Book - The Early Embryology of the Chick 7
|Embryology - 19 Oct 2020 Expand to Translate|
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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 Twenty-four Hour Chicks
The Formation of the Head
In embryos of 21 to 22 hours the anterior part of the embryonal area is thickened and elevated above the level of the surrounding blastoderm, with a well defined crescentic fold marking its anterior boundary. Between 21 and 24 hours this region has undergone rapid growth (Fig. 15). Its elevation above the blastoderm is much more marked and it has grown anteriorly so it overhangs the proamnion region. The crescentic fold which formerly marked its anterior boundary appears to have undercut the anterior part of the embryo and separated it from the blastoderm. The changes in relationships are due, however, not so much to a posterior movement of the fold as to the anterior growth of the embryo itself. This anterior region which projects, free from the blastoderm, may now properly be termed the head of the embryo. The space formed between the head and the blastoderm is called the subcephaUc pocket (Fig. 17, E).
In the mid-line the notochord can be seen through the overlying ectoderm. It is larger posteriorly near its point of origin than it is anteriorly. Nevertheless it can be readily traced into the cephalic region where it will be seen to terminate somewhat short of the anterior end of the head (Fig. 15).
The Formation of the Neural Groove
The neural plate in chicks of 18 hours was seen as a fiat, thickened area of the ectoderm. In embryos of 21 to 22 hours a longitudinal folding had involved it establishing the neural groove in the mid-dorsal Une flanked on either side by the neural folds. At 24 hours of incubation the folding of the neural plate is much more clearly marked. In a dorsal view of the entire embryo (Fig. 15) the neural folds appear as a pair of dark bands. The folding which establishes the neural groove takes place first in the cephalic region of the embryo. At its cephalic end the neural groove is therefore deeper and the neural folds are correspondingly more prominent than they are caudally. The folding has not, at this stage, been carried much beyond the cephalic half of the embryo. Consequently as the neural folds are followed caudad they diverge slightly from each other, and become less and less distinct.
Fig. 15. Dorsal view ( X 14) of entire chick embryo having 4 pairs of mesodermic somites (about 24 hours incubation).
Study of transverse sections of an embryo of this stage affords a clearer interpretation of the conditions in neural groove formation than the study of entire embryos. A section passing through the head region (Fig. 17, A) shows the neural plate folded so it forms a nearly complete tube. Dorsally the margins of the neural folds of either side have approached each other and lie almost in contact. The formation of the neural folds takes place first in about the center of the head region, and progresses thence cephalad and caudad. By following caudad the sections of a transverse series, the margins of the neural folds will be seen less and less closely approximated to each other.
The Establishment of the Fore-gut
In the outgrowth of the head, the entoderm as well as the ectoderm has been involved. As a result the entoderm forms a pocket within the ectoderm, much like a small glove finger within a larger. This entodermic pocket, or fore-gut, is the first part of the digestive tract to acquire a definite cellular floor. That part of the gut caudal to the fore-gut where the yolk still constitutes the only floor, is termed the mid-gut. The opening from the mid-gut into the fore-gut is called the anterior intestinal portal (fovea cardiaca).
Fig. 16. Ventral view ( X 37) of cephalic region of chick embryo having 5 pairs of somites (about 25-26 hours of incubation).
The topography of the fore-gut region at this stage can be made out very well by studying the ventral aspect of entire embryos. The margin of the anterior intestinal portal appears as a well defined crescentic line (Fig. 16). The lateral boundaries of the fore-gut can be seen to join the caudally directed tips of the crescentic margin of the portal. Considerably cephalic to the intestinal portal an irregularly recurved line can be made out. On either side it appears to merge with the ectoderm of the head. This Kne marks the extent to which the head is free from the blastoderm. It is due to the fold at the bottom of the subcephaUc pocket where the ectoderm of the under surface of the head is continuous with the ectoderm of the blastoderm. Comparison of Figure i6 with the sagittal section diagrammed in Figure 17, E, will aid in making clear the relationships of fore-gut to the head. From the sagittal section it will also be apparent why the margins of the intestinal portal and of the subcephalic pocket appear as dark lines in the wholemount. In viewing an entire embryo under the microscope by transmitted light one depends largely on differences in density for locating deep-lying structures. When a layer is folded so the light must pass through it edgewise, the fold stands out as a dark line by reason of the greater thickness it presents.
The Regional Divisions of the Mesoderm
The first conspicuous metamerically arranged structures to appear in the chick are the mesodermic somites. The somites arise by division of the mesoderm of the dorsal or segmental zone to form block-like cell masses. In the embryo shown in Figure 15 three pairs of somites are completely dehmited and a fourth pair can be made out which is not as yet completely cut off from the dorsal mesoderm posterior to it.
The regular addition of somites as embryos increase in age makes the number of somites the most reliable criterion of the stage of development. Chicks which have been incubated for a given number of hours show wide variation in the degree of development attained; chicks of a given number of somites vary but little among themselves.
Cross sections passing through the mid-body region show the formation of the somites and the beginning of other changes in the mesoderm (Fig. 17, C cf. also Fig. 28, E). Following the mesoderm from the mid-line toward either side three regions or zones can be made out:
- the dorsal mesoderm which at this level has been organized into somites
- the intermediate mesoderm, a thin plate of cells connecting the dorsal and lateral mesoderm and
- the lateral mesoderm which is distinguished from the intermediate by being split into two layers with a space between them.
The somites are compact cell masses lying immediately lateral to the neural folds. The cells composing them have a fairly definite radial arrangement about a central cavity which is very minute or wanting altogether when the somites are first formed but which later becomes enlarged (Fig. 38). Cephalic and caudal to the region in which somites have been formed the dorsal mesoderm is differentiated from the rest of the mesoderm simply by its greater thickness and compactness.
In 24-hour embryos the intermediate mesoderm shows very little differentiation. In the chick it never becomes segmentally divided as does the dorsal mesoderm. The fact that it is potentially segmental in character is indicated, however, by the way in which it later gives rise to segmentally arranged nephric tubules. Because of the part it plays in the establishment of the excretory system the intermediate mesoderm is frequently called the nephrotomic plate.
Fig. 17. Diagrams of sections of 24-hour chick.
- The sections are located on an outline sketch of the entire embryo. The conventional representation of the germ layers is the same as that employed in Fig. 13 except that here where its cells have become aggregated to form definite layers the mesoderm is represented by heavy solid black lines.
In the chick the lateral mesoderm like the intermediate mesoderm, shows no segmental division. In 24-hour embryos (Fig. 17, C) it is clearly differentiated from the intermediate mesoderm by being split horizontally into two layers with a space between them. The layer of lateral mesoderm lying next to the ectoderm is termed the somatic mesoderm, the layer next to the entoderm is termed the splanchnic mesoderm and the cavity between somatic and splanchnic mesoderm is the ccelom. Because in development the somatic mesoderm, and ectoderm are closely associated and undergo many foldings in common, it is convenient to designate the two layers together by the single term sxynatapleure. Similarly the splanchnic mesoderm and the entoderm together are designated as the splanchnopleure.
The coelom, like the cell layers of the blastoderm extends over the yolk peripherally beyond the embryonal area (Fig. 17, C). Later in development foldings mark off the embryonic from the extra-embryonic portion of the germ layers. This same folding process divides the coelom into intra-embryonic and extra-embryonic regions. In the 24-hour chick, however, embryonic and extra-embryonic coelom have not been separated.
It is evident from the manner in which the coelomic chambers arise in the lateral mesoderm that the ccelom of the embryo consists of a pair of bilaterally symmetrical chambers. It is not until later in development that the right and left coelomic chambers become confluent ventrally to form an unpaired body cavity such as is found in adult vertebrates.
The Pericardial Region
In the region of the anterior intestinal portal the coelomic chambers on either side show very marked local enlargements. Later in development these dilated regions are extended mesiad and break through into each other ventral to the fore-gut to form the pericardial cavity. In their early condition these enlarged regions of the coelomic chambers are usually called amnio-cardiac vesicles. With their later fate in mind we may avoid multiplication of terms and speak of them from their first appearance as constituting the pericardial region of the coelom.
The relationships of the pericardial region of the coelom in embryos of 24 hours can be most readily grasped from a study of transverse sections. Figure 17, B, shows the great dilation of the coelom on either side of the anterior intestinal portal as compared with its condition farther caudad (Fig. 17. C). Where the splanchnic mesoderm lies closely applied to the entoderm at the lateral margins of the portal it is noticeably thickened. It is from these areas of thickened splanchnic mesoderm that the paired primordia of the heart will later arise.
In entire embryos of this age the thickened splanchnic mesoderm can be made out as a dark band lying close against the crescentic entodermal border of the anterior intestinal portal (Fig. 16). If the preparation is favorably stained the boundaries of the pericardial regions of the coelom can be traced (see Fig. 16). Following mesiad from the easily located thickened areas, the mesodermic borders can be seen to extend from either side parallel to the entodermic margins of the portal nearly to the mid-line. They then turn cephalad. When they encounter the ectodermal fold which constitutes the posterior boundary of the subcephalic pocket they swing laterad parallel with it and can be traced outside the embryonic region where they constitute the cephahc borders of the anterior horns of the mesoderm (see also Fig. 27, A).
The portion of the coelom, the borders of which we have just located between the subcephalic pocket and the anterior intestinal portal, is an important landmark from another standpoint than the part it is destined to play in the formation of the pericardial region. It is the most cephalic part of the coelom. There is no coelom in the head. In the head region the mesoderm is not aggregated into definite masses or coherent cell layers. The mesodermic structures of the head are derived from cells which migrate into the cephalic region from the mesoderm lying farther caudally. These migrating cells are termed mesenchyme cells in distinction to the more definitely aggregated cell layers of the mesoderm. By careful focusing on. the whole-mount the mesenchyme of the head can be seen as an indefinite mass lying between the superficial ectoderm and the entoderm of the fore-gut. The distribution of the mesenchyme cells and the characteristic irregularity of shape correlated with their active amoeboid movement may be readily made out from sections (Fig. 17, ).
The Area Vasculosa
In a 24-hour chick the boundary between area opaca and area pellucida has the same appearance and significance as in chicks of 18 to 20 hours. There is, however, a very marked difference between the proximal portion of the area opaca adjacent to the area pellucida and the more distal portions of the area opaca. The proximal region is much darker and has a somewhat mottled appearance (Fig. 15). The greater density of this region is due to its invasion by mesoderm which makes it thicker and therefore more opaque in transmitted light (Fig. 17, D). The boundary between the inner and outer zones of the area opaca is established by the extent to which the mesoderm has grown peripherally. The distal zone is called the area opaca vitellina because the yolk alone underlies it. The proximal zone into which mesoderm has grown is known as the area opaca vasculosa, because it is from the mesoderm in this region that the yolk-sac blood vessels arise. The mottled appearance of this region is due to the aggregation of mesoderm into cell clusters, or blood islands, which mark the initial step in the formation of blood vessels and blood corpuscles. Later in development the formation of blood islands and vessels extends in toward the body of the embryo from its place of earliest appearance in the area opaca and involves the mesoderm of the area pellucida. The histological nature of the blood islands will be taken up in connection with later stages where their development is more advanced.
- Next: 24 to 33 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. (2020, October 19) Embryology Book - The Early Embryology of the Chick 7. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Early_Embryology_of_the_Chick_7
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