Difference between revisions of "Book - The Development Of The Human Body"

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The Development of the Human Body: A Manual of Human Embryology. By J. Playfair McMurrich. Philadelphia, P. Blakiston's Son and Company, fourth edition, 1913, pp. 8 + 495, 285 figures, $2.50 net.
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A new edition of McMurrich's Embryology has just appeared. The general character of the previous editions is retained, particular attention being given to the development of organs and less space devoted to the early stages of the embryo. Parts of the book have been re-written and other parts revised. The numerous typographical errors so conspicuous in the third edition have been eliminated. The volume is of pocket size with flexible binding. The large, clear type has been retained. Perhaps the most important feature of the book is the author's clearness of expression. If embryology is to be an aid to anatomy it would appear desirable to find some royal road to this science, which is at present perhaps more difficult than adult anatomy. The student is usually bewildered by serial sections unless guided by a very clear presentation of the subject. McMurrich has achieved this end at the expense of many details, and no doubt rightly. Yet, the large mass of facts contained in this small book seems remarkable. A few passages are ambiguous if not erroneous, and may deserve mention. McMurrich describes the observations of Will on gastrulation in the Gecko, but transposes Will's terminology of primary and secondar}^ endoderm. If gastrulation in vertebrates means anything, certainly the formation of endoderm by invagination is the primitive process and is 'primary' from the standpoint of phylogeny, whereas delamination is a secondary method of formation although it does occur earliest in the development of mammalian embryo. On page 60 the author describes the origin of the mesoderm as follows : "the layer of enveloping cells splits into two concentric layers, the inner of which seems to be mesodermal in its nature and forms a layer lining the trophoblast," and again on page 110: "the extra-embryonic mesoderm, instead of growing out from the embryo to enclose the yolk sac, splits off directly from the enveloping layer." And on page 55: "a splitting of the enveloping layer has occurred, so that the wall of the ovum is now formed of three layers, an outer one which may be termed the trophoblast, a middle one which probably is transformed into the extra-embryonic mesoderm of later stages, though its significance is obscure, and an inner one which is the primary endoderm." I know of no primate, however, in which it has been demonstrated that any 102 BOOK REVIEW 103 part of the mesoderm is delaminated from the trophoblast. McMurrich supposes a separate origin of the extra-embryonic mesoderm. He may be correct, since the origin of the mesoderm is unknown in man. The relation of bloodvessels to endoderm in the liver anlage is described on page 308 as follows: "Shortly after the hepatic portion has been differentiated, its substance becomes permeated by numerous blood vessels (sinusoids) and so divided into anastomosing trabecular" Since these blood vessels arise by a breaking up of the vitelline veins, which are present before the liver anlage appears, it might seem more logical to say that the liver invades the vitelline veins than that the veins invade the liver. McMurrich describes the separation of the coelomic cavities as they occur in the rabbit. The separation of the pericardium from the other cavities is simpler in man in that there are no openings ventral to the viteline veins that have to be closed. This is described in Keibeland Mall's Handbook (vol. 1, p. 526) as follows: "In the rabbit the pericardial coelom ends in two dorsal and two ventral recesses, all four of which connect subsequently with the peritoneal coelom. However, only the dorsal recesses break into the peritoneal coelom in the human embryo." A description of the simpler condition in the human embryo might be preferable in a textbook. Finally, there is a point which is of no evident importance in embryology, but since McMurrich has brought it up, we will consider it very briefly. He describes protoplasm as having a visible reticular or possibly alveolar structure. This reticular ' theory' is based on fixed material, although is it well known that fixing fluids produce the same appearance in clear gelatine jelly or clear albuminous fluids. Formalin and osmium tetroxide do not produce a reticular structure in gelatine jelly and protoplasm fixed with them appears homogeneous. Butschli observed an alveolar structure in 'living' protoplasm under certain conditions, but this appearance seemsto be exceptional or possibly abnormal. With the ultra-microscope of Siedentopf and Zigsmondy, particles smaller than the largest molecules may be made very evident. With this instrument the protoplasm of erythrocytes, and both nucleus and cytoplasm of frog erythrocytes, appear absolutely homogeneous, until injured by abnormal conditions. When transferred from the normal medium to even so good an imitation as Ringer's solution coagulations begin to appear. The numerous deutoplasmic granules in many cells prevent a universal application of this method. To sum up: McMurrich's Embryology may be considered as a very convenient and desirable text book for medical students. It might also be used as a brief reference book provided the subject matter be verified by looking up the literature cited at the end of each chapter. J. F. McClendon.
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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.

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   McMurrich 1914: General 1 Spermatozoon - Spermatogenesis - Ovum - Fertilization | 2 Ovum Segmentation - Germ Layer Formation | 3 Medullary Groove - Notochord - Somites | 4 Embryo External Form | 5 Yolk-stalk - Belly-stalk - Fetal Membranes Organogeny 6 Integumentary System | 7 Connective Tissues - Skeleton | 8 Muscular System | 9 Circulatory - Lymphatic Systems | 10 Digestive Tract and Glands | 11 Pericardium - Pleuro-peritoneum - Diaphragm | 12 Respiration | 13 Urinogenital System | 14 Suprarenal System | 15 Nervous System | 16 Organs of Special Sense | 17 Post-natal | Figures
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This is a draft version of McMurrich's 1914 embryology textbook.



PDF 1907 edition | Internet Archive - 1903 edition | 1910 edition | c1920 edition

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McClendon - Book Review (1913)  
Book Review (1913)

The Development of the Human Body: A Manual of Human Embryology. By J. Playfair McMurrich. Philadelphia, P. Blakiston's Son and Company, fourth edition, 1913, pp. 8 + 495, 285 figures, $2.50 net.

A new edition of McMurrich's Embryology has just appeared. The general character of the previous editions is retained, particular attention being given to the development of organs and less space devoted to the early stages of the embryo. Parts of the book have been re-written and other parts revised. The numerous typographical errors so conspicuous in the third edition have been eliminated. The volume is of pocket size with flexible binding. The large, clear type has been retained. Perhaps the most important feature of the book is the author's clearness of expression. If embryology is to be an aid to anatomy it would appear desirable to find some royal road to this science, which is at present perhaps more difficult than adult anatomy. The student is usually bewildered by serial sections unless guided by a very clear presentation of the subject. McMurrich has achieved this end at the expense of many details, and no doubt rightly. Yet, the large mass of facts contained in this small book seems remarkable. A few passages are ambiguous if not erroneous, and may deserve mention. McMurrich describes the observations of Will on gastrulation in the Gecko, but transposes Will's terminology of primary and secondar}^ endoderm. If gastrulation in vertebrates means anything, certainly the formation of endoderm by invagination is the primitive process and is 'primary' from the standpoint of phylogeny, whereas delamination is a secondary method of formation although it does occur earliest in the development of mammalian embryo. On page 60 the author describes the origin of the mesoderm as follows : "the layer of enveloping cells splits into two concentric layers, the inner of which seems to be mesodermal in its nature and forms a layer lining the trophoblast," and again on page 110: "the extra-embryonic mesoderm, instead of growing out from the embryo to enclose the yolk sac, splits off directly from the enveloping layer." And on page 55: "a splitting of the enveloping layer has occurred, so that the wall of the ovum is now formed of three layers, an outer one which may be termed the trophoblast, a middle one which probably is transformed into the extra-embryonic mesoderm of later stages, though its significance is obscure, and an inner one which is the primary endoderm." I know of no primate, however, in which it has been demonstrated that any 102 BOOK REVIEW 103 part of the mesoderm is delaminated from the trophoblast. McMurrich supposes a separate origin of the extra-embryonic mesoderm. He may be correct, since the origin of the mesoderm is unknown in man. The relation of bloodvessels to endoderm in the liver anlage is described on page 308 as follows: "Shortly after the hepatic portion has been differentiated, its substance becomes permeated by numerous blood vessels (sinusoids) and so divided into anastomosing trabecular" Since these blood vessels arise by a breaking up of the vitelline veins, which are present before the liver anlage appears, it might seem more logical to say that the liver invades the vitelline veins than that the veins invade the liver. McMurrich describes the separation of the coelomic cavities as they occur in the rabbit. The separation of the pericardium from the other cavities is simpler in man in that there are no openings ventral to the viteline veins that have to be closed. This is described in Keibeland Mall's Handbook (vol. 1, p. 526) as follows: "In the rabbit the pericardial coelom ends in two dorsal and two ventral recesses, all four of which connect subsequently with the peritoneal coelom. However, only the dorsal recesses break into the peritoneal coelom in the human embryo." A description of the simpler condition in the human embryo might be preferable in a textbook. Finally, there is a point which is of no evident importance in embryology, but since McMurrich has brought it up, we will consider it very briefly. He describes protoplasm as having a visible reticular or possibly alveolar structure. This reticular ' theory' is based on fixed material, although is it well known that fixing fluids produce the same appearance in clear gelatine jelly or clear albuminous fluids. Formalin and osmium tetroxide do not produce a reticular structure in gelatine jelly and protoplasm fixed with them appears homogeneous. Butschli observed an alveolar structure in 'living' protoplasm under certain conditions, but this appearance seemsto be exceptional or possibly abnormal. With the ultra-microscope of Siedentopf and Zigsmondy, particles smaller than the largest molecules may be made very evident. With this instrument the protoplasm of erythrocytes, and both nucleus and cytoplasm of frog erythrocytes, appear absolutely homogeneous, until injured by abnormal conditions. When transferred from the normal medium to even so good an imitation as Ringer's solution coagulations begin to appear. The numerous deutoplasmic granules in many cells prevent a universal application of this method. To sum up: McMurrich's Embryology may be considered as a very convenient and desirable text book for medical students. It might also be used as a brief reference book provided the subject matter be verified by looking up the literature cited at the end of each chapter. J. F. McClendon.

Historic Disclaimer - information about historic embryology pages 
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) 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, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

The Development of the Human Body - A Manual of Human Embryology

J. Playfair McMurrich
J. Playfair McMurrich (1859 – 1939)

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.

Book Review by Frederic T. Lewis - Anatomical Record 1 (1906) 
The Development of the Human Body by J. Playfair McMurrich. Third edition. P. Blakiston's Son & Co., Philadelphia, 1907. X + 528 pages, 277 figures. $3.00

McMurrich's "Development of the Human Body" is the best brief didactic American text-book of embryology. In its third edition it has been thoroughly revised, but is no larger than before. The Basle nomenclature has been introduced, but here and there are found such rejected terms as lymph follicles, discus proligems, and uterus masculinm. The word anlage does not occur. In English we may say- that "' an organ arises (or begins) as an outgrowth " rather than "' the anlage of the organ is an outgrowth " ; and that " the liver at first is a diverticulum " rather than " the anlage of the liver is a diverticulum." Apparently no new terms have been proposed.

The following changes and additions are of interest. In man the number of chromosomes is 24 (Duesberg) instead of 16. The studies of Heape on menstruation in lower mammals, of Marshall on the internal secretion of the ovaries, of Kirkham and of Gerlach showing that the mouse has a second polar body overlooked by Sobotta, and of several authors in favor of the epithelial origin of lutein cells have been added. Hubrecht's term trophohlast, corresponding with epiblast and hypoblast, is replaced by Minot's trophoderm, corresponding with ectoderm, etc.; it is described as a modified ectoderm adapted for implantation rather than as a distinct germ layer. The description of implantation has been modified with reference to Rejsek's study of Sperinophilus and Doria's account of a young human embryo. Assheton's and Peebles' experimental studies of growth in the chick's blastoderm have been substituted for the concrescence theory, which is not mentioned in the third edition. The existence of a primary chordal canal opening at Hensen's knot is now admitted (p. 60) ; in view of this change the secondary chordal canal (p. 100) should be more sharply defined.

In connection with the development of the skeleton, a new figure is introduced illustrating the relation of vertebrae to segments; ]\Iairs work on the ossification centers of the interparietal and maxilla, and Fawcett's on the pterygoid process have been cited.

Red blood corpuscles are described as discs. Since photographs of the cup-shaped form have not yet been published, two are insei-ted in this review. They represent a few corpuscles of the foi'm in question among a large number of shadows from which the luemoglobin has disappeared. It is improbable that IJetterer is correct in regarding the cup as being only apparent and due to the distribution of haemoglobin. Such cupshaped appearances as are shown in the photograph are characteristic of circulating and of well-preserved blood. They appear in embryos wlien the nuclei of the erythroblasts are eliminated.

Of the origin of blood plates it is said that " the most plausible suggestion is that they are the fragmented nuclei of broken-down leucocytes." The fine investigation of James 11. Wright is not mentioned.

There are many clianges in the chapter on blood-vessels. A new diagram shows the questionable fifth aortic arch as equivalent to the other five. Six aortic arches require the presence of five pharyngeal pouches, yet according to McMurrich, there are but four. The fifth arch is however, said to be " rudimentary." Mall's important work on the cerebral veins in man replaces Selzer's description of these veins in the guinea-pig, but the complex story of development is rendered confusing by a misplacement of figures, Fig. 151 B (p. 271) being younger than Fig. 151 A. The supracardinal veins of Huntington and McClure are described and figured. These include the dorsal limbs of the loop formed by the cardinal veins around the ureter, and other more anterior vessels of a different origin; the anterior portion is shown in the figures of the cat, but not of the rabbit. Tlie loop around the ureter is well known from the studies of Tloehstetter. His diagrams (Hertwig's Handhuch, Vol. 3, p. 142) show the extent of the loop more accurately than McMurrich's figure, and correctly indicate the relation of the genital veins to its ventral limb (compare also Anat. Anz., Vol. 25, p. 271). If the term supracardinal vein could be restricted in its application, so as to be synon}anous with dorsal limh of the ureteric loop, it would be more readily adopted. The veins of the limbs are described at greater length than in the previous edition, but without the necessary figures.

Photographs of human red blood corpuscles within a blood vessel. Fixing reagent unknown. Weigert's stain. Those retaining their haemoglobin are cup-shaped. A, X 315 diams. B, X 630 diams.


In the account of the lymphatic vessels interpretations rather than observations are considered, and thus the " discordant " element is emphasized. A figure of a section through the jugular lympli sac, such as Professor Sabin has published, is much needed, and reconstructions are better than diagrams. Sabin's studies of lymph glands and ]\rairs work on the spleen have been incorporated. The spleen has been transferred from the chapter on mesenteries to that on the circnlatorv system, and as a result of Stoerk's investigations, the coccygeal gland is placed with the lymphoid organs.

The description of the entodermal tract now includes Flint's conclusion that the hmg in the pig grows by lateral branching, and that the suppression of the left eparterial bronchus and the development of the right infracardiac bronchus are correlated with the position of the aortic arch and heart respectively.

Tandler's record of the pronephros in human embryos up to 20 mm. is noted, and the development of the ovary and testis liavc been rewritten with reference to Allen's work. Presumably McMurrich hesitates to accept the determination of the large cells appearing in the entoderm and supposedly migrating into the sexual glands, as germ cells; he does not refer to them.

The suprarenal glands liave been redescribed, with a figui'e and references to Wiesel's studios. Tliey are placed in a chapter witli the carotid glands and Zuckerkandl's organs.

Under the nervous system the bearing of Harrison's experiments on the interpretation of sheath cells and on the neurone theory is recorded. Streeter's reconstruction of the otocyst of a 20-mm. human embryo replaces that of His of a similar stage. Fuch's observation in the rabbit that the stapes is at first separate from the second branchial cartilage is regarded as an ontogenetic condition and not of general significance.

The large number of changes in this edition of McMurrich's book reflect the progress of embryology during the last three years. Students will like the small size of the volume, and teachers will appreciate Professor McMurrich's estimate of the value of recent researches.

Frederic T. Lewis.

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 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!


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.


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

Wilson EB. The Cell in Development and Inheritance. Second edition (1900) New York, 1900.

O. Hertwig: "Die Zelle und die Gewebe." Jena, 1893.


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   McMurrich 1914: General 1 Spermatozoon - Spermatogenesis - Ovum - Fertilization | 2 Ovum Segmentation - Germ Layer Formation | 3 Medullary Groove - Notochord - Somites | 4 Embryo External Form | 5 Yolk-stalk - Belly-stalk - Fetal Membranes Organogeny 6 Integumentary System | 7 Connective Tissues - Skeleton | 8 Muscular System | 9 Circulatory - Lymphatic Systems | 10 Digestive Tract and Glands | 11 Pericardium - Pleuro-peritoneum - Diaphragm | 12 Respiration | 13 Urinogenital System | 14 Suprarenal System | 15 Nervous System | 16 Organs of Special Sense | 17 Post-natal | Figures


McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.


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