Book - The Development Of The Human Body
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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The Development Of The Human Body - A Manual Of Human Embryology
By
J. Playfair McMurrich, A. M., Ph. D., Ll. D.
Professor Of Anatomy In The University Of Toronto
Formerly Professor Of Anatomy In The University Of Michigan
Fourth Edition, Revised And Enlarged
With Two Hundred and Eighty-five Illustrations Several of which are Printed in Colors
Philadelphia
P. Blakiston's Son & Co.
1012 Walnut Street
1914
Copyright, 1913, By P. Blakiston's Son & Co.
Contents
Introduction
PART I. - GENERAL DEVELOPMENT.
CHAPTER I. The Spermatozoon and Spermatogenesis; the Ovum and Its Maturation and Fertilization
CHAPTER II. The Segmentation of the Ovum and the Formation of the Germ Layers
CHAPTER III. The Medullary Groove, Notochord, and Mesodermic Somites
CHAPTER IV. The Development of the External Form of the Human Embryo
CHAPTER V. The Yolk-stalk, Belly-stalk, and Fetal Membranes
PART II. - ORGANOGENY.
CHAPTER VI. The Development of the Integumentary System
CHAPTER VII. The Development of the Connective Tissues and Skeleton
CHAPTER VIII. The Development of the Muscular System
CHAPTER IX. The Development of the Circulatory and Lymphatic Systems
CHAPTER X. The Development of the Digestive Tract and Glands
CHAPTER XI. The Development of the Pericardium, the Pleuro-peritoneum, and the Diaphragm
CHAPTER XII. The Development of the Organs of Respiration
CHAPTER XIII. The Development of the Urinogenital System
CHAPTER XIV. The Suprarenal System of Organs
CHAPTER XV. The Development of the Nervous System
CHAPTER XVI. The Development of the Organs of Special Sense
CHAPTER XVII. Post-natal Development
Preface to the Fourth Edition
The increasing interest in human and mammalian embryology which has characterized the last few years has resulted in many additions to our knowledge of these branches of science, and has necessitated not a few corrections of ideas formerly held. In this fourth edition of this book the attempt has been made to incorporate the results of all important recent contributions upon the topics discussed, and, at the same time, to avoid any considerable increase in the bulk of the volume. Several chapters have, therefore, been almost entirely recast, and the subject matter has been thoroughly revised throughout, so that it is hoped that the book forms an accurate statement of our present knowledge of the development of the human body.
To several colleagues the author is indebted for valuable suggestions, and in this connection he desires especially to thank Dr. J. C. Watt for much generous assistance in the revision of the manuscript and for undertaking the correction of the proof-sheets.
In addition to the works mentioned in the preface to the first edition as of special value to the student of Embryology, mention should be made of the Handbuch der vergleichenden mid experimentellen Entwickhmgslehre der Wirbeltiere edited by Professor Oscar Hertwig and especially of the Manual of Human Embryology edited by Professors F. Keibel and F. P. Mall. University of Toronto.
Preface to the First Edition
The assimilation of the enormous mass of facts which constitute what is usually known as descriptive anatomy has always been a difficult task for the student. Part of the difficulty has been due to a lack of information regarding the causes which have determined the structure and relations of the parts of the body, for without some knowledge of the why things are so, the facts of anatomy stand as so many isolated items, while with such knowledge they become bound together to a continuous whole and their study assumes the dignity of a science.
The great key to the significance of the structure and relations of organs is their development, recognizing by that term the historical as well as the individual development, and the following pages constitute an attempt to present a concise statement of the development of the human body and a foundation for the proper understanding of the facts of anatomy. Naturally, the individual development claims the major share of attention, since its processes are the more immediate forces at work in determining the conditions in the adult, but where the embryological record fails to afford the required data, whether from its actual imperfection or from the incompleteness of our knowledge concerning it, recourse has been had to the facts of comparative anatomy as affording indications of the historical development or evolution of the parts under consideration.
It has not seemed feasible to include in the book a complete list of the authorities consulted in its preparation. The short bibliographies appended to each chapter make no pretensions to completeness, but are merely indications of some of the more important works, especially those of recent date, which consider the questions discussed. For a very full bibliography of all works treating of human embryology up to 1893 reference may be made to Minot's Bibliography of Vertebrate Embryology, published in the "Memoirs of the Boston Society of Natural History," volume iv, 1893. It is fitting, however, to acknowledge an especial indebtedness, shared by all writers on human embryology, to the classic papers of His, chief among which is his Anatomie menschlicher Embryonen, and grateful acknowledgments are also due to the admirable text-books of Minot, O. Hertwig, and Kollmann.
Anatomical Laboratory,
University of Michigan.
Introduction
Somewhat more than seventy years ago (1839) one of the fundamental principles of biology was established by Schleiden and Schwann as the cell theory. According to this, all organisms are composed of one or more structural units termed cells, each of which, in multicellular organisms, maintains an individual existence and yet contributes with its fellows to the general existence of the individual. Viewed in the light of this theory, the human body is a community, an aggregate of many individual units, each of which leads to a certain extent an independent existence and yet both contributes to and shares in the general welfare of the community.
To the founders of the theory the structural units were vesicles with definite walls, and little attention was paid to their contents. Hence the use of the term "cell" in connection with them. Long before the establishment of the cell theory, however, the existence of organisms composed of a gelatinous substance showing no indications of a definite limiting membrane had been noted, and in 1835 a French naturalist, Dujardin, had described the gelatinous material of which certain marine organisms (Rhizopoda) were composed, terming it sarcode and maintaining it to be the material substratum which conditioned the various vital phenomena exhibited by the organisms. Later, in 1846, a botanist, von Mohl, observed that living plant cells contained a similar substance, upon which he believed the existence of the cell as a vital structure was dependent, and he bestowed upon this substance the name protoplasm, by which it is now universally known.
By these discoveries the importance originally attributed to the cell-wall was greatly lessened, and in 1864 Max Schultze reformulated the cell theory, defining the cell as a mass of protoplasm, the presence or absence of a limiting membrane or cell-wall being immaterial. At the same time the spontaneous origination of cells from an undifferentiated matrix, believed to occur by the older authors, was shown to have no existence, every cell originating by the division of a preexisting cell, a fact concisely expressed in the aphorism of Virchow - omnis cellula a cellula.
Interpreted in the light of these results, the human body is an aggregate of myriads of cells,* - i. e., of masses of protoplasm, each of which owes its origin to the division of a preexistent cell and all of which may be traced back to a single parent cell - a fertilized ovum. All these cells are not alike, however, but just as in a social community one group of individuals devotes itself to the performance of one of the duties requisite to the well-being of the community and another group devotes itself to the performance of another duty, so too, in the body, one group of cells takes upon itself one special function and another another. There is, in other words, in the cell-community a physiological division of labor. Indeed, the comparison of the cell-community to the social community may be carried still further, for just as gradations of individuality may be recognized in the individual, the municipality, and the state, so too in the cell-community there are cells; tissues, each of which is an aggregate of similar cells; organs, which are aggregates of tissues, one, however, predominating and determining the character of the organ; and systems, which are aggregates of organs having correlated functions.
It is the province of embryology to study the mode of division of the fertilized ovum and the progressive differentiation of the resulting cells to form the tissues, organs, and systems. But before considering these phenomena as seen in the human body it will be well to get some general idea of the structure of an animal cell.
- It has been estimated that the number of cells entering into the composition of the body of an adult human being is about twenty-six million five hundred thousand millions!
This (Fig. i), as has been already stated, is a mass of protoplasm, a substance which in the living condition is a viscous fluid resembling in many of its peculiarities egg-albumen, and like this being coagulated when heated or when exposed to the action of various chemical reagents. As to the structure of living protoplasm little is yet known, since the application of the reagents necessary for its accurate study and analysis results in its disintegration or coagulation. But even in the living cell it can be seen that the Fig. i. - Ovum of New-born protoplasm is not a simple homogeneous ?^ IL r ? WI n TH Follicle - cells ~ substance. What is termed a nucleus is
usually clearly discernible as a more or less spherical body of a greater refractive index than the surrounding protoplasm, and since this is a permanent organ of the .cell it is convenient to distinguish the surrounding protoplasm as the cytoplasm from the nuclear protoplasm or karyoplasm.
The study of protoplasm coagulated by reagents seems to indicate that it is a mixture of substances rather than a simple chemical compound. Both the cytoplasm and the karyoplasm consist of a more solid substance, the reticulum, which forms a network or feltwork, in the interstices of which is a more fluid material, the enchylema* The karyoplasm, in addition, has scattered along the fibers of its reticulum a peculiar material termed chromatin and usually contains embedded in its substance one or more spherical bodies termed nucleoli, which may be simply larger masses of chromatin or bodies of special chemical composition. And, finally, in all actively growing cells there is differentiated in the cytoplasm a peculiar body known as the archo plasm sphere, in the center of which there is usually a minute spherical body known as the centrosome.
- It has been observed that certain coagulable substances and gelatin, when subjected to the reagents usually employed for "fixing" protoplasm, present a structure similar to that of protoplasm, and it has been held that protoplasm in the uncoagulated condition is, like these substances, a more or less homogeneous material. On the other hand, Biitschli maintains that living protoplasm has a foam-structure and is, in other words, an emulsion.
It has been already stated that new cells arise by the division of preexisting ones, and this process is associated with a series of complicated phenomena which have great significance in connection with some of the problems of embryology. When such a cell as has been described above is about to divide, the fibers of the reticulum in the neighborhood of the archoplasm sphere arrange themselves so as to form fibrils radiating in all directions from the sphere as a center, and the archoplasm with its contained centrosome gradually elongates and finally divides, each portion retaining its share of the radiating fibrils, so that two asters, as the aggregate of centrosome, sphere and fibrils is termed, are now to be found in the cytoplasm (Fig. 2, A) . Gradually the two asters separate from one another and eventually come to rest at opposite sides of the nucleus (Fig. 2, C). In this structure important changes have been taking place in the meantime. The chromatin, originally scattered irregularly along the reticulum, has gradually aggregated to form a continuous thread (Fig. 2, A), and later this thread breaks up into a definite number of pieces termed chromosomes (Fig. 2, B), the number of these being practically constant for each species of animal. In man the number has been placed at twenty-four (Flemming, Duesberg) , but the recent observations of Guyer indicate that it is probably twenty-four in the female and twenty-two in the male. The significance of this difference in the two sexes will be considered in connection with the fertilization of the ovum (p. 32).
As soon as the asters have taken up their position on opposite sides of the nucleus, the nuclear reticulum begins to be converted into a spindle-shaped bundle of fibrils which associate themselves with the astral rays and have lying scattered among them the chromosomes (Fig. 2, C). To the figure so formed the term amphiaster is applied, and soon after its formation the chromosomes arrange themselves in a circle or plane at the equator of the spindle (Fig. 2, D) and the stages preparatory to the actual division, the prophases, are completed.
The next stage, the metaphase (Fig. 3, A), consists of the division, usually longitudinally, of each chromosome, so that the cell now contains twice as many chromosomes as it did previously. As soon as this division is completed the anaphases are inaugurated by the halves of each chromosome separating from one another and approaching one of the asters (Fig. 3, B), and a group of chromosomes, containing half the total number formed in the metaphase, comes to lie in close proximity to each archoplasm sphere (Fig. 3, C). The spindle and astral fibers gradually resolve themselves again into the reticulum and the chromosomes of each group become irregular in shape and gradually spread out upon the nuclear reticulum so that •two nuclei, each similar to the one from which the process started,
Fig. 2. - Diagrams Illustrating the Prophases of Mitosis. - (Adapted from E. B. Wilson.)
Fig. 3. - Diagrams Illustrating the Metaphase and Anaphases of Mitosis.: - (Adapted from E. B. Wilson.) are formed (Fig. 3, D). Before all these changes are accomplished, however, a constriction makes its appearance at the surface of the cytoplasm (Fig. 3, C) and, gradually deepening, divides the cytoplasm in a plane passing through the equator of the amphiaster and gives rise to two separate cells (Fig. 3, D).
This complicated process, which is known as karyokinesis or mitosis, is the one usually observed in dividing cells, but occasionally a cell divides by the nucleus becoming constricted and dividing into two parts without any development of chromosomes, spindle, etc., the division of the cell following that of the nucleus. This amitotic method of division is, however, rare, and in many cases, though not always, its occurrence seems to be associated with an impairment of the reproductive activities of the cells. In actively reproducing cells the mitotic method of division may be regarded as the rule.
Since the process of development consists of the multiplication of a single original cell and the differentiation of the cell aggregate so formed, it follows that the starting-point of each line of individual development is to be found in a cell which forms part of an individual of the preceding generation. In other words, each individual represents one generation in esse and the succeeding generation in posse. This idea may perhaps be made clear by the following considerations. As a result of the division of a fertilized ovum there is produced an aggregate of cells, which, by the physiological division of labor, specialize themselves for various functions. Some assume the duty of perpetuating the species and are known as the sexual or germ cells, while the remaining ones divide among themselves the various functions necessary for the maintenance of the individual, and may be termed the somatic cells. The germ cells represent potentially the next generation, while the somatic cells constitute the present one. The idea may be represented schematically thus:
First generation
Somatic cells + germ cells
II Second generation
Somatic cells + germ cells
II Third generation
Somatic cells + germ cells, etc.
It is evident, then, while the somatic cells of each generation die at their appointed time and are differentiated anew for each generation from the germ cells, the latter, which may be termed collectively the germ-plasm, are handed on from generation to generation without interruption, and it may be supposed that this has been the case ab initio. This is the doctrine of the continuity of the germ-plasm, a doctrine of fundamental importance on account of its bearings on the phenomena of heredity.
It is necessary, however, to fix upon some link in the continuous chain of the germ-plasm as the starting-point of the development of each individual, and this link is the fertilized ovum. By this is meant a germ cell produced by the fusion of two units of the germplasm. In many of the lower forms of life (e.g., Hydra and certain turbellarian worms) reproduction may be accomplished by a division of the entire organism into two parts or by the separation of a portion of the body from the parent individual. Such a method of reproduction is termed non-sexual. Furthermore in a number of forms (e. g., bees, Phylloxera, water-fleas) the germ cells are able to undergo development without previously being fertilized, this constituting a method of reproduction known as parthenogenesis. But in all these cases sexual reproduction also occurs, and in all the more highly organized animals it is the only method that normally occurs; in it a germ cell develops only after complete fusion with another germ cell. In the simpler forms of this process little difference exists between the two combining cells, but since it is, as a rule, of advantage that a certain amount of nutrition should be stored up in the germ cells for the support of the developing embryo until it is able to secure food for itself, while at the same time it is also advantageous that the cells which unite shall come from different individuals (cross-fertilization), and hence that the cells should retain their motility, a division of labor has resulted. Certain germ cells store up more or less food yolk, their motility becoming thereby impaired, and form what are termed the female cells or ova, while otners discard all pretensions of storing up nutrition, are especially motile and can seek and penetrate the inert ova; these latter cells constitute the male cells or spermatozoa. In many animals both kinds of cells are produced by the same individual, but in all the vertebrates (with rare exceptions in some of the lower orders) each individual produces only ova or spermatozoa, or, as it is generally stated, the sexes are distinct. It is of importance, then, that the peculiarities of the two forms of germ cells, as they occur in the human species, should be considered.
Literature
E. B. Wilson: "The Cell in Development and Inheritance." Third edition. New York, 1900. O. Hertwig: "Die Zelle und die Gewebe." Jena, 1893.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
Cite this page: Hill, M.A. (2024, April 24) Embryology Book - The Development Of The Human Body. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Development_Of_The_Human_Body
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