Book - The Early Embryology of the Chick 8
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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 Changes Between Twenty-four and Thirty-three Hours of Incubation
In dealing with developmental processes the selection of stages for detailed consideration is more or less arbitrary and largely determined by the phenomena one seeks to emphasize. There is no stage of development which does not show something of interest. It is impossible in brief compass to take up at length more than a few stages. Nevertheless it is important not to lose the continuity of the processes involved. By calling attention to some of the more important intervening changes, this brief chapter aims to bridge the gap between the 24-hour stage and the 3'3-hour stages of the chick both of which are taken up in some detail.
The Closure of the Neural Tube
In comparison with 24hour chicks, entire embryos of 27 to 28 hours of incubation (Fig. 18) show marked advances in the development of the cephalic region. The head has elongated rapidly and now projects free from the blastoderm for a considerable distance, with a corresponding increase in the depth of the subcephaUc pocket and in the length of the fore-gut.
In 24-hour chicks the anterior part of the neural plate is already folded to form the neural groove. Although the neural folds are at that stage beginning to converge mid-dorsally the groove nevertheless remains open throughout its length (Fig. 17, A J B, C). By 27 hours the neural folds in the cephaUc region meet in the mid-dorsal line and their edges become fused.
The fusion which occurs is really a double one. Careful following of Figures 26, A to E, will aid greatly in understanding the process. Each neural fold consists of a mesial component which is thickened neural plate ectoderm, and a lateral component, which is unmodified superficial ectoderm (Fig. 26, A), When the neural folds meet in the mid-dorsal line (Fig. 26, B^C) the mesial, neural plate components of the two folds fuse with each other and the outer layers of unmodified ectoderm also become fused (Fig. 26, D). Thus in the same process the neural groove becomes closed to form the neural tube and the superficial ectoderm closes over the place formerly occupied by the open neural groove. Shortly after this double fusion the neural tube and the superficial ectoderm become somewhat separated from each other leaving no hint of their former continuity (Fig. 26, E).
Fig. 18. Dorsal view ( X 14) of entire chick embryo having 8 pairs of somites (about 27-28 hours incubation).
The Differentiation of the Brain Region
By 27 hours of incubation the anterior part of the neural tube is markedly enlarged as compared with the posterior part. Its thickened walls and dilated lumen mark the region which will develop into the brain. The undilated posterior part of the neural tube gives rise to the spinal cord. Three divisions, the three primarybrain vesicles, can be distinguished in the enlarged cephalic region of the neural tube (Fig. 18). Occupying most of the anterior-part of the head is a conspicuous dilation known from its position as the fore-brain or prosencephalon. Posterior to the prosencephalon and marked off from it by a constriction is the mid-brain or mesencephalon. Posterior to the mesencephalon with only a very slight constriction marking the boundary is the hind-brain or rhombencephalon. The rhombencephalon is continuous posteriorly with the cord region of the neural tube without any definite point of transition.
Fig. 19. Ventral view ( a 45) of head and heart region of chick embryo of 9 somites (about 29-30 hours incubation).
In somewhat older embryos (Fig. 19) the lateral walls of the prosencephalon become out-pocketed to form a pair of rounded dilations known as the primary optic vesicles. When the optic vesicles are first formed there is no constriction between them and the lateral walls of the prosencephalon, and the lumen of each optic vesicle communicates mesially with the lumen of the prosencephalon without any definite line of demarcation.
The relation of the notochord to the divisions of the brain is of importance in later developmental processes. The notochord extends anteriorly as far as a depression in the floor of the prosencephalon known as the infundibulum (Fig. 19). Therefore, the rhombencephalon, mesencephalon, and that part of the prosencephalon posterior to the infundibulum he immediately dorsal to the notochord (are epichordal) while the infundibular region and the parts of the prosencephalon cephalic to. it project anterior to the notochord (are pre-chordal) .
The Anterior Neuropore
The closure of the neural folds takes place first near the anterior end of the neural groove and progresses thence both cephalad and caudad. At the extreme anterior end of the brain region closure is delayed. As a result the prosencephalon remains for sometime in communication with the outside through an opening called the anterior neuropore. The anterior neuropore is still open in chicks of 27 hours ' (Fig. 18). In embryos of 33 hours the neuropore appears much narrowed (Fig. 21). A little later it becomes closed but leaves for some time a scar-like fissure in the anterior wall of the prosencephalon (Fig. 23). The anteriof neuropore does not give rise to any definite brain structure. It is important simply as a landmark in brain topography. Long after it has disappeared as a definite opening the scar left by its closure serves to mark the point originally most anterior in the developing brain.
The Sinus Rhomboidalis
The myelencephahc region of the brain merges caudally without any definite line of demarcation into the region of the neural tube destined to become the spinal cord. The neural tube as far caudally as somite forma- ^ tion has progressed is completely closed and of nearly uniform diameter. Caudal to the most posterior somites the neural groove is still open and the. neural folds diverge to either side of Hensen's node (Fig. 18). In their later growth caudad the neural folds converge toward the mid-line and form the lateral boundaries of an unclosed region at the posterior extremity of the neural tube known because of its shape as the sinus rhomboidalis (Fig. 21). Hensen's node and the primitive pit lie in the floor of this as yet unclosed region of the neural groove and subsequently are enclosed within it when the neural folds here finally fuse to complete the neural tube.
This process in the chick is homologous with the enclosure of the blastopore by the neural folds in lower vertebrates. In forms where the blastopore does not become closed until after it is surrounded by the neural folds, it for a time constitutes an opening from the neural canal into the primitive gut known as the neurenteric canal or posterior neuropore. In the chick the early closure of the blastopore precludes the estabhshment of an open neurenteric canal but the primitive pit represents its homologue.
The Fate of the Primitive Streak
In embryos of about 27 hours the primitive streak is relatively much shorter than in younger embryos (Cf. Figs. 8, 11, 14, 15, and 18). This is due partly to its being overshadowed by the rapid growth of structures lying cephalic to it, and partly to actual decrease in the length of the primitive streak itself. The cells in the primitive stieak region would appear to be contributed to surrounding structures. Whatever the exact fate of its cells may be, the primitive streak becomes less and less a conspicuous feature in the developing embryo. By the time the caudal end of the body is delimited, the primitive streak as a definitely organized structure has disappeared altogether (Cf. Figs. 18, 21, 29, 34).
The Formation of Additional Somites
The division of the dorsal mesoderm to form somites begins to be apparent in embryos of about 22 hours. By the end of the first day three or four pairs of somites have been cut off (Fig. 15) . As development progresses new somites are added caudal to those first formed. In embryos which have been incubated about 27 hours eight pairs of somites have been estabhshed (Fig. 18).
It was formerly beheved that some new somites were formed anterior to the first pair. The experiments of Patterson would seem to indicate quite definitely that the first pair of completely formed somites remains the most anterior and that all the new somites are added posterior to them. The expeiiments referred to were carried out on eggs which had been incubated up to the time of the formation of the first somite. With thorough aseptic precautions the eggs were opened and the first somite marked, in some cases by injury with an '"electric needle" in other cases by the insertion of a minute glass pin. Following the operation the shell was closed by sealing over the opening a piece of egg shell of appropriate size. After being again incubated for varying lengths of time the eggs were reopened. In all cases the injured first somite was still the most anterior complete somite. All the new somites except the incomplete head somite" had appeared caudal to the first pair of somites formed.
The Lengthening of the Fore-gut
Comparison of the relations of the crescentic margin of the anterior intestinal portal in embryos between 24 and 30 hours shows it occupying progressively more caudal positions (Fig. 27). This change in the position of the anterior intestinal portal is the result of two distinct growth processes. The margins of either side of the poital are constantly converging toward the mid-line where they become fused with each other. Their fusion lengthens the foregut by adding to its floor and thereby displaces the crescentic margin of the portal caudad. At the same time the structures cephalic to the anterior intestinal poital are elongating rapidly so that the portal becomes more and more remote from the anterior end of the embryo with the further lengthening of the fore-gut.
As a result of these two processes the space between the subcephalic pocket and the margin of the anterior intestinal portal is also elongated (Fig. 27). This is of importance in connection with the formation of the heart for it is into this enlarging space that the pericardial portions of the coelom extend and in it that the heart comes to he.
The Appearance of the Heart and Omphalomesenteric Veins
Although the early steps in the formal ion of the heart take place in embryos of this range, detailed consideration of them has been deferred to be taken up in connection with later stages when conditions in the circulatory system as a whole are more advanced.
In dorsal views of entire embryos the heait is largely concealed by the overlying rhombencephalon (Fig. i8) but it may readily be made out by viewing the embryo from the ventral surface (Fig. 19). At this stage the heart is a nearly straight tubular structure lying in the mid-line ventral to the fore-gut. ^ Its mid-region has noticeably thickened walls and is somewhat dilated. Anteriorly the heart is continuous with the large median vessel, the ventral aorta, posteriorly it is continuous with the paired omphalomesenteric veins. The fork formed by the union of the omphalomesenteric veins in the posterior part of the heart lies immediately cephalic to the crescentic margin of the anterior intestinal portal, the veins lying within the fold of entoderm which constitutes its margin.
Organization in the Area Vasculosa
The extra-embryonic vascular area at this stage in undergoing rapid enlargement and presents a netted appearance instead of being mottled as in the ear her embryos. The peripheral boundary of the area vasculosa is definitely marked by a dark band, the precursor of the sinus terminahs (marginal sinus). Its netted appearance is due to the extension and anastomosing of blood islands. The formation of the network is a step in the organization of a plexus of blood vessels on the yolk surface which will later be the means of absoibing and transferring food material to the embryo. The afferent yolk-sac or viteUine circulation is established in the next few hours of incubation when this plexus of vessels developing on the yolk surface comes into communication with the omphalomesenteric veins already developing within the embryo and extending laterad. The efferent vitelline circulation is established somewhat later when the omphalomesenteric arteries arise from the aorta of the embryo and become connected with the yolk-sac plexus. (Cf. Figs. 15, 18, 21).
- Next: 33 to 39 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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Cite this page: Hill, M.A. (2021, June 14) Embryology Book - The Early Embryology of the Chick 8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Early_Embryology_of_the_Chick_8
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