Book - The Early Embryology of the Chick 9
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Those interested in historic chicken development should also see the earlier text The Elements of Embryology (1883).
Foster M. Balfour FM. Sedgwick A. and Heape W. The Elements of Embryology (1883) Vol. 1. (2nd ed.). London: Macmillan and Co.
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The Structure of Chicks Between Tihrty-three and Thirty-nine Hours of Incubation
Chicks which have been incubated from 33 to 39 hours are in a favorable stage to show some of the fundamental steps in the formation of the central nervous system, and of the circulatory system. In this chapter, therefoie, attention has been concentrated on these two systems.
During this period of incubation there are also changes in the fore-gut region and in the somites, and differentiation in the intermediate mesoderm which presages the formation of the urinary organs. Consideration of these structures has, however, been deferred until their development has progressed somewhat farther.
The Divisions of the Brain and Their Neuromeric Structure
The metameric arrangement of structures which is so striking a feature in the body organization of all vertebrates, is masked in the head region of the adult by superimposed specializations. In the brain of young vertebrate embryos, however, the metamerism is still indicated. Dissections of the neural plate of chicks at the end of the first day of incubation show a series of eleven enlargements marked off from each other by contrictions (Fig. 20, A). Concerning the precise homologies of individual enlargements with specific neuromeres in other forms there is not complete agreement. The controversies center about the question of neuromeric fusions in the anterior part of the brain. For the beginning student the fact that metamerism is present is to be emphasized rather than the controversies concerning the homologies of neuromeres. With the reservation that some of the anterior enlargements may represent fusions of more than one neuromere, the series of enlargements seen in the brain region of the chick may be regarded as neuromeric. For convenience in designation the neuromeres are numbered beginning at the anterior end.
Fig. 20. Diagrams to show the neuromeric enlargements in the brain region of the neural tube. (Based on figures by Hill.)
- A , lateral view of neural plate from dissection of chick of 4 somites (24 hours) ; B, dorsal view of brain dissected out of 7-somite (26 to 27-hour) embryo; C, dorsal view of brain trom lo-somite (30-hour) embryo; D, dorsal view of brain from 14-somite (36-hour) embryo.
With the closure of the neural tube and the establishment of the three piimary brain vesicles we can begin to trace the fate of the vaiious neuromeric enlargements in the formation of the brain regions. The three anterior neuromeres form the prosencephalon; neuromeres four and five are incorporated in the mesencephalon; and neuromeres six to eleven in the rhombencephalon (Fig. 20, B). Anteriorly the interneuromeric constrictions soon disappear except for two; namely, the one between the prosencephalon and mesencephalon, and the one between the mesencephalon and rhombencephalon. The rhombencephalic neuromeres, however, remain clearly marked fpr a considerable period.
By about 33 hours of incubation the optic vesicles are established as paired lateral outgrowths of the prosencephalon. They soon extend to occupy the full width of the head (Fig. 20, C and Fig. 21). The distal portion of each of the vesicles thus comes to lie closely approximated to the superficial ectoderm, a relatibnship of importance in their later development. At first the cavities of the optic vesicles (opticoeles) are broadly confluent with the cavity of the prosencephalon (prosocoele) . Somewhat later constrictions appear which mark more definitely the boundaries between the optic vesicles and the prosencephalon (Fig. 20, D and Fig. 22).
Fig. 21. Dorsal view ( X 14) of an entire chick embryo of 12 somites (about 33 hours incubation).
Fig. 22. Dorsal view ( X 45) of' head and heart region of a chick embryo of 17 somites (38-39 hours incubation).
There arises also at this stage a depression in the floor of the prosencephalon known because of its peculiar shape as the infundibulum (Figs. 23 and 24). The infundibular region is the site of important changes later in development. At this stage, conditions are not sufficiently advanced to warrant more than calling attention to its origin from, and relations to, the prosencephalon, and to the anterior end of the notochord as shown in the figures referred to.
Fig. 23. Diagrammatic ventral view of dissection of a 35-hour chick embryo. (Modified from Prentiss.)
- The splanchnopleure of the yolk-sac cephalic to the anterior intestinal portal, the ectoderm of the ventral surface of the head, and the mesoderm of the pericardial region, have been removed to show the underlying structures. Figure 24 should be referred to for the relations of the pericardial mesoderm.
In chicks of about 38 hours indications of the impending division of the three primary vesicles to form the five regions characteristic of the adult brain are already beginning to appear. In the establishment of the five-vesicle condition of the brain, the prosencephalon is subdivided to form the telencephalon and diencephalon. the mesencephalon remains undivided, and the rhombencephalon divides to form the metencephalon and myelencephalon.
The division of the prosencephalon into telencephalon and diencephalon is not completed until a much later stage of development, but the median enlargement at this stage extending anterior to the level of the optic vesicles indicates where the telencephalofi will be established (Fig. 20, D). The optic vesicles and that part of the prosencephalon lying between them go into the diencephalon.
The mesencephalon, as stated above, undergoes no subdivision. The original mesencephalic region of the three- vesicle brain gives rise to the mesencephalon of the adult. This region of the brain does not undergo any marked differentiation until relatively late in development.
At this stage the division of the rhombencephalon is clearly marked (Fig. 20, D and Fig. 22). The two most anterior neuromeres of the original rhombencephalon form the metencephalon and the posterior four neuromeres are incorporated in the myelencephalon.
The Auditory Pits
As is the case with the central nervous system; the organs of special sense arise early in development. The appearance of the optic vesicles which later become the sensory part of the eyes has already been noted. The first indication of the formation of the sensory part of the ear becomes evident at about 35 hours of incubation. At this age a pair of thickenings termed the auditory placodes arise in the^ superficial ectoderm of the head. They are situated on the dorso-lateral surface opposite the most ^posterior inter-neuromeric constriction of the myelencephalon. By 38 hours of incubation (Fig. 22) the auditory placodes have become depressed below the general level of the ectoderm and form 1 the walls of a pair of cavities, the auditory pits. When first formed the walls of the auditory pits are directly continuous with the superficial ectoderm, and their cavities are widely open to the outside. In later stages the openings into the pits become narrowed and finally closed so that the pits become vesicles lying between the superficial ectoderm and the myelencephalon. As yet they have no connection with the central nervous system.
The Formation of Extra-embryonic Blood Vessels
In dealing with the circulation of the chick we must recognize at the outset two distinct circulatory arcs of which the heart is the common center. One complete circulatory arc is established entirely within the body of the embryo. A second arc is established which has a rich plexus of terminal vessels located in the extra-embryonic membranes enveloping the yolk. These are the vitelline vessels. The vitelline vessels communicate with the heart over main vessels which traverse the embryonic body. The chief distribution of the vitelline circulation is however, extra-embryonic. Later in development there arises a third circulatory arc involving another set of extra-embryonic vessels in the allantois, but with that we have no concern until we take up later stages. Neither the intra-embryonic, nor the vitelline circulatory channels have as yet been completed but the heart and many of the main vessels have made their appearance.
The formation of extra-embryonic blood vessels is presaged by the appearance of blood islands in the vascular area of chicks toward the end of the first day of incubation (see Chapter Vll). Figure 25 shows the differentiation of blood islands to form primitive blood corpuscles and blood vessels. At their first appearance the blood islands are irregular clusters of mesoderm cells lying in intimate contact with the yolk-sac entoderm (Fig. 25, A). When the lateral mesoderm becomes split forming the somatic and splanchnic layers with the coelom jpetween, the blood islands lie in the splanchnic mesoderm adjacent to the eiitodermi. In embryos of 3 to 5 somites fluid filled spaces begin to appear in the blood islands with the result that in each blood island the peripheral cells are separated from the central ones (Fig. 25, 5). As the fluid accumulates and the spaces expand the peripheral cells become flattened and pushed outward, but they remain adherent to each other and completely enclose the central cells. At this stage the single layer of peripheral cells may be regarded as constituting the endothelial wall of a primitive blood channel (Fig. 25, C). Extension and anastomosis of neighboring blood islands which have undergone similar differentiation results in the establishment of a network of communicating vessels. Meanwhile the cells enclosed in the primitive blood channels have become separated from each other and rounded. They soon come to contain haemoglobin and constitute the primitive blood corpuscles. The fluid accumulated in the blood islands serves as a vehicle in which the corpuscles are suspended and conveyed along the vessels.
Fig. 25. Drawings to show the cellular organization of blood islands at three stages in their differentiation.
- The location of the areas drawn with reference to the body of the embryo and other structures of the blastoderm can be ascertained by reference to Fig. 17, D.
- A, from blastoderm of 18-hour chick; B, from blastoderm of 24-hour chick; C, from blastoderm of 33-hour chick.
The differentiation of the blood islands in the manner described begins first in the peripheral part of the area vasculosa and from there extends toward the body of the embryo. By 33 hours of incubation the extra-embryonic vascular plexus has extended inward and made connection with the omphalomesenteric veins which, originating within the body of the embryo have grown outward. Thus are established the afferent vitelline channels (Fig. 21).
The efferent vitelline channels have not yet appeared and there is no circulation of the blood corpuscles which are being formed in the area vasculosa. The intra-embryonic blood vessels remain empty until the extra-embryonic circuit is completed. The embryo meanwhile draws its nutrition from the yolk by direct absorption.
The Formation of the Heart
The structural relations of the heart and the way in which it is derived from the mesoderm can be grasped only by the careful study of sections through the heart region in several stages of development (Fig. 26). The fact that the heart, itself an unpaired structure, arises from paired primordia which at first lie widely separated on either side of the mid-line, is likely to be troublesome unless its significance is understood at the outset. The paired condition of the heart at the time of its origin is due to the fact that the early embryo lies open ventrally, spread out on the yolk surface. The rudiments of all ventral structures which appear at an early age are thus at first separated, and lie on either side of the mid-line.
As the embryo develops, a series of foldings undercut it and separate it from the yolk. This folding off process at the same time establishes the ventral wall of the gut and the ventral body wall of the embryo by bringing together in the mid-line the structures formerly spread out to right and left. The primordia of the heart arise in connection with layers which are destined to form ventral parts of the' embryo, but at a time when these layers are still spread out on the yolk. As the embryo is completed ventrally the paired primordia of the heart are brought together in the mid-line and become fused (Fig. 27).
The first indication of heart formation is to be seen in transverse sections passing through a 25-hour chick immediately caudal to the anterior intestinal portal. Where the splanchnopleure of either side bends toward the mid-line along the lateral margin of the intestinal portal there is a marked regional thickening in the splanchnic mesoderm of either side (Figs. 26, A and 27, ^). This pair of thickenings indicates where there has been rapid cell proliferation preliminary to the differentiation of the heart. Loosely associated cells can already be seen somewhat detached from the mesial face of the mesoderm layer. These cells soon become organized to form the endocardial primordia.
In a chick of about 26 hours, sections through a corresponding region show distinct dfferentiation of the endocardial and ppimyocardial primordia (Fig. 26, B). The endocardial primordia are a pair of delicate tubular structures, a single cell in thickness, lying between the entoderm and mesoderm. They arise from the cells seen separating from the adjacent thickened mesoderm in the 25-hour chick. As their name indicates they are destined to give rise to the endothelial lining of the heart. By far the greater part of each of the original mesodermic thickenings becomes applied to the lateral aspects of the endocardial tubes as the epi-myocardial primordium which is destined to give rise to the external coat of the heart (epicardium) and to the heavy muscular layers of the heart (myocardium).
In chicks of 27 hours the lateral margins of the anterior intestinal portal have been undergoing concrescence lengthening the fore-gut caudally and involving the heart region. In this process the former lateral margins of the portal swing in to meet each other and fuse in the mid-line, and the endocardial tubes of the right and left side are brought toward each other beneath the newly completed floor of the fore-gut (Figs. 26, C and 27, B). In the 28-hour chick the endocardial primordia, are approximated to each other (Figs. 26 and 27, C) and by 29 hours they fuse in their mid-region to form a single tube (Figs. 26, E and 27, D).
At the same time the epi-myocardial areas of the mesoderm are brought together first ventrally (Fig. 26, D) and then dorsally to the endocardium (Fig. 26, E). Where the splanchnic mesoderm of the opposite sides of the body comes together dorsal and ventral to the heart it forms double layered supporting membranes called respectively the dorsal mesocardium and the ventral mesocardium. The ventral mesocardium is a transitory structure, disappearing almost as soon as it is formed (Fig. 26, E). The dorsal mesocardium, although the greater part of it disappears in the next few hours of incubation, persists in embryos of the stage under consideration, suspending the heart in the pericardial region of the coelom. Conditions reached in the heart region at 33 hours of incubation are shown in section in Figure 28, C. The heart here is enlarged and displaced somewhat to the right of the mid-line but its fundamental relations are otherwise the same as in a 29-hour embryo (Fig. 26, E).
Fig. 26. Diagrams of transverse sections through the pericardial region of chicks at various stages to show the formation of the heart. For location of the sections consult Fig. 27.
- A, at 25 hours; B, at 26 hours; C, at 27 hours; D, at 28 hours; E, at 29 hours.
The gross shape of the heart and its positional relations to other structures are best seen in entire embryos. The fusion of the jpaired cardiac primordia establishes the heart as a nearly straight tubular structure. It lies at the level of the rhombencephalon in the mid-line, ventral to the fore-gut (Fig. 19). By 33 hours-of incubation the mid-region of the heart is considerably dilated and bent to the right (Fig. 21). At 38 hqurs the heait is bent so far to the right that it extends beyond the lateral body margin of the embryo (Fig. 22). This bending process is. correlated with the rupture of the dorsal mesocardium at the mid-region of the heart. The breaking through of the dorsal and ventral mesocardia is of interest aside from the fact that it leaves the heart free to undergo changes in shape. It makes the right and left coelomic chambers confluent, the pericardial region thus being the first part of the coelom to acquire the unpaired condition characteristic of the adult.
Fig. 27. Ventral- view diagrams to show the origin and subsequent fusion of the paired primordia of the heart.
- The lines A, C, D, and E indicate the planes of the sections diagrammed in Fig. 26, Af, C, D, E, respectively.
- A, chick of 25 hours; B, chick of 27 hours; (Schick of 28 hours; D, chick of 29 hours.
Although there, are as yet no sharply bounded subdivisions of the,heart, it is convenient to distinguish four regions which later become clearly marked off from each other (Fig. 23). The most caudal part of the heart where the omphalomesenteric veins join is the sinus venosus; the caudal part of the region of the heait which is dilated and bent to the right will become the atrium; the cephalic part of the heart bend is the ventricular region; and the region where the ventricle swings into the midline and becomes narrowed is known as the bulbo-conus arteriosus. Approximately at the stage of development indicated in Figure 23 irregular twitchings occur in the heart walls, but regular pulsations are not established until about the 44th hour of incubation.
The Formation of the Intra-embryonic Blood Vessels
Coincident with the establishment of the heart, blood vessels have arisen within the body of the embrye. Concerning the exact nature of the process of blood vessel formation there is some disagreement. The weight of evidence seems to indicatethat the early vessels are formed from mesodermal cells which lie in the path of their development. They grow by organization of cells in situ as a drain might be built from bricks already deposited along its projected course. In later stages it seems probable that vessels extend by the formation of budlike outgrowths from their walls, as well as by organization of cells in their path of development. When first formed, the blood vessel walls are but a single cell in thickness. There is no structural differentiation between arteries and veins until a considerably later period. Recognition of the vessels depends wholly, therefore, on determining their course and relationships.
The large vessels connecting with the heart are the first of the intra-embryonic channels estabhshed. From the bulboconus arteriosus the paired ventral aortic roots extend cephalad ventral- to the fore-gut (Fig. 23). At the cephalic end of the
fore-gut the ventral aortic roots turning dorsad curve around it, and then extend caudad, dorsal to the gut, as the paired 4 dorsal aortae (Figs. 23, 24 and Fig. 28, B). Few conspicuous branches arise from the aortae at this stage but as development jprogresses branches extend to the various parts of the embryo the embryonic circulation. Both the ventral aortic roots and the omphalomesenteric veins are direct continuations of the paired endocardial primordia of the heart. The epi-myocardial coat is formed about the original endothelial tubes only where they are fused in the region destined to become the heart. The development of the heart at this stage is an epitome of its phylogenetic origin. The local investment of the endocardial tubes by the epi-myocardium, as seen in the formation of the chick heart, is a recapitulation of the evolutionary origin of the heart by the local addition of a heavy muscular coat about the walls of a blood vessel.
Fig. 28. Diagrams of sections of 33-hour chick. The location of each section is indicated on a small outline sketch of the entire embryo.
During early embryonic life the cardinal veins are the main afferent vessels of the intra-embryonic circulation. The main cardinal trunks are paired vessels symmetrically placed on either side of the mid-line. There are two pairs, the anterior cardinals which return the blood to the heart from the cephalic region of the embryo, and the posterior cardinals which return the blood from the caudal region. The anterior and posterior cardinal veins of the same side of the body become confluent dorsal to the level of the heart. The vessels formed by the junction of the anterior and posterior cardinals are the ducts of Cuvier or common cardinal veins. The right and left ducts of Cuvier turn ventrad, one either side of the fore-gut, and enter the sinus-venosus along with the right and left omphalomesenteric veins, respectively (Fig. 24).
In chicks of 33 hours the anterior cardinal veins can usually be made out in sections (Fig. 28, B, C). By 38 hours the anterior cardinals and the ducts of Cuvier are readily recognized. The posterior cardinals appear somewhat later than the anterior cardinals but are ordinarily discernible in the region of the duct of Cuvier by 33 to 35 hours and well established by 38 hours. For the sake of simplicity and clearness the cardinal veins have been represented in Figure 24 larger and more regularly formed than they are in actual specimens. Like all the other blood vessels of the embryo they arise as irregular anastomosing endothelial tubes, only gradually taking on the regularity of shape characteristic of fully formed vessels.
- Next: 40 to 50 Hours
The Early Embryology of the Chick: Introduction | Gametes and Fertilization | Segmentation | Entoderm | Primitive Streak and Mesoderm | Primitive Streak to Somites | 24 Hours | 24 to 33 Hours | 33 to 39 Hours | 40 to 50 Hours | Extra-embryonic Membranes | 50 to 55 Hours | Day 3 to 4 | References | Figures | Site links: Embryology History | Chicken Development
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