryo at the end of the fifth day of incubation. The amnion has been removed, and the vitelline vessels cut short a little distance from the embryo, x 12.
A, dorsal aorta. A3, A4, A5, third, fourth, aud fifth aortic arches of the right siile, lying in the first, second, and third branchial arches respectively. AA, allantoic artery. AB, basilar artery. AC, carotid artery. AH, caudal artery, the terminal portion of the dorsal aorta. AL, lingual artery. AP, pulmonary artery. AV, vitelline artery. El, auditory vesicle. RA, right auricle. RS, sinus venosus. RT, truncus arteriosus. RV, right ventricle. TA, allantois. VA, allantoic vein. VB, anterior cardinal vein. VC, posterior cardinal vein. VD, Cuvierian vein. VE, meatus venosus. VH, efferent hepatic vessel. VI, posterior vena cava. VJ, jugular vein. VO, afferent hepatic vessel. VV, vitelline vein.
On the establishment of the visceral clefts and arches, the aortic arches acquire definite relations with the latter ; the first, or most anterior aortic arch of each side (Figs. 113 and 124, Ai), lying in the first or mandibular arch ; the second aortic arch, A2, lying in the hyoid arch ; and the third aortic arch, A3, in the first branchial arch. During the fourth day, fourth and fifth pairs of aortic arches (Fig. 128, A4, AS) appear in the second and third branchial arches.
There are thus altogether five pairs of aortic arches in the chick, corresponding to the five anterior of the six pairs present in the tadpole. These arches, however, differ from those of the tadpole inasmuch as (i) they never have any gills developed in connection with them ; (ii) they form from their first appearance direct connections between the truncus arteriosus and the aorta, there being no separation into afferent and efferent vessels. A further difference lies in the fact that the aortic arches in the mandibular and hyoid arches are complete in the chick, while they never become so in the tadpole.
The condition of the aortic arches in the chick is comparable to that of a frog after the metamorphosis ; but it is at present a matter of doubt whether this indicates an entire omission of the earlier stages in the chick, owing to the absence of gills ; or whether the continuous aortic arch from heart to aorta does not rather represent a still earlier ancestral condition, prior to the acquisition of gills.
From the five pairs of aortic arches of the embryo the adult arterial system is derived, in the following manner.
During the fourth, or fourth and fifth days, the first or mandibular, and the second or hyoidean aortic arches disappear along the middle portions of their lengths ; their ventral and dorsal portions persist, the ventral portions remaining of comparatively small size as the lingual or mandibular arteries (Fig. 128, AL); while the dorsal portion, which is much larger, extends forwards into the head as the carotid artery (Fig. 128, AC), which divides into internal and external carotid arteries, supplying the brain and face respectively.
By the end of the fifth day the ventricular septum is completed, and has fused with the longitudinal or spiral septum of the truncus arteriosus (cf. p. 302). This latter septum arises, in front, between the roots of the fourth and fifth pairs of aortic arches, and divides the truncus arteriosus in such manner that the right division of the truncus arteriosus, and consequently the right ventricle, from which this division arises, sends all its blood into the fifth pair of aortic arches ; while the left ventricle and left division of the truncus arteriosus conduct blood to the third and fourth pairs of arches. It follows from this that the supply of blood to the head and anterior part of the body is derived from the left ventricle ; while the right ventricle supplies the whole of the body behind the heart, as well as the yolk-sac and allantois.
About the seventh day the two divisions of the truncus arteriosus separate completely from each other at their bases ; the right branch, or pulmonary trunk, remaining in connection with the right ventricle and the fifth pair of aortic arches ; and the left branch, or systemic trunk, with the left ventricle and the third and fourth pairs of aortic arches.
The part of the aorta connecting the dorsal ends of the third and fourth pairs of aortic arches (Fig. 128) becomes very slender, and is finally obliterated altogether, while the branch of the truncus arteriosus from which the third aortic arch arises elongates very considerably, and carries this arch forwards some distance in front of the next, or fourth aortic arch. The subclavian arteries, supplying the fore-limbs, arise from the ventral ends of the third aortic arches, during the third day ; they grow backwards, lying ventral to the other vessels, and reach the limbs during the fourth or fifth day.
The fourth pair of aortic arches is, from the fifth day onwards, much the largest of the three persistent pairs. The arches of the two sides of the body are at first of equal size ; but the arch of the right side soon becomes much larger than that of the left side ; and the latter ultimately becomes obliterated along the greater part of its length, while the arch of the right side persists as the arch of the aorta in the adult bird.
The pulmonary arteries appear, in the walls of the lungs, about the middle of the third day, before the two hinder pairs of aortic arches are formed. On the appearance of the fifth pair of aortic arches the pulmonary arteries (Fig. 128, AP) become connected with their ventral ends. Each fifth aortic arch thus consists of two parts : a proximal part running from the truncus arteriosus, i.e, from the right ventricle, to the lung ; and a distal or dorsal part which connects the root of the pulmonary artery with the aorta. This distal portion is spoken of as the ductus Botalli, or ductus arteriosus ; and so long as it remains open the blood from the right ventricle can avoid the lung circulation, and pass to the aorta direct. Shortly after the hatching of the chick, however, the ductus Botalli shrivels up, and its cavity becomes obliterated ; from this time the fifth arch communicates with the lungs alone, and all the blood from the right ventricle must pass through the pulmonary capillaries.
b. The Dorsal Aortse and their branches. The two dorsal aortse are at first separate along their whole length, running parallel to the notochord and some little distance from the median plane. Each aorta is. from the first, continuous along its outer side at several places with the vascular reticulum of the area pellucida and area vasculosa. By coalescence of the vessels of the reticulum at one place, with shrinking and disappearance of the reticulum in front of and behind this spot, the definite vitelline artery of each side is formed (Figs. 113 and 128, AV) : this is a large vessel running outwards from the aorta, between the splanchnopleuric mesoblast and the hypoblast, and passing out beyond the embryo to open distally into the vascular reticulum of the area vasculosa.
Before the end of the second day the two aorta3 have met and fused for a short distance along the middle part of their course, separating again towards their hinder ends, and giving off at intervals along their length small arteries to supply the various parts of the body.
By the fourth day the union of the two aortae has extended much further back than before, and involves the part from which the vitelline arteries arise. The two vitelline arteries have themselves coalesced at their proximal ends, and now spring from the aorta as a single trunk, which divides almost at once into the right and left vitelline arteries, of which the left one is much the larger.
The allantoic, or, as they are often called, umbilical arteries (Fig. 128, AA), arise from the aortas just beyond their point of bifurcation, and run outwards to the allantois. The left allantoic artery is usually the larger of the two from the first, and becomes ultimately the sole one, the right allantoic artery disappearing.
4. The Veins
The chief peculiarities in the veins of the chick, as distinguished from those of the tadpole, consist in the large size and great importance of the vitelline and allantoic veins, which return to the embryo the blood from the yolk-sac and the allantois respectively ; the blood of the vitelline veins being laden with food matter absorbed from the yolk, while that of the allantoic veins is charged with oxygen, and freed from its excess of carbonic acid.
The blood is returned to the heart by three chief veins, as in the tadpole : (i) and (ii) the right and left Cuvierian veins (Fig. 128, VD), which return blood from the head and body of the embryo, and which afterwards become the right and left anterior venae cavse : and (iii) the meatus venosus (Fig. 128, VE), a median posterior vein, which is formed in the first instance by the union of the right and left vitelline veins, vv ; is joined a little later by the allantoic veins, VA ; and, later still, receives in addition the posterior vena cava, vi. At the time of hatching of the chick the vitelline and allantoic veins disappear, or become comparatively insignificant vessels, and the posterior vena cava acquires its adult relations.
a. The System of the Anterior Venae Cavae.
Towards the end of the second day a pair of longitudinal vessels, the anterior cardinal veins (of. Fig. 128, VB), are formed in the mesoblast of the sides of the head, slightly ventral to the level of the notochord. They collect the blood from the sides of the head, and carry it backwards.
A similar pair of vessels, the posterior cardinal veins, vc, appear about the same time in the trunk of the embryo. They return the blood from the hinder part of the body, and more especially from the Wolffian bodies or embryonic kidneys, as soon as these are formed.
The anterior and posterior cardinal veins of each side unite, opposite the heart, to form a short transverse vessel, the Cuvierian vein (Fig. 128, VD), sometimes called the ductus Cuvieri, which opens into the sinus venosus, RS, or most posterior division of the heart.
Throughout the earlier stages of development the two cardinal veins return the blood from practically the whole of the body of the embryo, excepting the alimentary canal and the liver. The anterior cardinal veins persist throughout life as the jugular veins, and are joined at an early stage by pectoral veins from the wing, and vertebral veins from the head and neck. The posterior cardinal veins remain of large size so long as the Wolflfian bodies are in functional activity; but when these begin to diminish in size, on the appearance of the permanent kidneys, the posterior cardinal veins also shrink up, and ultimately disappear.
The Cuvierian veins, as already noticed, persist and become the anterior vense cavae of the adult bird.
b. The System of the Posterior Vena Cava.
The meatus venosus. The heart, as already described, is formed by the union of the right and left vitelline veins (Fig. 127, vv), which lie along the edge of the fold of the splanchnopleure, marking the posterior limit of the fore-gut (cf. Fig. 112).
The vitelline veins remain at the edge of the splanchnopleuric fold, and consequently travel backwards with this fold as the embryo becomes more definitely constricted from the yolksac. This shifting backwards of the point of meeting of the right and left vitelline veins causes, first, a lengthening of the heart ; and then, after the heart is definitely established, the formation of a median vitelline vein lying posteriorly to the heart, between the hinder end of the heart and the point of meeting of the two vitelline veins.
This median vitelline vein may be divided into an anterior part or sinus venosus (Fig. 128, ES), which really forms the posterior chamber of the heart and receives the Cuvierian veins ; and a posterior part or meatus venosus, VE.
The meatus venosus has, from the first, very close relations with the liver. The two primary liver diverticula lie one on each side of it, and as the liver increases in size it completely surrounds the vein. Blood-vessels appear in the substance of the liver mass, and soon acquire openings into the meatus venosus. At first these vessels are very irregularly disposed, but by about the fifth day a definite arrangement can be made out. The meatus venosus on entering the liver gives off afferent hepatic vessels (Fig. 128, vo), which open into the capillary plexus of the liver substance ; from this plexus, efferent hepatic vessels (Fig. 128, VH) arise, which open into the meatus venosus shortly before this emerges from the anterior end of the liver. Thus, while the main stream of blood, entering the liver from the vitelline veins, passes along the meatus venosus direct to the heart, a small part of it is diverted through the capillary system of the liver, joining the meatus venosus again further on in its course.
The part of the meatus venosus in the substance of the liver, between the openings of the afferent and efferent hepatic vessels, is usually spoken of as the ductus venosus.
The vitelline veins return to the embryo the blood from the capillary network of the vascular area of the blastoderm (cf. Fig. 99, AV). From the greater part of the area vasculosa the blood is returned directly by the right or left vitelline veins ; but from the more peripheral part of the area the blood is collected by a circular marginal or terminal vein, which runs round the outer edge of the area vasculosa, and from which anterior and posterior veins carry the blood to the vitelline veins.
Of the two vitelline veins, the left is, almost from the first, larger than the right. After the embryo has turned so as to lie on its left side, the difference between the two veins becomes more pronounced, and ultimately the right one disappears.
The allantoic veins. As soon as the allantois is definitely formed, on the fourth day, two allantoic veins are developed, returning the blood from it to the embryo. These unite on entering the embryo to form a single allantoic vein, which runs forwards in the splanchnopleure of the left side to join the left vitelline vein. With growth of the allantois, the allantoic vessels increase greatly in size, but their relations remain practically the same. In the earlier stages (Fig. 128), the vitelline vein is much larger than the allantoic ; but later on, as the yolk becomes smaller through absorption of its contents, while the allantois continues to increase, the proportions are reversed, and the allantoic vein becomes the larger of the two.
The mesenteric vein is formed by the union of the veins which return the blood from the intestine of the embryo. It appears during the fourth day, and is at first very small, but it becomes larger and more important as the alimentary canal lengthens.
The mesenteric vein joins the vitelline vein just before this reaches the liver. The blood entering the liver by the meatus venosus is thus derived from three sources : (i) the vitelline vein returns blood, highly charged with nutritious matter, from the yolk-sac ; (ii) the allantoic vein returns blood from the allantois, rich in oxygen and freed from carbonic acid ; (iii) the mesenteric vein brings venous blood from the walls of the alimentary canal of the embryo. Of these, the vitelline and allantoic veins are, during the period of incubation, much larger than the mesenteric vein ; but inasmuch as the two former vessels return blood from parts outside the embryo, they become obliterated at the time of hatching ; while the mesenteric vein, which alone persists of the three, retains its relations with the liver, and becomes the hepatic portal vein of the adult bird.
The posterior vena cava appears about the fourth day (cf. Fig. 128, vi). It arises in the mesoblast between the hinder ends of the Wolffian bodies, and runs forwards in the median line, ventral to the aorta ; at its anterior end it joins the meatus venosus, as this emerges from the liver, and just before it reaches the heart.
As the permanent kidneys become definitely established, the posterior vena cava acquires special relations with them, and increases in size as they develop. At first (Fig. 128), the posterior vena cava appears as a comparatively unimportant tributary of the meatus venosus, but with growth of the embryo, and especially with the enlargement of the hind limbs, it steadily increases in size, and towards the close of incubation becomes the larger vessel of the two.
After the posterior vena cava has attained some size the hepatic efferent veins shift so as to open into it directly, instead of into the meatus venosus, and it is from this time that the vena cava becomes a larger and more important vessel than the meatus venosus.
5. The Course of the Circulation
It will render the description of the development of the blood-vessels easier to follow if a connected account is given of the course of the circulation on the third day, and during the later stages of incubation.
a. The circulation at the end of the third day (Fig. 113). The heart, at the end of the third day, is a single twisted tube, slightly constricted at internals which mark the boundaries of the successive chambers, but with no trace of a division into right and left sides. The blood enters the hinder end of the heart by the two vitelline veins, and passes out in front through the truncus arteriosus ; this at once splits into right and left branches, each of which again divides into the three anterior aortic arches, which encircle the pharynx and open dorsally into the aortae. The two aortas are widely separate in the head, but approach each other further back, and are fused for a short distance in the body. Behind this point they separate again, and each aorta gives off a large vitelline artery (Fig. 113, AV), which, pasiing out beyond the embryo, opens into the capillary network of the area vasculosa ; from this network the blood is collected again by vessels which unite to form the vitelline veins, and so returns once more to the heart.
The great purpose of the circulation at this stage is to insure the absorption of nutriment from the yolk-sac, and its conveyance along definite channels to the embryo. The vascular system of the embryo itself is as yet only imperfectly formed, and there is no special respiratory organ.
b. The circulation during the latter half of the period of incubation. The heart is now fully formed. The sinus venosus has become absorbed into the right auricle, of which it now forms part : the auricular septum is still incomplete, the large foramen ovale allowing blood to pass freely from the right auricle to the left auricle. The ventricular septum is complete ; and the truncus arteriosus is divided into two entirely separate vessels, of which one, the pulmonary trunk, arises from the right ventricle, and the other or systemic trunk from the left ventricle.
Three pairs of aortic arches are present, but these are the third, fourth, and fifth of the complete series, the first and second having disappeared along the greater part of their length. The systemic trunk, arising from the left ventricle, leads to the third and fourth pairs of aortic arches, and through these to the head and fore-limbs. The pulmonary trunk, arising from the light ventricle, leads to the fifth pair of aortic arches, which andirectly continuous with the dorsal aorta of the body of the embryo, and from which also the small pulmonary arteries arise. From the aorta a vitelline artery carries blood to the yolk-sac ; and a still larger allantoic artery runs from the aorta to the allantois.
The blood is brought back to the heart by three great veins : the right and left anterior venae cavae, and the posterior vena cava. The right and left anterior venae cavae return venous blood from the head and fore-limbs of the embryo. The posterior vena cava returns blood from the hinder part of the body, the hind limbs, and the kidneys ; just before reaching the heart it is joined by the ductus venosus, which returns the blood from the yolk-sac, from the allantois, and from the alimentary canal of the embiyo, by the vitelline, allantoic, and mesenteric veins respectively. The blood in the vitelline vein is arterial as regards nutrient matter ; the blood in the allantoic vein is arterial as regards its gaseous components ; and the blood in the mesenteric vein is venous. The blood in the posterior vena cava is venous as regards nutriment, and as regards gaseous components, but, having just passed through the kidneys, is arterial as regards freedom from nitrogenous excretory matters.
The blood brought to the heart by the posterior vena cava may therefore be spoken of as arterial, and stands in this respect in marked contrast to the venous blood brought to the heart by the right and left anterior venae cavae.
All three vena3 cava3 open into the right auricle of the heart ; but, owing to the position and direction of the opening, and to the Eustachian valve, the arterial blood from the posterior vena cava is directed at once through the foramen ovale into the left auricle, while the venous blood from the right and left anterior vena? cava? remains in the right auricle. The right auricle is thus filled with venous blood, and the left auricle with arterial blood.
On contraction of the auricles, the blood they contain is driven into the ventricles, so that the right ventricle will be filled with venous, and the left with arterial blood.
The left ventricle drives its arterial blood along the systemic trunk, and through the third and fourth pairs of aortic arches to the head and fore-limbs ; while the right ventricle forces its venous blood through the pulmonary trunk and the fifth pair of aortic arches into the dorsal aorta, from which part goes to supply the body and hind limbs of the embryo, and part, in the earlier stages by far the larger part, passes out along the vitelline and allantoic arteries to the yolk-sac and allantois, where it takes up nutriment and oxygen.
The enormously disproportionate size of the head and anterior part of the embryo and the stunted condition of the hinder part during the earlier stages are to be ascribed, at any rate in part, to this arterial supply of the anterior half as contrasted with the venous supply of the posterior half of the embryo.
c. The changes in the circulation on hatching. The changes which occur in the blood-vessels at or about the time of hatching of the chick are comparatively slight, but suffice to convert the circulation into that of the adult bird. The more important of these changes are the following :
(i) Closure of the ductus arteriosus of each side : i.e. obliteration of the distal parts of the fifth pair of aortic arches, beyond the points of origin of the pulmonary arteries (cf. Fig. 128, A.5). The effect of this change is that the blood from the right ventricle can no longer pass into the aorta, but is sent entirely, through the pulmonary arteries, to the lungs.
(ii) Obliteration of the vitelline and allantoic veins. This is a necessary consequence of the complete absorption of the yolksac, and the casting off of the allantois. Its effect is to reduce the blood supply of the liver to the venous blood brought by the mesenteric vein, or, as it may now be called, the hepatic portal vein.
(iii) Closure of the ductus venosus. The effect of this change , is to render it impossible for the blood brought to the liver to pass straight through it to the heart. All the blood entering the liver by the portal vein must now pass through the capillaries of the liver in order to get to the posterior vena cava, and so to the heart. The short cut by which the liver capillaries could previously be avoided is now stopped.
(iv) Closure of the foramen ovale. This is not completed until some little time after hatching. Its effect is to absolutely prevent the passage of blood from the right to the left auricle. From the time of its completion, the blood brought to the heart by all three venae cava? is discharged into the right auricle, and passes from this into the right ventricle ; while the only blood entering the left auricle is that brought to it from the lungs by the pulmonary veins.
On the completion of these changes the circulation becomes that of the adult bird. The arterial and venous streams of blood are kept quite distinct, and the so-called double circulation is completely established.
Development of the Urinary Organs
1. General Account
The urinary organs of the chick, while agreeing in their general relations and mode of development with those of the frog, yet present considerable and important points of difference.
The head kidneys, which in the young tadpole are of large size, and for a considerable time are the sole excretory organs present, are in the chick extremely rudimentary structures, which appear later than the WolfBan bodies, and disappear again almost at once.
The Wolffian bodies of the chick are developed early : they soon attain a large size, and form the functional excretory organs during embryonic life. Shortly after the time of hatching, they lose their kidney structure and excretory function completely, though parts of them persist as accessory portions of the reproductive apparatus.
The Wolffian and Miillerian ducts develop independently in the chick, at any rate so far as their anterior ends are concerned. As in the frog, the Miillerian duct becomes the oviduct of the female, while the vas deferens of the male is formed from the Wolffian duct.
2. The Wolffian Duct
The Wolffian duct, which is the first part of the kidney system to be developed, appears early in the second day, in embryos with about eight pairs of rnesoblastic somites (cf. Fig. 110). The duct arises on either side as a ridge-like projection of the mesoblast, immediately beneath the dorsal epiblast, and a little to the outer side of the mesoblastic somites. At the stage mentioned, it lies opposite the three hindmost of the eight somites.
As the embryo grows, the ridge lengthens, extending slowly forwards, and more rapidly backwards. It also becomes free from the mesoblast ; and in embryos with fourteen pairs of somites, i.e. of about the thirty-sixth hour (cf. Fig. Ill), the Wolffian duct is present along each side of the body as a solid rod of cells, lying free between the epiblast and the mesoblast, and extending from the fourth to the fourteenth somite.
The rod soon becomes tubular, by acquiring a central lumen ; and at the same time its position becomes changed, the mesoblast growing rapidly, and spreading over the duct, between it and the epiblast. The Wolffian duct (Fig. 129, KG) consequently gets driven down into the mesoblast, where it lies, about the level of the dorsal aortal, imbedded in the mass of mesoblast, sometimes spoken of as the intermediate cell mass, which projects into the dorsal part of the body cavity as a longitudinal ridge between the somatopleuric and splanchnopleuric folds.
The Wolffian duct continues its growth backwards during the succeeding stages, and towards the end of the fourth day (cf. Fig. 123, KC) reaches the cloaca, or terminal dilatation of the alimentary canal, and opens into its dorsal surface.
3. The Wolffian Body
The Wolffian body is developed entirely from mesoblast, and agrees closely in its structure and mode of development with that of the frog. In its fully formed condition (Fig. 123, KM) it consists of a complicated mass of convoluted Wolffian tubules, each of which commences with a Malpighian body, and opens at its other end into the W'olffian duct.
The Wolffian body extends along the greater part of the length of the dorsal body-wall, as far back as the thirtieth somite. Its anterior end is imperfectly developed, or even rudimentary from the first, but from about the sixteenth to the thirtieth somite the Wolffian body remains of large size almost up to the time of hatching.
In their mode of development the Wolffian tubules present certain differences in the anterior and posterior parts of the Wolffian body respectively, and must be described separately.
In front of about the sixteenth somite, a number of small funnel-like depressions of the peritoneal epithelium appear, towards the end of the second day (Fig. 129, KS), below and a little to the inner side of the Wolffian duct. The bottoms of these funnels are in connection with slightly twisted cellular cords, which form in the mesoblast. These cords soon become tubular, and acquire communications at one end with the funnel-like depressions, and at the other end with the Wolffian duct ; so that the peritoneal funnels, or nephrostomes, now lead from the coelom through these Wolffian tubules into the Wolffian duct. The nephrostomes, and the Wolffian tubules into which they open, are usually from the first rather more numerous than the somites in which they lie.
FIG. 129. A transverse section across the body of a Chick Embryo at the forty-eighth hour of incubation, x 150.
A, aorta. AN", amiiion. C, body cavity or coeloin. CH, notochord. CM, myocoel, or cavity of mcsoblastic somite. TT, hypoblast forming roof of mid-gut. KG, Wolffian duct. 3Z.S, nepbrostome. ME, somatopleuric layer of mesoblast. MH, Splanchnopleuric layer of mesoblast. MP, muscle plate. N"E, rudiment of spinal ganglion. NS, spinal cord. OE, genital epithelium. VC, posterior cardinal vein. W, vitelline vein.
The anterior Wolffian tubules are imperfect from the first, and soon undergo degenerative changes. The mouths of the nephrostomes first dilate greatly. A vascular process, or glomerulus, then arises from the side wall of each tubule, and projects into its cavity ; by further enlargement, the glomeruli grow out through the expanded mouths of the tubules, or nephrostomes, so as to hang freely into the body cavity. These changes are probably of a degenerative character, for shortly afterwards both the glomeruli and the tubules disappear completely. Further back in the body, but still in front of the sixteenth somite, the Wolffian tubules develop differently ; the nephrostomes close, the tubules separate from the peritoneum, and then become dilated to form Malpighian bodies, into which little vascular tufts or glomeruli. derived from the aorta, soon penetrate.
In the posterior part of the Wolffian body, from about the sixteenth to the thirtieth somites, there are no nephrostomes. The Wolffian tubules in this region have no connection with the peritoneal epithelium, but arise from the first in the mesoblast, appearing as oval vesicles, which by elongation become the Wolffian tubules. These acquire openings into the Wolffian duct, and dilate at their opposite ends to form Malpighian bodies. After the first-formed tubules are completed, others arise in the same manner, and usually nearer the dorsal surface. Partly owing to this increase in the number of the Wolffian tubules, and partly owing to each tubule increasing greatly in length and becoming much convoluted, the Wolffian body soon attains a considerable size, causing a marked ridge-like projection of the intermediate cell mass into the body cavity, along each side of the mid-dorsal line.
4. The Head Kidney and the Mullerian Duct
Towards the end of the fourth day, three pit-like involutions of the peritoneal epithelium appear, one behind another, close to the outer side of the Wolffian duct, and three or four somites behind its anterior end. A ridge-like thickening of the peritoneal epithelium connects the three pits of each side with one another, and grows backwards behind the third pit as a solid rod of cells, lying along the outer side of the Wolffian duct, and very close to this.
This rod soon becomes tubular, ending blindly behind, but opening in front into the body cavity through the three pits. These three pits form the head-kidney of the chick embryo, and the tube into which they open is the commencement of the Miillerian duct.
Towards the end of the fifth day the two hinder pits close up and disappear. The anterior pit persists, and forms the peritoneal opening of the Miillerian duct or oviduct. The Miillerian duct itself grows rapidly backwards ; it? lies in close contact with the outer wall of the Wolffian duct, and in its hinder part appears to be formed from cells derived from the wall of the Wolffian duct.
About the end of the sixth day, the Miillerian duct has grown backwards as far as the cloaca. It remains blind at its hinder end in the male, but in the female opens, at a later stage, into the cloaca.
5. The Permanent Kidney, or Metanephros ; and the Ureter
The cloacal opening of the Wolffian duct (Fig. 123, TC) is opposite the thirty-fourth somite, while the hinder end of the Wolffian body itself does not extend behind the thirtieth somite. The permanent kidney is formed in the mesoblast immediately behind the Wolffian body, and is at first confined to the somites from the thirty-first to the thirty-fourth inclusive, i.e. to the somites between the hinder end of the Wolffian body and the cloacal opening of the Wolffian duct.
Towards the end of the fourth day, the ureter arises on each side (cf. Fig. 123, KD) as a forwardly directed diverticulum from the dorsal surface of the hinder end of the Wolffian duct. From the ureter lateral outgrowths arise, which become continuous with masses of cells in the mesoblast around it ; and from these cells the tubules of the kidney, with their Malpighian bodies, are formed at a later stage. The kidney is at first very small as compared with the Wolffian body, but towards the close of incubation it increases very considerably in size, growing forwards dorsal to the Wolffian body. The ureters, which at first open into the dorsal wall of the cloaca, through the hinder ends of the Wolffian ducts (Fig. 123), acquire independent openings into the cloaca about the sixth day.
6. The Genital Ducts
a. In the male, or cock bird, the Miillerian ducts, although they reach the cloaca, never open into it. In the later stages they undergo degenerative changes, and ultimately they become almost completely obliterated on both sides of the body.
The Wolffian body aborts in great part : a portion of it. however, persists as the epididymis ; while outgrowths from the Malpighian bodies of this portion penetrate into the testis, and become the vasa efferentia. The Wolffian duct persists as the vas deferens, becoming bent on itself into short transverse folds along almost its entire length, in the manner so frequently seen in the male genital duct both of Vertebrates and of Invertebrates.
b. In the female, or hen bird, the Wolffian body atrophies, a small part alone persisting as the parovarium. a body lying in the mesentery between the ovary and the kidney, and in which the tubular structure of the Wolffian body remains recognisable even in the adult bird. The Wolffian duct disappears.
The Miillerian duct of the right side, like the right ovary, disappears, though traces of it may persist in the adult. The left Miillerian duct becomes the oviduct : its peritoneal opening becomes the fimbriated mouth of the oviduct, and its walls thicken greatly, especially at the cloacal end.
7. The Supra-renal Bodies
The development of the supra-renal or adrenal bodies is not very satisfactorily determined. Each consists of two parts, cortical and medullary. Of these, it is generally agreed that the medullary portion arises as a part of the sympathetic nervous system. The cortical portion is developed from groups of cells which appear, about the end of the fourth day, in the mesoblast along the inner side of the Wolffian body, between this and the aorta ; and which increase during the fifth and sixth days so as to form cell masses of considerable size. On the seventh day these masses become closely attached to the Malpighian capsules of the Wolffian body, but whether this is to be interpreted as indicating any essential connection between the two bodies is very doubtful.
The Development of the Body Cavity and Muscular System
1. The Body Cavity or Coelom
The body cavity or coelom is formed, as described on p. 243, towards the end of the first day, by the splitting of the mesoblast, or rather by rearrangement of the mesoblast cells into two layers, upper or somatic and lower or splanchnic respectively ; the body cavity being the narrow chink-like space between the two layers.
In the later stages, the body cavity enlarges considerably, by further separation of its walls. It extends the whole length of the body, but does not reach into the head, stopping at the first mesoblastic somite or proto vertebra.
The two halves, right and left, of the body cavity are at first separate ; but as the ventral body-wall of the embryo is completed, by constriction of the embryo from the yolk, the two halves meet and open into each other across the mid-ventral plane. From the mode of its formation, the body cavity is directly continuous with the space between the two layers, inner and outer, of the amnion (Figs. 112 and 129, AN) ; and it is owing to this continuity that the allantois is able to pass out beyond the limits of the embryo, and spread over its back (Figs. 100 and 101).
The mesoblast cells lining the body cavity become the peritoneal epithelium ; from this epithelium the genital organs, i.e. the ovary or testis, are developed ; and from it, or in close relation with it, the tubules of the Wolffian body are formed.
2. The Pericardial Cavity
There is at first no separate pericardial cavity ; the heart lying in the anterior part of the general body cavity, ventral to the oesophagus and pharynx (Fig. 112). About the end of the second day a septum begins to form, which shuts off the ventral portion of the body cavity, in which the heart lies, at first partially and ultimately completely from the general body cavity ; and this shut-off portion becomes the pericardial cavity. The septum is formed in the following manner.
Opposite the hinder end of the heart the Cuvierian veins cross the body cavity transversely, in order to reach the sinus venosus from the somatopleure ; and it is from the walls of the Cuvierian veins that the pericardial septum arises, as a thin sheet of connective tissue which grows forwards and upwards to the under surface of the fore-gut, and obliquely downwards and backwards to the ventral body wall, which it meets a little way behind the heart.
The lungs lie in two pocket-like diverticula of the bodycavity which extend forwards along the sides of the oesophagus (cf. Fig. 163). These pleural cavities at first lie dorsal to the pericardial cavity, but, as they gradually enlarge to make room for the lungs, they spread ventralwards over the pericardial cavity, between it and the body walls. The pleural cavities in the bird remain throughout life continuous at their hinder ends with the general body cavity.
3. The Muscular System
The mesoblastic somites or proto-vertebree (cf. p. 244) are paired, cubical blocks of naesoblast, arranged in series along the sides of the spinal cord, and formed by transverse division of the vertebral plates of the mesoblast (Figs. 110, 111, 113, MS).
The somites are at first hollow ; their cavities are to be regarded as parts of the ccelom ; and in the case of the anterior three or four somites the cavities are for a time actually continuous with the general body cavity. The walls of the somites are at first of nearly uniform thickness on all sides, but during the second and following days they thicken very unequally, and undergo further changes, by which they give rise to the greater part of the voluntary muscles of the trunk, and to the elements from which the vertebral column is developed. These changes are effected as follows.
During the second and third days (Fig. 129) the ventral walls of the somites thicken very considerably, so that the whole embryo is increased greatly in depth, from the dorsal to the ventral surface, and the cavities of the somites, CM, become situated, not in their centres, but close to their dorsal surfaces. The dorsal part of each somite, inclosing the cavity, or myoccel, CM, now separates off from the underlying mesoblast, as the muscle-plate, MP : the walls of the muscle-plate consist of cells of an epithelial character, closely packed side by side, and contrast strongly with the loosely arranged stellate cells of the ventral part of the somite.
The cells forming the deeper, or ventral, walls of the muscleplates become elongated parallel to the axis of the embryo, and very soon become converted into a system of longitudinal muscle fibres. In this way a broad band of longitudinal muscle fibres is formed, stretching along the whole length of each side of the body of the embryo, each band being divided transversely, by the original protovertebral lines of segmentation, into paired muscle segments or myotomes. These muscle bands in the chick correspond to the great longitudinal body muscles of Amphioxus or of Fishes ; and from them a great part of the voluntary muscular system is developed.
The muscle-plates at first lie nearly horizontally, their inner borders being at a level slightly dorsal to their outer edges. As the body of the embryo increases in thickness, and assumes more definite shape, the muscle-plates become placed more and more obliquely (c/. Fig. 129); and by the end of the third day (Fig. 124, MP) they lie almost vertically, the original inner border becoming dorsal, and the original outer border ventral in position.
The outer, or ventral, edge of each muscle-plate rapidly extends into the somatopleure, and by the fifth day has spread half way down to the ventral surface of the embryo ; the greater part of its component cells become converted into fusiform musclecells, from which the muscles of the back and trunk are developed. At the dorsal and ventral borders of the muscle-plates the cells retain their epithelial character so long as the plate continues to grow.
In the segments opposite the limb buds (Fig. 115), the muscle-plates stop at the bases of the limbs ; the muscles of the limbs themselves are formed independently of the muscleplates, about the fifth or sixth day, an arrangement which is probably to be regarded as a modification of a more primitive one by which the musculature of the limbs is derived directly from that of the body.
The Development of the Skeleton
The skeleton of the chick, like that of the tadpole, consists in its earliest stages of densely packed mesoblast cells, which soon give rise, by the formation of an intercellular matrix, to the primary or cartilaginous skeleton. This, in the adult, is almost completely replaced by the secondary or bony skeleton, but persists in places, especially in the skull.
As in the frog, bone may appear either in direct relation with the pre-existing cartilage, or independently of it, and hence a distinction may be drawn between cartilage-bones and membrane-bones ; the latter, which appear independently of the cartilage, being almost confined to the head.
1. The Vertebral Column
The vertebral column is formed from the ventral portions of the mesoblastic somites or proto vertebrae. After the muscleplates have separated off, the remaining or ventral portions of the somites of each pair, which consist of indifferent and rather loosely compacted niesoblast-cells, grow inwards towards the median plane, and meet both above and below the spinal cord, and below the notochord as well ; so that by the end of the third day (Fig. 124) both the spinal cord and the notochord have mesoblastic investments, which during the fourth day increase considerably in thickness.
Early on the fifth day, the transverse lines of demarcation between the successive pairs of somites disappear, the mesoblast becoming a continuous mass the whole length of the body, and forming continuous investments to the notochord and the spinal cord. This fusion only concerns the ventral portions of the somites, the muscle-plates retaining their original distinctness.
In the course of the fifth day, the mesoblast immediately around the notochord becomes cartilaginous, forming a continuous unsegmented cartilaginous tube, ensheathing the notochord along the whole length of the body. At the sides of the spinal cord, paired cartilaginous bars appear, which soon fuse with the cartilaginous investment of the notochord, and become the neural arches of the vertebrae.
A little later, but before the close of the fifth day, further histological changes occur in the cartilaginous tube surrounding the notochord. Opposite the places of attachment of the neural arches, the matrix becomes more abundant, and the cartilage cells fewer; while between the successive neural arches the matrix remains comparatively scanty, and the cartilage cells more numerous. In this way the cartilaginous tube round the notochord, while still remaining a continuous unsegmented structure, becomes marked into alternate vertebral and intervertebral rings, the vertebral rings being the parts to which the neural arches are attached, and in which the cartilage is of a more mature type ; and the intervertebral rings being the parts between successive neural arches, in which the cartilage remains of a more embryonic character.
FIG. 130. The left half of the skeleton of the Common Fowl. The skull, vertebral column, and sternum are bisected in the median plane. (From Marshall and Hurst.)
iscliium. L, lacrymal bone. MC3, inetacarpal bone of tliird digit. MW, mandible. carpal bone. UP, uncinate process of rib. Z. infra-orbital bar. 1, 2, 3, 4, the first, second, third, and fourth digits.
Each intervertebral ring, about the end of the fifth day, divides into two portions, anterior and posterior, which attach themselves to the vertebral rings in front of, and behind, them respectively. By this division the originally continuous cartilaginous sheath of the notochord becomes cut up into a series of segments ; each segment consisting of a vertebral ring, fused with the posterior and anterior halves of successive intervertebral rings, and fused also with a cartilaginous neural arch. The segments so formed become the adult vertebrae (Fig. 110).
The original vertebral rings, and the neural arches, lie opposite the intervals between successive pairs of muscle-plates ; while the intervertebral rings lie opposite the muscle-plates themselves. As the division into vertebrae takes place across the centres of the intervertebral rings, it follows that the planes of division between the vertebraa do not coincide with the planes of division between the muscle-plates, i.e. with the original protovertebral planes of division, but are midway between these. Hence the division, or segmentation, of the vertebral column has been spoken of as secondary or permanent segmentation, in contrast to the primary or protovertebral segmentation which is retained by the muscle-plates or myotomes.
It must be borne in mind, however, that the permanent segmentation is the only one ever shown by the skeletal elements themselves. The primary segmentation is essentially a division into myotomes or muscle segments, and occurs at a time when the notochord is the only skeletal structure present. The cartilaginous skeleton is at its first appearance unsegmented, and the only segmentation it ever shows is the permanent or ' secondary ' one.
The reason why the permanent or vertebral segmentation alternates with the primary or myotomic segmentation is probably to be found in mechanical considerations. The longitudinal muscle fibres of the myotomes are attached at their ends to the vertebras or to their processes, and the strain 011 the axial skeleton, and consequent tendency to lateral bending, caused by alternate contractions of the muscles of the two sides of the body, will be greatest, not opposite the attachments of the muscles, but midway between these ; and it is at these midway points that the intervertebral joints are formed when the axial skeleton becomes too rigid to allow of free bending of the body, without segmentation. The mechanical advantage of the arrangement, by which each vertebra is acted on by two myotomes on each side, one pulling it forwards and one backwards, is sufficiently clear ; and the actual segmentation is probably due, in the first instance, to the direct action of the muscles themselves, causing bending, and subsequently jointing, of the originally continuous cartilaginous tube at points midway between the attachments of the muscles.
Up to the sixth or seventh day (c/. Fig. 116, CH), the notochord remains of full size and nearly uniform diameter ; but from this time it becomes gradually encroached on by the vertebral centra surrounding it, which grow inwards, constricting it, and causing its gradual absorption and final disappearance. Ossification of the vertebras begins about the twelfth day, in the centrum of the second or third cervical vertebra, and gradually extends backwards along the column; the neural arches ossify rather later than the centra, and independently of them ; each having two centres of ossification.
At an early stage, about the seventh day, the true centrum of the first, or atlas, vertebra separates from the outer ring, and becomes attached to the second, or axis vertebra, as its odontoid process. The atlas and axis vertebras have no rib elements, but these are present in the remaining cervical vertebras ; they lie to the outer sides of the vertebral arteries, and they are from the first continuous with the centra of the vertebras to which they belong, except in the case of the hindmost two or three cervical vertebrae, in which the rib elements are for a time independent.
In a seven-day chick embryo there are forty-five vertebra? present, of which the hindmost five or six fuse at a later stage to form the pygostyle.
Of the ' sacral ' vertebras, the first four have ribs, which in the first are long, and in the remaining three are much shorter. The fifth, sixth, seventh, and eighth vertebras have no ribs ; the ninth and tenth have ribs, and are for this reason regarded by many zoologists as the true sacral vertebras. The remaining five, or ' urosacral,' vertebras have no rib elements. Up to the seventh day these latter are quite distinct from the ilia, which stop at the tenth vertebra of the sacral series : in the later stages the ilia gradually extend further backwards, and ultimately overlap and fuse with all five ' urosacral ' vertebras.
2. The Skull
The skull of the chick consists of the same morphological elements as that of the frog, viz. :
(i) The cranium, or brain case.
(ii) The sense capsules, olfactory and auditory.
(iii) The visceral skeleton.
The sense capsules and the cranium are, however, so closely united from their earliest appearance that it will be convenient to describe them together.
The main factors of the skull may be recognised, in the form of tracts of condensed mesoblast, as early as the fourth day, but it is not until the sixth day that cartilage is definitely established.
a. The Cartilaginous Skull.
(i) and (ii) The Cranium and the Sense Capsules.
At the end of the sixth day, when cartilage first appears in definite form, the structure of the skull is as follows (cf. Figs. 116 and 123). The notochord extends forwards in the median plane, beneath the brain, as far as the hinder end of the pituitary body, where it stops. At the sides of the notochord, and in close contact with it, are a pair of horizontal cartilaginous plates, the parachordal plates, which, with the notochord, form a broad floor to the hinder part of the skull, underlying the hind- and midbrains. Imbedded in the parachordal plates, and continuous with them from their first appearance, are the cartilaginous auditory capsules, inclosing the auditory organs.
In front of the notochord, the parachordals are continued forwards as a pair of short and rather slender rods, the trabeculse cranii : these lie at the sides of the pituitary body, and unite in front of this to form the ethmoidal plate, which underlies and supports the fore-brain.
By the eighth day (Figs. 131 and 116) important changes have occurred in the skull, mainly associated with the growth forwards of the beak.
FIG. 131. The skull of a Chick Embryo at the end of the eighth day of incubation ; seen from the right side. The head and eye are represented in outline, x 10.
AN", ansrulare. AR, articular ]>ortion of mainlibular bar. BB, basi-branchial cartilage. BK, cerato-branchial cartilage. CL, columella. EH, external or horizontal semicircular canal. EP, posterior vertical semicircular canal; ET, rnescthruoid cartilage. 3PR, fenestra ovalis. HR, ceratohyal cartilage. MC, Meokel's cartilage. OC, occipital condyle. OK, slit-like aperture of olfactory capsule. OL, outline of lens. ON", outline of eyeball. PG, pterj-goid. Q,, quadrate cartilage. Q,J", quadratojugul. RL, trabecula cranii. SE, pre-'sphenoidal region. SF, ali-sphenoidal region. SL, supra-occipital region. SR. supra-angular.
At the hinder end of the skull the two parachordal cartilages (Fig. 116, EC) have united, above and below the notochord, to form the basilar plate ; and the sides of the basilar plate, including the auditory capsules which are fused with them, have grown upwards to form the side walls of the skull (Fig. 131) ; the exoccipital, ali-sphenoidal, and orbito-sphenoidal regions being already established.
In front of the pituitary body the ethmoidal plate (Fig. 131, ET) has grown enormously ; it extends forwards to the tip of the beak, and is fused in front with the cartilaginous capsules of the olfactory organs, OK. From the dorsal surface of the ethmoidal plate, along its whole length, a huge vertical crest, the interorbital plate, has arisen, which supports the fore part of the brain (Fig. 11 G) along its upper edge, and is notched in front for the passage of the olfactory nerves, I.
(iii) The Visceral Skeleton.
In the chick embryo, cartilaginous elements, corresponding to the cartilaginous bars of the tadpole's skull, are developed in the mandibular, hyoidean, and first branchial arches.
The mandibular arch. In the mandibular arch two cartilages appear, proximal and distal respectively, which are from the first independent. The proximal, or dorsal, one (Fig. 131, Q) is the quadrate cartilage, a stout tri-radiate cartilage of which the longest arm, or otic process, is directed backwards, and articulates with the auditory or periotic capsule ; while the ventral, and stoutest limb furnishes the articular surface for the mandible. The distal cartilage of the mandibular arch is a slender rod, Meckel's cartilage, MC, which forms the basis of the lower jaw ; its hinder end, AR, which articulates with the quadrate, is expanded and thickened.
The hyoid arch. The bar of cartilage belonging to the hyoid arch is imperfect or absent along the greater part of its length, its dorsal and ventral ends alone being present. The uppermost or dorsal end is believed to be represented by the columella (Fig. 131, CL), a slender rod of cartilage which very early fuses with the stapes, a small plug of cartilage formed in the membrane closing the fenestra ovalis. The ventral end of the hyoid bar forms the cerato-hyal, or lesser cornu of the hyoid, HR ; and the median element of the hyoid, or basihyal, appears also to belong to this arch.
The first branchial arch. In the ventral part of the first branchial arch a slender cartilaginous bar, the cerate-branchial, or greater cornu of the hyoid (Fig. 131, I?K), is formed ; and in the mid-ventral plane, where the arches of the two sides meet, a median basibranchial cartilage, BB, is developed.
In the hinder branchial arches no skeletal elements are formed in the chick.
b. The Osseous or Bony Skull.
(i) The cartilage-bones developed in connection with the skull are as follows :
From the parachordal cartilages are formed the basi-occipital, ex-occipitals, and supra-occipital.
FIG. 132. The skull of the Fowl, from the right side. (From Marshall and Hurst.)
A, articular surface of the mandible. AT, anterior tympanic recess, leading to Eustachian tube. B, pterygoid. C, occipital condyle. D, palatine. E. rostrum. F, mandibular foramen. FO, feuestra ovalis. FR, "fenestra rotunda. FZ, zygomatic process of frontal bone. GJ-, supra-angular. TT, dentarv. IS, inter-orbital septum. J, jugal. L, lacrymal. M, maxilla. MP, maxillo-palatine process of maxilla. If, nasal. OF, optic foramen. PM, premaxilla. PT, iwsterior tympanic recess. Q,. quadrate. Q,J, quadrato-jugal. SF, olfactory foramen. SZ, zygomatic process of squamosal. TF, foramen for fifth nerve.
From the periotic capsules are formed the prootics, epiotics, and opisthotics.
From the trabeculas cranii, the ethmoidal cartilages and the olfactory capsules, are formed the alisphenoids, orbitosphenoids, presphenoid, and mesethmoid.
In the mandibular arch of each side are formed the quadrate, and the articular e.
In the hyoid arch are formed the columella, the ceratohyal, and basihyal.
In the first branchial arch are formed the ceratobranchial, and basibranchial.
(ii) The membrane-bones are less closely connected with the cartilaginous skull than are the cartilage-bones, and can only be grouped somewhat arbitrarily according to the primary divisions of the cartilaginous skull.
In connection with the cranium and the sense capsules are formed the parietals, squamosals, frontals, lacrymals, nasals, vomer, basitemporal, and parasphenoid.
In connection with the upper jaw are formed the pterygoids, palatines, quadratojugals,jugals, maxillce, and premaxlllce.
In connection with the lower jaw are formed on each side the dentary, angulare, supra-angulare, and splenial.
3. The Pectoral Girdle and Sternum
The sternum develops as two separate halves, apparently formed by fusion of the ventral ends of the ribs, which meet and unite in the median plane on the ninth or tenth day. Both halves contribute to the formation of the keel, which is formed by fusion of their adjacent edges. The keel is very small until towards the close of development. The manubrium of the sternum is formed rather later, and is apparently a secondary outgrowth.
The sternum shows, in its development, evidence of having been originally of greater length, and associated with a larger number of ribs than in the adult fowl. In embryos of the sixth day the two hindmost cervical ribs are attached to the sternum ; on the seventh day they have lost their sternal attachments, but are still greatly elongated. At the hinder end of the thoracic series there is, during the sixth and seventh days, a rudiment of an eighth rib, which disappears shortly afterwards.
In the shoulder girdle, the scapula and coracoid are almost at right angles to each other on the seventh day, the scapula being long and blade-like, and the coracoid short and stout. At the beginning of the sixth day the scapula and coracoid are continuous with each other, but before the end of the day they separate.
The clavicles are membrane-bones ; the median part of the furcula has been compared to an interclavicle, but the embryological evidence does not support this view.
4. The Fore-limb or Wing
The humerus, and the radius and ulna, present no points of special importance in their development ; but the carpus and the manus show peculiar modifications in all birds, and are of much greater interest (Fig. 130).
a. The carpus. On the seventh day the carpus consists of: (i) a proximal row of two cartilages, of which the larger one is situated opposite the end of the radius, and is regarded by Parker as corresponding to the radiale and intermedium of the typical carpus ; while the smaller one, placed opposite the end of the ulna, is commonly regarded as the ulnare, but, according to Parker, corresponds to the ulnare and centrale of the typical carpus, (ii) A distal row of two cartilages, a larger one on the radial side and a smaller one on the ulnar side. A little later, about the tenth day, the radial cartilage divides into two, giving three cartilages in the distal row of the carpus, of which the middle one is the largest.
The carpus remains in this condition until some time after the hatching of the chick. The two proximal carpals persist as the two free carpals of the adult bird ; they begin to ossify in chicks about five weeks after hatching. The distal carpals remain free for some time, but ultimately, in chicks eight or nine months old, they unite with the metacarpals.
b. The digits. In the manus of the fowl there are three wellformed digits, which at first are quite independent of one another, and which correspond to the three radial digits, pollex, index, and medius, of the typical Vertebrate manus. The fourth digit is rather doubtfully represented by a small rudiment.
On the seventh day the first three metacarpals are welldeveloped cartilaginous rods, completely separate from one another, and from the carpus. The first metacarpal is short ; the second long and thick ; the third about the same length as the second, but much thinner. The first digit or pollex has two short phalanges ; the second digit or index has three ; and the third digit or medius has two, of which the terminal one ultimately disappears.
During the tenth day a small nodule of cartilage, the prepollex, appears on the radial side of the first metacarpal, with which it ultimately fuses. Another small bar of cartilage appears, about the same time, on the outer side of the third metacarpal, at its proximal end ; this, which perhaps represents the fourth metacarpal, remains distinct until some time after hatching, ultimately fusing with the base of the third metacarpal.
The further changes in the manus are effected very slowly. The three metacarpals begin to ossify about the tenth or twelfth day, but remain distinct from one another until about a month after hatching, when they begin slowly to unite together ; the fusion of the metacarpals with the distal row of carpals does not occur until some time later.
5. The Pelvic Girdle
The pelvic girdle, about the sixth day, consists of a somewhat squarish plate on either side of the body, the central part of which is at first directly continuous with the femur. The dorsal border of the plate corresponds with the iliac region, which at this stage does not extend over more than about three somites, differing in this respect very markedly from its condition in the adult. From the ventral and anterior border of the plate two processes, prepubic and pubic, project downwards and forwards ; and from the ventral and posterior border a broad ischiatic process projects downwards and inwards.
During the seventh day, the femur becomes separated off from the hip girdle ; the ilium extends rapidly backwards along the vertebral column, and more slowly forwards ; the ischium grows backwards ; and the pubis also begins to grow backwards as a slender bar, lying parallel to the ventral border of the ischium, and a little way below this. The prepubic process is relatively much less conspicuous than in the earlier stages.
During the later stages of development these changes become more and more marked, and the pelvis gradually acquires its adult shape. The ilium elongates, both anteriorly and posteriorly, becoming ultimately attached to no less than fifteen vertebras. The ischium also lengthens greatly : its hinder end becomes expanded, and, shortly before the time of hatching, fuses with the posterior end of the ilium, to complete the boundary of the ilio-sciatic foramen. The pubes elongates still more than the ischium, and forms a long slender rod, lying parallel to the ventral edge of the ischium and projecting backwards some distance beyond this. The prepubic process, which, both in its cartilaginous condition and when ossified, appears to belong to the ilium rather than to the pubes, forms in the adult a small blunt process, projecting from the anterior and ventral border of the acetabulum .
6. The Hind-limb or Leg
The femur, and the tibia and fibula, present no special points of interest in their development.
The tarsus consists, on the seventh day, of a proximal row of two tarsal cartilages, of which one represents the tibiale and intermedium, and the other the fibulare of the typical tarsus; and a single distal cartilage, which shows indications of its formation from three centres. At a later stage the proximal tarsal cartilages fuse together ; and about the fourteenth or fifteenth day the proximal tarsals fuse with the distal end of the tibia to form the tibio-tarsus, while the distal tarsal cartilage fuses with the metatarsals to form the tarso-metatarsus.
In the pes, the first digit, or hallux, is represented by a short metatarsal, of which the proximal end is never present ; and two phalanges. The second, third, and fourth digits are approximately equal in size, having well-formed metatarsals, and three, four, and five phalanges respectively. The fifth digit is represented by a small nodule of cartilage on the outer, or fibular, side of the proximal end of the fourth metatarsal.
The three fully-developed metatarsals, i.e. the second, third, and fourth, remain distinct, though closely apposed, until about the beginning of the third week of incubation, when they fuse with one another and with the distal tarsal cartilage.
Development Of The Feathers
The feathers are formed by special modification of the epidermal coverings of papillge, which appear as projections of the skin about the eighth day of incubation.
Each of the permanent feathers is preceded by an embryonic or down feather, the mode of development of which is as follows. About the eighth day small conical projections of the skin appear, the feather papillae, each consisting of a central core of vascular connective tissue, covered by a cap of epidermis. As the papillas increase in height, their bases become depressed below the general surface of the skin. This depression is most marked on the surface of the papillae towards the tail end of the embryo, and gives rise to the characteristic backward slant of the feathers. The epidermal cap of each papilla consists of a superficial, or epitrichial, layer of flattened pavement cells, and a deeper or Malpighian layer of short cylindrical or cubical cells. At first the epidermal cap is of uniform thickness all over the papilla ; but, as the papilla lengthens, the inner surface of the epithelium thickens along certain lines, so as to form ridges projecting into the papilla. The thickening is due to the formation of a third or intermediary layer of cells, spherical or polygonal in shape, and lying between' the epitrichial and Malpighian layers ; and it is from the ridges formed in this way that the feather is developed.
The thickest and best marked of the ridges runs longitudinally, along the upper or anterior surface of the papilla, from base to apex ; while the other ridges, which develop in order from the apex towards the base of the papilla, arise from the sides of this main ridge, and run very obliquely round the papilla to its lower or posterior surface, the ridges of the two sides not quite meeting on the lower surface of the papilla.
In each ridge, which is thus a solid rod of epithelial cells, the outer cells become elongated and cornified, while the central or axial cells remain comparatively soft. The vascular connective tissue, forming the core of the papilla, now shrinks away from its apex. The outer, or epitrichial, layer of the epidermis, which merely forms a sheath inclosing the papilla, is cast off; and the epithelial ridges or rods, which now alone remain, spread out to form the feather ; the main longitudinal ridge becoming the shaft, and the diverging lateral ridges the barbs ; while minor or tertiary ridges, which arise from the barbs, give rise to the barbules.
Towards the base of the papilla the epithelial ridges die out, and the entire epithelial investment of the papilla, including both epitrichial and Malpighian layers, becomes converted, by cornification of its cells, into the quill of the feather, which remains open at its lower end for the admission of blood-vessels.
At the lower end of the shaft, immediately above the quill, the main epithelial ridge widens, and the two sides bend inwards towards each other, and ultimately meet to form a tube, which is continuous below with the quill, and the upper aperture of which persists as the superior umbilicus of the feather.
The down feathers do not involve the entire length of the papillse, but only their distal or apical portions. The basal portions of the papillae sink deeply below the skin ; a second set of epithelial ridges is formed on them, and gives rise to the permanent feathers, the development of which is essentially similar to that of the down feathers. The permanent feathers, as they are developed, gradually make their way to the surface, replacing the down feathers, the nutrition of which is cut off by further narrowing of the opening, or inferior umbilicus, at the base of the quill.
Feathers do not develop uniformly over the entire surface of the body, but along certain definite lines or tracts spoken of as pterylia, and separated from one another by naked areas or apteria ; the actual arrangement of these feathered and featherless patches varying considerably in different groups of birds.
List of the more important Publications dealing with the development of the Chick.
Von Baer, K. E. : ' Ueber Entwickelungsgeschichte der Thiere.' Konigsberg,
Balfour, F. M. : ' The Development and Growth of the Layers of the Blastoderm ; and on the Disappearance of the Primitive Groove in the Embryo
Chick.' Quarterly Journal of Microscopical Science, vol. xiii. 1873.
'A Treatise on Comparative Embryology.' 1880-81. Balfour, F. M., and Sedgwick, A. : ' On the Existence of a Head-kidney in the
Embryo Chick, and on Certain Points in the Development of the
Miillerian Duct.' Quarterly Journal of Microscopical Science, vol. xix.
1879. Balfour, F. M., and Deighton, F. : 'A Renewed Study of the Germinal Layers
of the Chick.' Quarterly Journal of Microscopical Science, vol. xxii. 1882. Beard, J. : ' The Development of the Peripheral Nervous System of Vertebrates.'
Quarterly Journal of Microscopical Science, vol. xxix. 1888. Brandt, A. : ' Ueber den Zusammenhang der Glandula suprarenalis mit dem
Parovarium resp. der Epididymis bei Huhnern.' Biologisches Central blatt, ix. 1889. Budge, A. : ' Untersuchungen iiber die Entwickelung des Lymphsystems beim
Hiihuerembryo.' Archiv fur Anatomic und Entwickelungsgeschichte.
1887. Cajal, R. : ' A quelle Epoque apparaissent les Expansions des Cellules Nerveuses
de la Moe'lle Epiniere du Poulet ? ' Anatomischer Anzeiger, v. 1880. Cazin, M. : ' Recherches Anatomiques, Histologiques, et Embryologiques sur
1'Appareil Gastrique des Oiseaux.' Annales des Sciences Naturelles,
serie 7, tome iv. 1 888. Corin, G., et Berard, E. : ' Contributions a FEtude des Matieres Albuminoides
du Blanc d'CEuf.' Archives de Biologie, ix. 1889. Coste, M. : ' Histoire Generate et Particuliere du Developpement des Corps
Organises.' Paris, 1847-59. Davies, H. R. : ' Die Entwicklung der Feder und ihre Beziehungen zu anderen
Integumentgebilden.' Morphologisches Jahrbuch, xv. 1889.
Dexter, S. : ' The Somites and Coelome in the Chick.' Anatomischer Anzeiger,
vi. 1891. Duval, M. : ' Etudes Histologiques et Morphologiques sur les Annexes des
Embryons d'Oiseau.' Journal de 1'Anatomie et de la Physiologic, xx.
' De la Formation du Blastoderme dans 1'CEuf d'Oisean.' Annales
des Sciences Naturelles, serie 6, tome xviii. 1885.
' Atlas d'Embryologie.' Paris. 1889. Fabricius ab Aquapendente : ' De Formato Fcetu,' 1600. ' De Formatione
Foetus,' 1604. The earliest descriptions, with figures, of the Development of the Chick and other Vertebrates. Felix, W. : ' Zur Entwickelungsgeschichte der Vorniere des Hiihnchens.'
Anatomischer Anzeiger, v. 1890. Foster, M., and Balfour, F. M. : ' The Elements of Embryology.' Second
edition, by Sedgwick and Heape. 1883. Gadow, H. : 'On the Modifications of the First and Second Visceral Arches,
with especial Reference to the Homologies of the Auditory Ossicles.'
Philosophical Transactions, vol. 179. 1888. Gasser, E. : 'Der Primitivstreifen bei Vogelembryonen.' Marburg, 1878.
' Beitrage zur Kenntnis der Vogelkeimscheibe.' Archiv fur Anatomie
und Entwickelungsgeschichte.' 1882. Gerlach, L. : ' Ueber die entodermale Entstehungsweise der Chorda dorsalis.'
Biologisches Centralblatt, i. 1881. Golowine, E. : ' Sur le DeVeloppement du Systeme Ganglionnaire chez le
Poulet.' Anatomischer Anzeiger, v. 1890. Holl, M. : ' Ueber die Reifung der Eizelle des Huhns.' ' Sitzungsberichte d. k.
Akademie d. Wiss. in Wien,' xcix. 1890. Johnson, Alice : ' On the Development of the Pelvic Girdle and Skeleton of
the Hind Limb in the Chick.' Quarterly Journal of Microscopical
Science, xxiii. 1883. Kastschenko, N. : ' Das Schlundspaltengebiet des Huhnchens.' Archiv fur
Anatomie und Entwickelungsgeschichte. 1887. Kolliker, A. : ' Entwicklungsgeschichte des Menschen und der hoheren Thiere.'
Leipzig, 1879. Kowalevsky, R. : ' Die Bildung der Urinogenitalanlage (des Wolff 'schen Ganges)
bei Huhnerembryonen.' Warsaw, 1875.
Lahousse, E. : 'Recherches sur FOntogenese du Cervelet.' Archives de Biologic, viii. 1888. Liessner, E. : Untersuchungen betreffend die Entwicklung der Kiemenspalten
bei Vertretern der drei oberen Wirbelthierklassen.' Sitzungsberichte
d. Nat.-Gesellschaft. Dorpat. Band viii. 1887. Lindsay, Beatrice : ' On the Avian Sternum.' Proceedings of the Zoological
Society of London. 1885. Mackay, T. J. : ' The Development of the Branchial Arterial Arches in Birds,
with special Reference to the Origin of the Subclavians and Carotids.'
Philosophical Transactions, vol. 179. 1888. Mall, F. P. : ' Entwickelung der Branchialbogen und Spalten des Hiihnchens.'
Archiv fur Anatomie und Entwickelungsgeschichte. 1887.
' Development of the Eustachian Tube, Middle Ear, Tympanic
Membrane, and Meatus of the Chick.' Studies from the Biological
Laboratory of the Johns Hopkins University, vol. iv. 1888.
Marshall, A. Milnes : ' On the Early Stages of Development of the Nerves in Birds.' Journal of Anatomy and Physiology, vol. xi. 1877.
' The Development of the Cranial Nerves in the Chick.' Quarterly Journal of Microscopical Science, xviii. 1878.
'The Morphology of the Vertebrate Olfactory Organ.' Quarterly Journal of Microscopical Science, xix. 1879. Masius, J. : ' Quelques Notes sur le Developpement du Cceur chez le Poulet.'
Archives de Biologie, ix. 1889. Mehnert, E. : ' Untersuchungen iiber die Entwickelung des Os pelvis der
Vogel.' Morphologisches Jahrbuch, xiii. 1887.
Meuron, P. de: 'Sur le Developpement de 1'CEsophage.' Comptes Rendus, tome 102. 1886.
' Recherches sur le Developpement du Thymus et de la Glande Thyroide.' Geneve, 1886.
Mihalkovics, G. V. von : ' Untersuchungen uber die Entwicklung des Harnund Geschlechtsapparates der Amnioten.' Internat. Monatsschr. Anat. Histol., Bd. ii. 1885. Moldenhauer, W. : ' Die Entwicklung des mittleren und des ausseren Ohres.'
Morphologisches Jharbuch, iii. 1877. Onodi, A. D. : ' Ueber die Entwickelung des sympathischen Nervensystems.'
Archiv fiir mikroskopische Anatomic, xxvi. 1885. Pander, C. : ' Beitrage zur Entwicklungsgeschichte des Hiihnchens im Eie.'
Parker, W. K. : ' On the Structure and Development of the Skull of the Common Fowl.' Philosophical Transactions. 1869.
'On the Structure and Development of the Bird's Skull.' Transactions of the Linnean Societj T , series 2, Zoology, vol. i. 1875.
' On the Structure and Development of the Wing in the Common Fowl.' Philosophical Transactions, 1888.
' On the Morphology of the Gallinaceas.' Transactions of the Linnean Society, series 2, Zoology, vol. v. 1891.
' On the Vertebral Chain of Birds.' Proceedings of the Royal Society, vol. xliii. 1888. Rathke, H. : ' Abhandlungen zur Bildung und Entwicklungsgeschichte des
Menschen und der Thiere.' Leipzig, 1833.
Ravn, E. : ' Ueber die mesodermfreie Stelle in der Keimscheibe des Hiihner embryos.' Archiv fur Anatomie und Entwickelungsgeschichte. 1886.
Remak, R. : ' Untersuchungen iiber die Entwickelung der Wirbelthiere.'
Sedgwick, A. : ' Development of the Kidney in its Relation to the Wolffian Body in the Chick.' Quarterly Journal of Microscopical Science, xx. 1880.
' On the Development of the Structure known as the Glomerulus of the Head-kidney in the Chick.' Quarterly Journal of Microscopical Science, xx. 1880,
' On the Early Development of the Anterior Part of the Wolffian Duct and Body in the Chick, together with some Remarks on the Excretory System of the Vertebrata.' Quarterly Journal of Microscopical Science, xxi. 1881.
Semon, R. : ' Die indifferente Anlage der Keimdriisen beim Huhnchen und
340 THE CHICK.
ilire Differenzirung zutn Hoden.' Jenaische Zeitschrift f iir Naturwissen schaft, Band xxxi. 1887. Shore, T. W. : ' Notes on the Origin of the Liver.' Journal of Anatomy and
Physiology, vol. xxv. 1891. Shore, T. W., and Pickering, J. W. : ' The Proaranion and Amnion in the
Chick.' Journal of Anatomy and Physiology, vol. xxiii. 1889. Studer, T. : ' Beitrtige zur Entwicklungsgeschichte d. Feder.' Zeitschrift fur
wissenschaftliche Zoologie,' xxx. 1878. Uskow, N. : ' Die Blutgefiisskeime und deren Entwickelung bei einem Hiihner embryo." Mmoires de 1'Academie Imperiale des Sciences de St.
Petersbourg, s6rie 7, tome xxxv. 1887. Vialleton, L. : ' Developpement des Aortes chez 1'Embryon duPoulet.' Journal
de 1'Anatomie et de la Physiologic, xxviii. 1892. Wijhe,_ J. W. van : ' Ueber Somiten und Nerven im Kopfe von Vogel- und
Reptilien-embryonen.' Zoologischer Anzeiger, ix. 1886. Wolff, C. F. : 'Theoria Generationis.' Halle. 1759.
' De Formatione Intestinorum.' Halle. 1768-69.
|Vertebrate Embryology 1893: 1 Introduction | 2 Amphioxus | 3 Frog | 4 Chick | 5 The Rabbit | 6 Human Embryo | Illustrations|
Marshall AM. Vertebrate Embryology: A Text-book for Students and Practitioners. (1893) Elder Smith & Co., London.
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