Book - A Laboratory Manual and Text-book of Embryology

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Prentiss CW. and Arey LB. A laboratory manual and text-book of embryology. (1918) W.B. Saunders Company, Philadelphia and London.

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This 1918 historic embryology textbook by Charles Prentiss and Leslie Arey.

Also by this author: Prentiss CW. On the development of the membrana tectoria with reference to its structure and attachments. (1913) Amer. J Anat. 14(4): 425-461.

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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)
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A Laboratory Manual And Text-Book Of Embryology


Charles William Prentiss, A. M., Ph. D.

Late Professor Of Microscopic Anatomy, Northwestern University Medical School, Chicago

Revised And Extensively Rewritten By

Leslie Brainerd Arey, Ph. D.

Associate Professor Of Anatomy In The Northwestern University Medical School, Chicago

Second Edition, Enlarged With 388 Illustrations (Many In Color)

Philadelphia and London

W. B. Saunders Company


Copyright, 1915, by W. B. Saunden Company. Reprinted August. 1915. Revised, entirely reset, reprinted, and recopyrighted October 1917. Copyright 1917 by W. B. Saunders Company. Reprinted July 1918.

Human Embryology 1917: The Germ Cells | Germ Layers | Chick Embryos | Fetal Membranes | Pig Embryos | Dissecting Pig Embryos | Entodermal Canal | Urogenital System | Vascular System | Histogenesis | Skeleton and Muscles | Central Nervous System | Peripheral Nervous System | Historic Embryology Textbooks | Embryology History

Preface to the Second Edition

The untimely death of Professor Charles William Prentiss (1874 - 1915) has made necessary the transfer of his Embryology into other hands. In this second edition, however, the general plan and scope of the book remain unchanged although the actual descriptions have been extensively recast, rewritten, and rearranged. A new chapter on the Morphogenesis of the Skeleton and Muscles covers briefly a subject not included hitherto. Forty illustrations replace or supplement certain of those in the former edition.

In preparing the present manuscript a definite attempt has been made to render the descriptions as clear and consistent as is compatible with brevity and accuracy. It has likewise been essayed to properly evaluate the embryological contributions of recent years, and, by incorporating the fundamental advances, to indicate the trend of modern tendencies. Since no page remains in its entirety as originally penned by Professor Prentiss, the reviser must assume full responsibility for the subject-matter as it now stands.

It is hoped that those who read this text will co-operate with the writer by freely offering criticisms and suggestions.

L. B. A.



This book represents an attempt to combine brief descriptions of the vertebrate embryos which are studied in the laboratory with an account of human embryology adapted especially to the medical student. Professor Charles Sedgwick Minot, in his laboratory textbook of embryology, has called attention to the value of dissections in studying mammalian embryos and asserts that "dissection should be more extensively practised than is at present usual in embryological work" The writer has for several years experimented with methods of dissecting pig embryos, and his results form a part of this book. The value of pig embryos for laboratory study was first emphasized by Professor Minot, and the development of my dissecting methods was made possible through the reconstructions of his former students. Dr. F. T. Lewis and Dr. F. W. Thyng.

The chapters on human organogenesis were partly based on Keibel and Mall's Human Embryology. We wish to acknowledge the courtesy of the publishers of Kollmann's Handatlas, Marshall's Embryology, Lewis-Stohr's Histology and McMurrich's Development of the Human Body, by whom permission was granted us to use cuts and figures from these texts. We are also indebted to Professor J. C. Heisler for permission to use cuts from his Embryology, and to Dr. J. B. De Lee for several figures taken from his *' Principles and Practice of Obstetrics." The original figures of chick, pig and hiunan embryos are from preparations in the collection of the anatomical laboratory of the Northwestern University Medical School. My thanks are due to Dr. H. C. Tracy for the loan of valuable human material, and also to Mr. K. L. Vehe for several reconstructions and drawings.

C. W. Prentiss.

Northwestern University Medical School.


Fig. 1. Human ovum examined fresh in the liquor folliculi
  • Index


The study of human embryology deals with the development of the individual from the origm of the germ cells to the adult condition. To the medical student human embryology is of primary importance because it affords a comprehensive understanding of gross anatomy. It is on this account that only recently a prominent surgeon has recommended a thorough study of embryology as one of the foundation stones of surgical training. Embryology not only throws light on the normal anatomy of the adult, but it also explains the occurrence of many anomalies, and the origin of certain pathological changes in the tissues. From the theoretical side, embryology is the key with which we may unlock the secrets of heredity, of the determination of sex, and, in part, of organic evolution.

There is, unfortunately, a view current among graduates in medicine that the field of embryology has been fully reaped and gleaned of its harvest. On the contrary, much productive ground is as yet unworked, and all well-preserved human embryos are of value to the investigator. An institute of embryology for the purpose of collecting, preserving, and studying human embryos has recently been established by Professor F. P. Mall of the Johns Hopkins Medical School. Aborted embryos and those obtmned by operation in case of either normal or ectopic pregnancies should always be saved and preserved at once by immersing them intact in 10 per cent, fprmalin or in Zenker's fluid.


The science of modern embryology is a comparatively new one, originating with the use of the compound microscope and developing with the improvement of microscopical technique. Aristotle (384-322 b. c), however, centuries before had followed the general development of the chick day by day. The belief that slime and decajdng matter was capable of giving rise to living animals, as asserted by Aristotle, was disproved by Redi (1668).

A few years after Harvey and Malpighi had published their studies on the chick embryo, Leeuwenhoek reported the discovery of the spermatozoon by Ham in 1677. At this period it was believed either that fully formed animals existed in miniature in the egg, needing only the stimulus of the spermatozoon to initiate development, or that similarly preformed bodies, male and female, constituted the spermatozoa and that these merely enlarged within the ovimi. According to this doctrine of preformation all future generations were likewise encased, one inside the sex cells of the other, and serious computations were made as to the probable number of progeny (200 million) thus present in the ovary of Mother Eve, at the exhaustion of which the human race would end! Dalenp>atius (1699) believed that he had observed a minute human form in the spermatozoon.

The preformation theory was strongly combated by Wolff (1759) who saw that the early chick embryo was differentiated gradually from unformed living substance. This theory, known as epigenesiSy was proved correct when, in 1827, von Baer discovered the mammalian ovum and later demonstrated the germ layers of the chick embryo.

About twenty years after Schleiden and Schwann (1839) had shown the cell to be the structural unit of the organism, the ovum and spermatozoon were recognized as true cells. O. Hertwig, in 1875, was the first to observe and appreciate the events of fertilization. Henceforth all multicellular organisms were believed to develop each from a single fertilized ovum, which by continued cell division eventually gives rise to the adult body, containing, it is estimated, 26 million million cells. In the case of vertebrates, the segmenting ovum differentiates first three primary germ layers. The cells of these layers are modified in turn to form tissues, such as muscle and nerve, of which the various organs are composed, and the organs together constitute the organism, or adult body.

Primitive Segments -Metamerism

In studying vertebrate embryos we shall identify and constantly refer to the primitive segments or metameres. These segments are homologous to the serial divisions of an adult earth worm's body, divisions which, in the earth worm, are identical in structure, each containing a ganglion of the nerve cord, a muscle segment, or myotome, and pairs of blood vessels and nerves. In vertebrate embryos the primitive segments are known as mesodermal segments, or somites. Each pair gives rise to a vertebra, to a pair of myotomes, or muscle segments, and to paired vessels; each pair of mesodermal segments is supplied by a pair of spinal nerves, consequently the adult vertebrate body is segmented like that of the earth worm. As a worm grows by the formation of new segments at its tail-end, so the metameres of the vertebrate embryo begin to form in the head and are added tail ward. There is this difference between the segments of the worm and the vertebrate embryo. The segmentation of the worm is complete, while that of the vertebrate is incomplete

A muIticeUuIar embryo develops by the division of the fertilized ovum to form daughter cells. These are at first similar in structure, and, if separated, any one of them may develop into a complete embryo, as has been proved by the experiments of Driesch on the ova of the sea urchin. The further development of the embryo depends: (1) upon the multiplication of its cells by division; (2) upon the growth in size of the individual cells; (3) upon changes in their form and structure.

The first changes in the form and arrangement of the cells give rise to three definite plates, or germ layers, which are termed from their positions the ectoderm (outer skin) , mesoderm (middle skin) and entoderm (inner skin) . Since the ectoderm covers the body it is primarily protective in function, but it also gives rise to the nervous system through which sensations are received from the outer world. The entoderm, on the other hand, lines the digestive canal and is from the first nutritive in function. The mesoderm, Ijdng between the other two layers, naturally performs the functions of circulation, of muscular movement, and of excretion; it also gives rise to the skeletal structures which support the body. While all three germ layers form definite sheets of cells known as epithelia, the mesoderm takes also the form of a diffuse network of cells, the mesenchyma.

The Anlage

This German word, which lacks an entirely satisfactory English equivalent, is a term applied to the first discernible cell, or aggregation of cells, which forms any distinct part or organ of the embryo. In the broad sense the fertilized ovum is the anlage of the entire adult organism; furthermore, in the early cleavage stages of certain embryos it is possible to recognize single cells or cell groups from which definite structures will indubitably arise. The term anlage, however, is more commonly applied to the primordia that differentiate from the various germ layers. Thus the thickening of the epithelium over the optic vesicle is the anlage of the lens.

Differentiation of the Embryo

The developing embryo exhibits a progressively complex structure, the various steps in the production of which occur in orderly sequence. There may be recognized in development a number of component mechanical processes which are used repeatedly by the embryo. The general and fundamental process conditioning differentiation is cell multiplication and the subsequent growth of the daughter cells. The more important of the specific developmental processes are the following: (1) cell migration; (2) localized growth, resulting in enlargements and cotistrictions ; (3) cell aggregation, forming (a) cords y (b) sheets y (c) masses; (4) delamination, i. e., the splitting of single sheets into separate layers; (5) folds, including circumscribed folds which produce (a) evaginations, or out-pocketings, e. g., the intestinal villi, (b) invaginations, or in-pocketings, e. g., the intestinal glands.

The production of folds, including evaginations and invaginations, due to unequal rapidity of growth, is the essential factor in moulding the organs and hence the general form of the embryo.

Differentiation of the Tissues

The cells of the germ layers which form organic anlages may be at first alike in structure. Thus the evagination which forms the anlage of the arm is composed of a single layer of like ectodermal cells, surrounding a central mass of diffuse mesenchyma (Fig. 136). Gradually the ectodermal cells multiply, change their form and structure, and give rise to the layers of the epidermis. By more profound structural changes the mesenchymal cells also are transformed into the elements of connective tissue, tendon, cartilage, bone, and muscle, aggregations of modified cells which are known as tissues. The development of modified tissue cells from the undifferentiated cells of the germ layers is known as histogenesis. During histogenesis the structure and form of each tissue cell are adapted to the performance of some special function or functions. Cells which have once taken on the structure and functions of a given tissue cannot give rise to cells of any other type. In tissues like the epidermis, certain cells retain their primitive embryonic characters throughout life, and, by continued cell division, produce new layers of cells which are later cornified. In other tissues all of the cells are differentiated into the adult type, and, during life, no new cells are formed. This takes place in the case of the nervous elements of the central nervous system.

Throughout life, tissue cells are undergoing retrogressive changes. In this way the cells of certain organs like the thymus gland and mesonephros degenerate and largely disappear. The cells of the hairs and the surface layer of the epidermis become cornified and eventually are shed. Thus, normally, tissue cells may constantly be destroyed and replaced by new cells.

The Continuity of the Germ Plasm

According to this important conception of Weismann the body-protoplasm, or soma, and the reproductive-protoplasm differ fundamentally. The germinal material is a legacy that has existed since the beginning of life, from which representative portions are passed on intact from one generation to the next. Around this germ plasm there develops in each successive generation a short-lived body, or soma, which serves as a vehicle for insuring the transmission and perpetuation of the former. The reason, therefore, why ofiFspring resembles its parents is because each develops from portions of the same stuff.

The Law of Biogenesis

Of great theoretical interest is the fact, constantly observed in studying embryos, that the individual in its development tends to repeat the evolutionary history of its own species. This law of recapitulation was first stated clearly by Miiller in 1863 and was termed by Haeckel the law of biogenesis. According to this law, the fertilized ovum is compared to a unicellular organism like the Amoeba; the bias tula is supposed to represent an adult Volvox; the gastrula, a simple sponge; the segmented embryo a worm-like stage, and the embryo with gill slits may be regarded as a fish-like stage. The blood of the human embryo in development passes through stages in which its corpuscles resemble in structure those of the fish and reptile ; the heart is at first tubular, like that of the fish; the kidney of the embryo is like that of the amphibian, as are also the genital ducts. Many other examples of this law may readily be observed.

Methods of Study

Human embryos not being available for individual laboratory work, the embryos of the lower animals which best illustrate certain points are employed instead. Thus the germ cells of Ascaris, a parasitic round worm, are used to demonstrate the phenomena of mitosis and maturation; the larvae of echinoderms, or of worms, are frequently used to demonstrate the cleavage of the ovum and the development of the blastula and gastrula larvae; the chick embryo affords convenient material for the study of the early vertebrate embryo, of the formation of the germ layers and of the embryonic membranes, while the structure of a mammalian embryo, similar to that of the human embryo, is best observed in the readily procured embr>'os of the pig. An idea of the anatomy of embryos is obtained first by examining the exterior of whole embryos and studying dissections and reconstructions of them. Finally, each embryo is studied in serial sections, the level of each section being determined by comparing it with figures of the whole embryo.

Along with his study of the embryos in the laboratory, the student should do a certain amount of supplementary reading. Only the gist of human organogenesis is contained in the following chapters. A very complete bibliography of the subject is given in Keibel and Mall's "Human Embryology to which the student is referred. Below are given the titles of some of the more important works on vertebrate and human embryology, to which the student is referred and in which supplementary reading is reconmended.

Titles for Reference

Duval, M. Atlas D'Embryologie. Masson, Paris, 1889.


His, W. Anatomie menschlicher Embryonen. Vogel, Leipzig, 1885.

Keibel, F. Normentafel zur Entwicklungsgeschichte der Wirbelthiere. Bd. I. Fischer, Jena, 1897.

Keibel and Elze. Normentafel zur Entwicklungsgeschichte des Menschen, Jena, 1908.

Keibel and Mall. Human Embryology. Lippincott, 1910-1912.

Kellicott, W. E. A Textbook of General Embryology. Henry Holt, 1913.

Kollmann JKE. Atlas of the Development of Man (Handatlas der entwicklungsgeschichte des menschen). (1907) Vol.1 and Vol. 2. Jena, Gustav Fischer. (1898).

Kollmann, J. Handatlas der Entwicklungsgeschichte des Menschen. Fischer, Jena, 1907. Volume 1 | Volume 2

Lee, A. B. The Microtomist's Vade Mecum. Blakiston, 1913.

Lewis, F. T. Anatomy of a 12 mm. Pig Embryo. Amer. Jour. Anat., vol. 2.

Lillie, F. R. The Development of the Chick. Henry Holt, 1908.

Minot CS. A Laboratory Text-Book Of Embryology. (1903) Philadelphia:P. Blakiston's Son & Co.

Thyng FW. The anatomy of a 7.8 mm pig embryo. (1911) Anat. Rec. 5: 17 - 45.

Wilson, E. B. The Cell in Development and Liheritance. Macmillan, 1911.

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