Book - Aids to Embryology (1948) 2

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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.

Contents: 1. Germ Cells | 2. Segmentation and Germ Layer Formation | 3. Changes in Female Genital Tract | 4. Implantation and Placentation | 5. Formation of the Embryo | 6. Skin and Accessory Structures | 7. Nervous System | 8. Special Sense | 9. Alimentary Canal | 10. Circulatory System | 11. Coelomic Cavities | 12. Urogenital System | 13. Muscular and Skeletal Systems | 14. Hereditary
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter II Segmentation and Germ Layer Formation

Segmentation

The fertilized ovum, after a short: period of rest, enters upon a phase of repeated cell division. This is known as cleavage or segmentation and results in the transformation of the fertilized ovum to a solid mass of cells (morula). The most striking result is that the volume of the cytoplasm diminishes as compared with that of the nucleus and a normal nuclear — cytoplasmic ratio for the cells of the morula is restored.


Information about cleavage stages is lacking in the human subject. The process has been extensively studied for other mammalian forms both in fixed material and with living specimens. The macaque monkey shows cleavage as it presumably is in the human, and the account given here is based on the work of Lewis and Hartman (1933) on this form. The first cleavage has not been observed in the monkey but the two-cell stage was found and studied in tissue culture. The two cells were somewhat unequal ; the larger cell divided first, giving a three-cell stage, and subsequent division of the smaller resulted in a cross-like pattern of four cells. Further divisions of the larger and smaller cells took place, the larger always first, so that stages with five, six, seven, eight and more cells were observed. At the sixteencell stage, the fertilized ovum could be termed a morula. During the process of segmentation the developing ovum passes along the tube to the uterine cavity. It arrives there four days after ovulation in the sixteen-cell stage. We have no absolute data for the human, but it is reasonable to suppose that transport through the tube requires about the same time (three days) as in the macaque monkey, and also that the human ovum attains the morula stage of development on, or shortly before, arrival in the uterus The Blastocyst. On arrival in the uterine cavity the actively growing morula undergoes important changes in cell orientation. These are mainly due to passage of fluid from the uterine lumen into the intercellular spaces of the morula. This is converted into a hollow vesicle having an outer layer of flattened cells, the trophoblast, with an inner cell mass attached at one pole of the inner aspect (Fig. 4). About this time, the zona pellucida disappears. The trophoblast is concerned with implantation of the developing ovum in the uterine mucous membrane and the later formation of an organ, the placenta, for nutritive exchanges between the developing individual and the mother. All these will be dealt with in the chapter on Implantation and Placentation (p. 26).


The inner cell mass, with which we are at present concerned, gives rise to the embryo, the amnion and the yolk sac. The early formation of these must necessarily be described together.


The youngest human ovum so far discovered is in the eighth day of development (Hertig and Rock, 1945)- It is just commencing to implant in the endometrium and already shows differentiation of the inner cell mass. There is an embryonic disc consisting of a flattened ovoid mass of cells. It is subdivided into a dorsal layer of fairly large ectodermal cells, and small, dark-staining entodermal cells. Between the primitive ectoderm and the inner surface of the trophoblast there is a very small, slit-like space, the future amniotic cavity. The roof of this is closed in by flattened amniogenic cells which are being split off from the true trophoblast of the embryonic pole.


The further development of these parts of the inner cell mass may be understood from study of the diagram of a twelve-day human embryo shown in Fig* 5- This embryo is already implanted in the endometrium but details of its uterine relationships have been omitted from the drawing.


This human blastocyst has an outer trophoblastic wall with irregular projections from the external surface. These are the early villi concerned with implantation. The embryonic disc is bilaminar with a layer of large columnar ectodermal cells dorsally and a layer of much smaller, cubical entodermal cells ventrally. The amniotic cavity is complete and lies between the embryonic ectoderm and a layer of flattened cells distinct from the overlying trophoblast. Cells continuous with the embryonic entoderm have extended round the inner aspect of the blastocyst wall to bound a second cavity, the primary yolk sac. Between this and the inner aspect of the trophoblast are a number of loosely arranged cells, the extraembryonic mesoblast. These have probably arisen by proliferation from the inner aspect of the trophoblast. In slightly later stages of development, cavities appear in the extra -embryonic mesoblast which become confluent and split these cells into a layer applied to the inner aspect of the trophoblast (which, with the trophoblast forms the chorion), and a second covering the amnion and the primary yolk sac. These two layers are continuous at one place, thus suspending the intrachorial structures from the outer wall of the blastocyst. This portion of the extra-embryonic mesoderm will become the body stalk.


Fig. 5. - Diagram of a Transverse Section through a Human Blastocyst of 12 days. (Adapted from Hertig and Rock.)

1, Trophoblast ; 2, amniotic cavity ; 3, embryonic ectoderm ; 4, embryonic entoderm ; 5, primary yolk sac ; 6, extraembryonic mesoblast.


The Embryonic Axis

Until this time the embryonic disc has been a more or less rounded bilaminar structure. When the embryo is about fifteen days old an antero-posterior axis becomes established with the formation, at the posterior edge of the embryonic disc, of an elongated area where rapid proliferation of ectodermal cells takes place. This is the primitive streak. At its anterior end is a rounded knot of ectodermal cells, Hensen’s or the primitive node. In the centre of this is an invagination of the ectoderm, the primitive pit, which corresponds with the blastopore of lower forms. The cells budded off from the primitive streak and from Hensen’s node migrate laterally, insinuating themselves between the ectoderm and the entoderm of the disc. This is the mesoderm and its appearance makes the germ disc a trilaminar structure. A cord of cells grows forward in the middle line from Hensen’s node between the ectoderm and entoderm. It is called the notochordal, or head process. The tip of this process becomes relatively fixed at the anterior margin of the embryonic disc, and further growth in length of it causes the embryonic disc to become pear-shaped. Such elongation of the disc causes Hensen’s node and the primitive streak to retreat towards its posterior, or future caudal margin. The invagination known as the primitive pit extends into the notochordal process as the notochordal canal. The floor of this breaks down allowing temporary communication between the amniotic and yolk sac cavities (see Fig. 6). Later, the entodermal continuity is restored ventral to the notochordal plate which transforms into a cylindrical bar of cells, the notochord. The anterior tip of this lies just behind the situation of the future hypophysis. The notochord is a vestigial structure in man and begins to disappear at an early date.


Further Differentiation of Embryonic Mesoderm

The mesodermal cells budded off from Hensen’s node and the sides of the primitive streak not only spread laterally but also extend forwards as two wings, one on each side of the notochord. With elongation of the embryonic disc a mid-line ectodermal thickening arises in front of Hensen’s node. This thickening rises up as a lip on each side, owing to differential growth, and a median groove, the neural groove, is formed. The further fate of this is dealt with in Chapter VII. What we are concerned with now, is that the mesoderm on each side of the neural groove and notochord becomes thickened as a longitudinal mass, the paraxial mesoderm. This is connected with the mesoderm towards the periphery of the embryonic disc, the lateral plate mesoderm, by the intermediate cell mass. The lateral plate mesoderm is continuous at the margins of the disc with the extra-embryonic mesoderm (p. 16).


Fig. 6. - Diagram of a Longitudinal Section through an i8-day Human Embryo. (Adapted from Heuser.)

1, Amnion; 2, primitive streak ; 3, notochordal canal ; 4, blastopore ; 5, yolk sac ; 6, allanto-enteric diverticulum.



About the twenty-first day of development in the human, the paraxial mesoderm begins a process of sub-division from before backwards into paired cubical masses, the somites. The first ones laid down are in the future occipital region of the embryo and some forty-two pairs are successively formed until the embryo is about 4 mm. in length. The mesodermal tissue of the somites is concerned essentially with the elaboration of parts of the muscular and skeletal systems (see Chapter XIII).


The mesodermal tissues of the intermediate cell mass give rise to the excretory sytem and the sex glands. This phase of development is discussed in Chapter XII. The intermediate cell mass does not exhibit an early segmental arrangement comparable to that of the somites, but hints of its segmental origin may be seen in the formation of the proand meso-nephros.


The lateral plate mesoderm continues with the extra-embryonic mesoderm and is split into two layers by the formation of a cavity, the intra-embryonic coelom. One layer is applied to the inner surface of the embryonic ectoderm and may therefore be termed somatopleure ; the second is in contact with the embryonic entoderm and is designated as splanchnopleure. The cavities extend forward on each side and eventually fuse with each other in front of the anterior extremity of the embryonic disc in the region of later formation of the primitive heart.


Expansion of the extra-embryonic coelom from the twelfth day onward for a few days is accompanied by diminution in size of the primary yolk sac (Heuser, Rock and Hertig, 1945). It becomes about equal in size to the amniotic sac at the fifteenth day.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Contents: 1. Germ Cells | 2. Segmentation and Germ Layer Formation | 3. Changes in Female Genital Tract | 4. Implantation and Placentation | 5. Formation of the Embryo | 6. Skin and Accessory Structures | 7. Nervous System | 8. Special Sense | 9. Alimentary Canal | 10. Circulatory System | 11. Coelomic Cavities | 12. Urogenital System | 13. Muscular and Skeletal Systems | 14. Hereditary

Cite this page: Hill, M.A. (2019, September 15) Embryology Book - Aids to Embryology (1948) 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_2

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