Book - Aids to Embryology (1948) 5
Embryology - 19 Apr 2024    Expand to Translate
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter V Formation of the Embryo - Determination of Age
At the end of the third week the human embryo consists of three layers of cells, ectoderm, mesoderm and entoderm. The notochord has been laid down, thus establishing an antero-posterior axis and bilateral symmetry upon the germ disc. Passing reference has been made to a series of foldings which convert the trilaminar germ disc into a cylindrical embryo and certain changes connected with this process must now be further considered.
Formation of Head Fold
At the anterior end of the notochord the embryonic ectoderm and entoderm are in contact, no mesoderm intervening. This is the future bucco-pharyngeal membrane. The lateral plate mesoderm of each side becomes continuous in front of this area as the protocardiac area in which the heart tubes develop. In the early somite stages of development these two areas become bent ventrally so that the protocardiac area becomes tucked in under the anterior end of the notochord and separated from it by a portion of the intra-embryonic entoderm, the fore-gut. This is bounded anteriorly by the bucco-pharyngeal membrane which forms the floor of a little surface depression of the anterior end of the embryo called the stomatodaeum. These changes are shown in Fig. 9.
Another result of the folding at the head end of the embryo is that the mesoderm which in early stages lay anterior to the protocardiac area comes now to occupy a position caudal to the pericardium and ventral to the fore-gut. This mass of mesoderm is known as the septum transversum.
Formation of Tail Fold
At the posterior end of the embryonic disc behind the primitive streak there is a second area where no mesoderm intervenes between ectoderm and entoderm. This is the cloacal membrane. Commencing in the early somite stages this area becomes folded in, ventral to the caudal portion of the intra-embryonic entoderm. This is the tail fold and the entoderm dorsal to it is the hindgut.
While the head and tail folds are being formed, the lateral parts of the embryonic disc also fold ventrally and now the general form of the embryo becomes established. The cells of the germ layers begin to take on special characters and to form the primordia of the definitive structures of the adult. In these changes, which are termed organogenesis, various processes may be observed ; multiplication and migration of cells takes place, and this leads to localized enlargements and constrictions ; to cell aggregations and the formation of cords, sheets and masses. Unequal rate of growth produces foldings, invaginations and evaginations.
Fig. 9. - Diagram to show the Formation of the Head Fold. (Adapted from Hamilton, Boyd and Mossman.
A.— Late presomite stage. 1, Neural plate ; 2, notochord ;
3, yolk sac entoderm ; 4, bucco-pharyngeal membrane ; 5, pericardial cavity. B. — 20 somite stage. 1, Neural tube ; 2, bucco-pharyngeal membrane ; 3, pericardial cavity ; 4, septum transversum.
Each of the three primary germ layers normally gives rise to certain structures. Thus, from the ectoderm there arise the epidermal component of the skin and its accessory glands, hair, nails, the nervous system and parts of the sense organs. The entoderm is responsible for the lining epithelium of the alimentary canal and its secreting organs, the liver and pancreas, together with glands derived from the primitive pharynx ; the epithelium of the larynx, trachea and lungs are also entodermal in origin as is the epithelium of parts of the lower urinary tract. From the mesoderm arise the connective tissues, the skeletal and muscular system, the blood and lymph vascular systems, and a large part of the urogenital system. Although the work of experimental embryologists has shown that the germ layers are not so absolutely specific as was formerly held, nevertheless the above-mentioned conception of their fate is of great use in descriptive embryology.
Determination of the Age of Embryos
There are three main phases in the period of pre-natal development, that of the ovum, the embryo and the foetus. During the first three weeks after fertilization the developing individual is referred to as an ovum. At the beginning of the fourth week the somites commence to appear, and from this time until the end of the eighth week is the embryonic phase. During this time, the main organ systems are established. The foetal phase extends from the end of the second month until birth and during this time histogenetic changes are the most marked feature of development.
Embryos of the same age vary somewhat in their degree of development so that estimations of age from the dimensions of embryo cannot be absolutely accurate. The most usual measurement employed is the crown -rump (C.R.) length, i.e., the distance from the vertex to the breech. A convenient rule is that the embryo of thirty-five days is 5 mm. C.R. length and until the end of the eighth week it adds 1 mm. daily to that measurement. Before the end of the fifth week measurements are not a reliable guide to embryonic age.
Estimation of Foetal Age
There are several methods of estimating foetal age. That most commonly employed is based on measurement of the body length (C.R. length) and body weight. The following table (based on data by Streeter) gives the average crown-rump length and the body weight at intervals from the end of the second lunar month until birth at the end of the tenth lunar month. The age of a foetus may be determined with a fair degree of accuracy by making the two observations.
TABLE I
Table showing the average length and weight of the human fcetus at the end of each lunar month.
Age in Lunar Months.
C.R. Length in mm.
Weight in grams.
End of 2nd month
23-0
1-25
End of 3rd month
64-0
20-0
End of 4th month
116-0
108-0
End of 5th month
164-0
316-0
End of 6th month
207-0
630-0
End of 7th month
245-0
1050-0
End of 8th month
284-0
1680-0
End of gth month
324-0
2470-0
End of 10th month
363-0
3400-0
An estimation of foetal age may also be made by studying the centres of ossification as seen, for example, by X-ray examination. Some data on ossification periods are given on page 175, but this method is of limited application and the results are probably not so accurate as simple estimation of length and weight. If details of the menstrual history of the mother are available, the age of the foetus may be calculated if it is remembered that ovulation occurs about fourteen days before/ the first day of the first missed menstrual period. There is sometimes, however, a scanty menstrual flow after conception has occurred which may introduce an error in the patient's recollection of her menstrual history.
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Cite this page: Hill, M.A. (2024, April 19) Embryology Book - Aids to Embryology (1948) 5. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_5
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
