1897 Human Embryology 7

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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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 VII. General Remarks on the Germ-Layers

In this chapter the general morphology and rule of the germlayers, the histor}' of the theory of the germ-layers, and the laws of differentiation are briefly considered.

I. Role op the Germ-Layers

It has long been known that the bodies of embryos consist of distinct layers, which, in many cases, are separable from one another, so as to be recognize<l in gross as discrete membranes. It is now known that all such layers may be reduced to three primitive ones, named the ectoderm, mesoderm, and entoderm (by certain writers, epiblast, mesoblast, and hypoblast). The ectoderm is a layer of epithelium ; so also is the entoderm ; the mesoderm is more complex, and as we ascend the animal scale the mesoderm gradually acquires a greater predominance until in mammals nearly the whole bulk consists of mesoderm. But in spite of this change, the throe layers are preserved throughout, and their essential relations are not altered, so that we are able to assert that imity of organization without which it would be impossible to accept the doctrine of evolution. The demonstration of the homologies of the germ-layers is the most important morphological generalization since the establishment of the cell doctrine.

As the history of all th^ organs is given in detail in other chapters, it is imnecessary to do more here than classify the tissues and organs of the human body according to the germ-layers from which they arise. Now, in classifying organs, it is best to rank them as belonging to that layer from which their functionally essential and characteristic part is derived. Thus, although the pancreas, ovary, and spinal cord all contain connective tissue, we do not call them mesenchymal, but respectively entodemial, mesothelial, and ectodermal. The gland cells of the pancreas come from the entoderm; the ova and the Graafian follicles come from the mesothelium ; the ganglion cells and nerve fibres (axis cylinders) from the ectoderm. Adopting this principle we may classify the organs of the human bodv as follows :


Ectodermal

Skin (epidermis). Epidermal structures : — Hairs. Nails. Glands : — Sebaceous. Sudorific. Salivary. Mammary. Corneal epithelium. Lens of eye. Central ner\'ous system :Ganglia. Nerves. Eye :— Optic vesicle. Optic nerve. Olfactory organ. Auditory organ. Mouth cavity : — Teeth.

Hypophysis. Anus.

Chorion : — Placenta. Amnion.

Mesodermal

1. MeBothdium. Peritoneum. Pleurae. Pericardium. Urogenital.

Wolffian body. Kidney. Testes. Ovary. Oviduct. Uterus. Vagina, etc. Striated muscle.

2. Mesenjchyjna. Connective tissues. Blood.

Blood-vessels. Lymphatics. Spleen.

Smooth muscle. Fat- cells. Marrow. Skeleton.

Entodermal

Epithelium (of digestive tract). Thyroid. Thymus. Tonsils.

Trachea and lungs. (Esophagus. Stomach. Liver. Pancreas. Intestine. Yolk -sack. Caecum. Vermix. Ck)lon. Rectum. Allantois : —

(Bladder) . Notochord.


The human body may be defined as two tubes of epitheliimi, one inside the other; the outer tube (epidermal or ectodermal) is very irregular in its form ; the inner tube (entodermal) is much smaller in diameter, but much longer than the outer and has a number of branches (lung, pancreas, etc.), and is placed within the ectodermal tube. Between these two tube« is the verj' bulky mesoderm, which is divided by large cavities (abdominal and thoracic) into two main layers, one of which is closely associated with the epidermis and forms the body- wall, the somatopleure of embryologists ; the other joins with the entoderm to complete the walls of the splanchnic viscera, and constitutes the splanchnopleure of embryologists. The mesoderm is penneated by two sets of cavities: 1, the heart and blood-vessels; 2, the lymphatic system. It is also differentiated into numerous tissues, muscle, tendon, bone, etc., and organs, urogenital system. The nervous system, although developed from the ectoderm, is found separated from its site of origin, and completely encased in mesoderm.


As we ascend the animal scale, wo discover in all parts an increasing complexity ; especially in the nervous system is this marked, but it IS even more strikingly shown by the evolution of the mesoderm in relative size and differentiation. This important correspondence between the organiziition of the mesoderm and the degree of evolution of animals has not, to my knowledge, hitherto attracted express attention.

II. Differentiation

The fundamental law of embryology is that the simple precedes the complex, the general and typical the special. Each germ-layer is at first a simple layer of cells o{ nearly uniform character. In order to develop out of the genn-layers the complex organs of the adult the layers have to be folded into various forms by unequal growth of their parts, and the cells composing them have to be specialized some in one way, some in another. This double process results in the diflferentiation of the organs. Differentiation may be defined as the process of change from homogeneous to heterogeneous structure, or as an increase of heterogeneity, since in living organisms there is no real homogeneity. From what this been just said it will be understood that under the present head we have to consider, 1 , the laws of unecjual growth; *2, the general laws of cellular differentiation, or, as it is called, histogenesis — the development of tissue.


The Relations of Surface to Mass.— However much the weight of an animal increases during its development, the ratio of the free surface to the mass alters but slightly from the ratio established when the embryo begins to take food from outside. It is only for convenience that I express this law in this precise form; in reality, about it our knowledge is scanty and our conceptions vague. According to a geometriccil principle, when the bulk of a body bounded by a simple surface increases, the surface enlarges less than the mass — in the simplest case of a cube, the surface increases as the 8<juare, the mass as the cube, of the diameter. If in a cube of unit diameter one unit of surface bounds one unit of mass, then in a cube of three units diameter nine units of surface will l)ound twenty-seven units of maas; the proportion in the first cube is 1 : 1, in the second 1 : 3. To maintain the pro|)er proportion in the embryo, simple enlargement is insufficient, therefore the surface increases by becoming more and more irregular. The iiTcgularities are characteristic of each organ and part, and may ho either large or niicroscoi)ic. They may be conveniently grouped under two main heads — projections ami invaginations.


Projections are illustrated by the limbs, filaments of the gills in fishes, the villi of the intestine, t\)lds of the stomach in ruminants, etc. In everj' case the projection is covered by an epithelium and has a core of mesodermic tissue.


InviKjinntious exist in much more varit^l form and play the principal part in the differentiation of the animal body. They may Ik? classified under four principal heads: 1, Dilatations: '2, Diverticula; 3, Glands; 4, Vesicles. iI)//r(/(f//o//.v have considerable iml>ortance in embryology'; the stomach, lungs, bladder, and uterus arise as gradual dilatations of canals or tu'oes of originally nearly uniform diameters. I)irerti('nla in the senst* of relatively hirge blind p;)uches also fonn im|>ortant organs, siich as the ciecinn and appendix venniformis, k)T the gall bladder; these structures arise, each as a blind outgrowth of a canal, the walls of whi(*h at a certain point rapidly grow to form the j>ouch. (tiifnds, which are, as first shown by Joliaimes iliiller's classic researcln^s, only small diverticula, which end blindly and appear in an innnensi^ variety of modifi(.'ations; the manifold types of glands are discussed Wlowin a separate paragraph; they constitute the largest class of organs with which we have t4) deal. The glands are develoiX'd from (epithelium and push their way into the ines<Klerni ii;)on which th(» epithelium n»sts, while in dilatations, and in diverticula, the epithelium ajid mesoderm expand together. Vesicles we call those epithelial sacs, which develop somewhat like glands by growing into the mesoderm, but the mouth of the invagination closes by the coalescence of the epithelium, thus shutting the c*ivity. The closed sac separates from the epithelium from which it arose, and connective tissue grows between the two ; the sac may then undergo various ^lodifications. The membraneous labyrinth of the ear is developed from the ectoderm in this way, as is also the lens of the eye. We might perhaps also class the medullary canal under this head (c/. Chap. VIII.) if we choose to consider it as a vesicle so much lengthened tnat it has become a tube.


The Law of Unequal Growth

The changing shapes- of the embryo and the development of those irregularities — projections and invaginations, which preserve the proper proportion between the surface and mass of the body, both depend upon the unequal growth of the germ-layers, especially in superficies. The expansion of a germlayer having the epithelial type of structure* may take place by three means: 1, the multiplication of the cells; 2, the flattening out of the cells ; 3, enlargement of the cells. In the early stages of development the influence of the first two factors predominates; during the later stages, especially after birth, the latter. Of the three factors the first is the most important.


The unequal multiplications of the cells in all embryonic epithelia is the fundamental factor of development, and we see it shaping out the embryo, its organs, and the parts of organs, before histological differentiation really begins. The distinct areas and centres of growth which are necessary to develop the hmnan body out of the germ-layers are innumerable, and their distribution, limitations, and interactions make up a large part of the subject-matter of embryolog>\ At every turn of our studies we encounter fresh illustrations. If in a limited area of a cellular membrane there occurs a growth or expansion more rapid than in the neighboring parts, then that area is, as it were, bounded by a fixed ring, and can, therefore, find room for its own expansion only by rising above the level of the membrane; thus when in the embryonic region of the blastodermic vesicle the growth becomes more rapid, the embryo begins to rise above the level of the vesicle; when, at a certain point of the surface of the embryo, a steady and long-continued growth occurs, the limb appeiirs, gradually lengthens out, and enlarges from a small bud at first to a complete arm or leg. If the departure takes place the other way we have an invagination produced ; thus for every hair and every gland of the intestine there is a separate centre of growth.

The reason for the unequal growth is unknown. We have not even an hypothesis to offer as to why one group of colls multiplies or expands faster than another group of apparently similar cells close by in the same gerni-layer. It is no real explanation to say that it is the result of heredity, for that leaves us as completely in the dark as ever as to the physiological factors at work in the developing individual.

The conception that the development of an animal depends fundaroentfilly upon the unequal* expansion and consequent foldings and bendings of the germ-layers was first suggested by the researches of C. F. Wolflf on the development of the intestine, and was more clearly recognized by Pander, who definitely asserted that the formation of the embryo is affected by foldings of the germ-layers. In recent times His has studied the problem very intently, and in his memoir on the chick, 68. 1, discussed it minutely. In this memoir is to be found most of what little we know concerning embryological mechanics.

  • By this liiuitatiuu we exclude the mesM^Dchyuia, but uot the luesothelium.

The Classification of Glands

For a long time it has been customary to divide glands into tubular and acinous. W. Flemming, in an admirable article, 88. 1, has shown that this classification as currently applied is imtenable, and he proposes in its stead another, the basis of which is the branching of the glands; he makes three primar}' divisions: Single glands (Einzeldriisen), which are unbranched; Single branching glands (verastelte Einzeldriisen), with a smgle duct" and the secretory portion branched ; Compound glands (zusammengesetzte Driisen), with both the ducts and the secretory portions branched. Under the first head he includes the follicles of the ovary, under the last the seminiferous tubules; but the so-called sexual glands are not, properly speaking, glands at all, since their products arise as differentiations of the cells, not as secretions; it can, I think, only perpetuate confusion to class them with the true glands. So, too, with regard to the principal organs of excretion — the lungs and the kidneys; the former can certainly not be regarded as a gland, since it produces no secretion, for the water and gases given off by the respiratory organs are not produced by the pulmonarj' epithelium. The kidneys have more claim to be classed with the glands, since their excretion is the direct product of the epithelium of the renal tubules ; the ureter represents the duct and the secretory portions (collecting tubules) branching, thus bringing them under the second of Flemming's headings. It seems to me more convenient to give the kidneys a place apart. Under the head of compound glands Flemming ranks the liver, but inasmuch as the gland cavities (gall-capillaries) of the liver form an anastomosing system of canals, it is better to put the liver in a class by itself, especially as its development is unlike that of any other gland. For the sake of completeness we may add also the unicellular glands, such as are found in the lower vertebrates and in man}- invertebrates; these constitute a group by themselves, distinct from the multicellular glands. The latter may be divided into four sub-groups : Simple, Branching, Compound, Anastomosing. A simple gland is one consisting of a single unbranched epithelial tube, ending blindly and opening upon the epithelial surface from which the gland has been developed; a simple gland may be iubnlar^ that is, a canal of approximately even diameter; or alveolar^ that is, with the blind end somewhat dilated; or vesicular, that is, with the opening small, but the rest of the gland distended like a cyst. Even in the simple glands we usually find the portion of the epithelial tube near the orifice acting simply as a duct, while the deeper part alone performs the secretory office, or acts as the gland proper. The differentiation of the duct is to be regarded, generally speaking, as the earliest and most primitive specialization of a gland. A branching gland is a simple gland with the addition of branching of the secretory portion proper ; under this head also we have tiihmar and alveolar glands. A compound gland is a branching gland with the addition of branching of the duct. An anastomosing gland is a compound gland with the additional feature of the branches of the secretory portion united together so as to form a network.

If we apply this classification to the glands of man, the result may be presented in a tabular form, as follows :

GLANDS

A. Unicellular

(Found in ichthyopsida and invertebrata) .

B. Mlt-ticellular

1. Simple glands.

a. Tubular.

1. Lieberkiihn*8 follicles.

2. Peptic glands.

3. Sweat glands.

b. Alveolar

Small sebaceous glands. r. Vesicular.

(Sub-epidermal glands, amphibia). J. H ranching glands.

a. Tubular.*

1. Pyloric glands.

2. Bninuer's Kl&nds.

3. Mucous glands.

4. Uterine gland.

b. Alveolar.

1. Large sebaceous glands.

2. Meibomian glands. li. Cnmjxmnd glands.

a. Tubular.

1. Salivary glands. 2 Pancreas.

3. Tear glands.

4. Cowper's glands. T). Prostate glands.

b. Alveolar, f

Milk glands. 4. Anastomosing glands. Liver.

This classification cannot lie regarded as final, since it is based solely on the general shape of the epithelial invagination forming the glancLs. We may expect in its stead a better classification, based on other and more essential characteristics. The defects of the above arrangement are serious, fis is strikingly illustrated by the unnatural separation of large and small sebaceous glands. The basis of classification ought to be the phylogeny of the glands.

Histological Differentiation. — The genesis of the tissues del^ends upon — 1, the multiplication of cells; 2d, the siXK.nalization of cells; ;ki, the development of intercellular substance. The first of the fiictors will be discussed in a later chaptiT. The secoml and third are to be considered here.

The first tissue to appear is the epithelium of the ectoderm and entoderm; the second form of tissue is the mesenchyma, for the mesothelial portion of the mesoderm is also epithelium. Histological differentiation, therefore, begins with epithelium and mesenchyma; these two primitive tissues we must consider separately.

  • If the kidneys be oonsid«*red qr glands they would voiur under this head, as liranchinf^ tubular f^lands.

t If we oonsitler the luii^ as a gland and the bronchi as ducts, the lung nouhl come under Uiis head as a compound alveolar gland



A. Epithelium

In invertebrates the ectoderm and entoderm as soon as they become cellular consist each of a single row of polyhedral cells, which in the most primitive type are of equal height. The cells when viewed from the surface are always irregular in outline, usually five and six-sided, sometimes seven-sided or more, but probably never four-sided, except occasionally isolated cells, whicn assume that outline. When the cells are not modified by the presence of yolk, the round or nearly round nucleus lies in the centre of each ceil. In every epithelial cell three axes may be distin^ished, two parallel, with one perpendicular to the surface of the myer, of which the cell forms a part. In the primitive epithelium the three axis are approximately equal in leneth, hence the tissue is said to be composed of " cubical" (cuboidal) cells. There is very little substance between the cells, and it always remains relatively insignificant in epithelium in marked contrast to its development in the mesenchyma.


In probably all vertebrates the ectoderm and entoderm during segmentation are both several-layered, but after the close of segmentation they soon become each single-layered, as we have seen. The significance of this modification of the course of development is imknown.


The further differentiation of the epithelial germ-layers depends on — 1, the formation of folds, already discussed, p. IGl; 2, changes in the proportion of the cellular axes; 3, structural changes in the cells; 4, arrangement of the cells* in several strata. Concerning the latter factors a few words are necessary. The horizontal axis usually remain approximately equal in length, while the perpendicular axis varies independently and to a much greater extent. That epithelial cells are primitively equiaxial may be accepted as an axiom. Yet in vertebrates there are marked departures from this type during very early stages. From the cuboidal type arise the principal modifications known as the " cylinder" epithelium and the " pavement" epithelium — names which are unfortunate. As regards the structural differentiation^ we must distinguish between the specialization of single cells and that of groups of cells. The former is presumably the primitive form, since it predominates in ccelenterates ; the laticr has been evolved, we must assume, by the grouping of specialized cells ; but in the development of a vertebrate we see always a cluster of cells gradually differentiated from their fellows, and never the cells first specialized and then collected by migration or otherwise. Speaking generally we may say that the higher we ascend the animal scale the less specialization do we find of isolated cells, and the more of groups of cells. This noteworthy fact will, I think, be ultimately found to possess an important significance at present hidden from us. The development of additional strata^ which is especially characteristic of the vertebrate ectoderm, is described in the chapter on the epidermis.

B. Mesenchyma

The first histological differentiation of the mesenchyma in vertebrates is the separation of a certain number of cells from all attachment to their fellows; these cells are capable of changing their site, and during further development they increase in number and variety. The first of these cells to appear are the blood-cells of the so-called blood islands. For all mesodermic cells not mechanically united to others, but capable of change of site, I have assumed that the primitive type was a cell capable of independent amoeboid movements, and have proposed for them (Minot, 23^ 207), the collective name of Mesamcehoids — as a term at once appropriate and corresponding to a natural cla^s of tissues. The mesamoeboids, then, I regard as a primitive form of the cells of the mesoderm, thus implying that when amoeboid cells are found in the higher metazoa we are dealing with those free mesodermic elements which have been least modified in the course of development. According to this view the wander cells and white corpuscles in vertebrates represent one of the earliest tissues of the mesoderm. As already pointed out, the essential feature of the mesenchyma is that its cells lie somewhat apart and are connected together by protoplasmatic processes running from cell to cell ; the space between the cells is filled with a homogeneous, structureless, transparent substance, which is at first perhaps merely a serous fluid, and which is known as the basal substance (Grundsubstanz) or matrix. The mesenchymal matrix is the seat of numerous modifications, varying according to the special tissue formed out of the mesenchyma; each modification of the matrix is associated with the corresponding specific change of the cells.

III. History of the Theory op the Germ-Layers

The fundamental facts of the construction of the vertebrate body out of distinct layers of cells are collectively designated as the theory of the germ-layers. The theory is as important as the cell theory for the comprehension of the morphology of animals. The establishment of it is due principally to Carl Ernst von Baer, although it was first suggested half a century earlier by C. F. Wolff, and more clearly developed by Pander, from whom Von Baer drew his immediate inspiration. Since Von Baer's time numerous investigators have contributed to our knowledge of the germ-layers. If we leave out of consideration the introduction of the cell doctrine, which had a profound influence on embryolog}% as upon every dejiartment of biology, we may distinguish three principal steps in the acquisition of our present notions concerning the germ-layers ; the first step was the recognition by Huxley that the. coelenterates are built up of two layers, and the suggestion that these two layers are homologous with the germ-layers of the higher animals ; the second step was the formulation of the gastrula theory by Kowalewsky, and the third step was the discovery by His that the middle germ-layer comprises two distinct groups of tissues.

C. F. Wolff was the first investigator to recognize the embryonic ^rm-layers, which he did in the course of his study of the development of the digestive canal of the chick. His article was published in Latin in the ** Commentaries of St. Petersburg Acatl.," XII., XIII., 17f58-1709, and shows that he suspected the far-reaching significance of th#observations which taught him that the intestine is evolved out of a leaf-like sheet in the embryo. Wolff's article secured very little notice from his contempi)rarie8, nor w«is it until it was translated into German by the elder Meckel, and published at Halle, in 1812, that its extraordinary merit became recognized. The translation seems to have awakened the interest of Dollinger, a professor at Wiirzburg in the early part of this century, who, though little known by his own works, has nevertheless become distinguished through his pupils, foremost among whom are Pander and Von Baer. The former in his dissertation (Wiirzburg, 1817) gives a history of the metamorphosis of the hen's ovum during the first five days of incubation, and shortly after piiblished his chief work (" Beitrage zur Entwickelimgsgeschichte des Hiihnchens im Eie," Wiirzburg, 1817), the beautiful plates of which were prepared by his friend, D'Alton. Pander distinguished in the blastoderm at first a single layer, das Schleimblattj external to which, after the twelfth hour, appears the serous layer, which is thinner and more transparent, and nnally, at the end of the first day, a third layer, the Gefdssschicht^ between the mucous and the serous hiyers. Pander appears not to have continued his embryological researches, but to have left that to his friend and fellow-student, Von Baer, who began his own studies in 1819, and continueil them with some interruptions for ten years, extending them gnidually to other vertebrates. In Von Baer's work we have the most profound, exhaustive, and original contribution to embrj'()log}\ which has ever been made, and it is unquestionably one of the gi*eatest tichievements in the history of science. It ought to be read and pondered upon by everj' embryologist. The work itself was entitlecl " Uel)er Entwickelimgsgeschichte der Thiere, Beobachtung und Reflexion. " Never again have observation and thought been so successfully comlrined in embryological work. The first part of Von Baer's " Entwickelungsgeschichte" appeared in 1828, the second part in 18:37. The second part was, however, incomplete and appeared with the announcement of the publishers, stating that they had begun to print the work in 18*20, and after waiting five years for manuscript had carrieil the printing to the 315th page, and finally, after three years more waiting, published the incomplete sec*ond part. In 1 888 the missing termination of Von Baer's work was published by Stieila. It seems that Von Baer had kept it back in the hope of filling up some gaps ; not suc(leeding in this he waited t(X) long, and after the incomplete work had been issued, Von Baer seems to have lost his interest and to have laid aside his manuscript ft)r the remainder of his long life. Von Baer worked out, almost as fully as was iH)ssible at this time, the genesis of all the principal organs from the germ-layers, instinctively getting at the truth as only a great genius could have done. Von Baer recognized the somatopleure, which he called animnles Blattf and splanclmopleure, which he called veqefatives Blatty and further (as each of these Blatter consists of two layers) the animates Blatt had a Hautschicht (ectoderm) and a Fleischachicht (mesoderm), while the vegetatives Blatt had its Si'hleimschicht (entoderm) and Gefiissschicht (mesoderm). With this generalization, and with the detail of development which he added, Von Baer created modem embryology. It was not until aftSr the cell doctrine was announced in 1838 by Schwann that any important progress was made; C. B. Reichert, 40.1, 43.1, added something to our knowledge, but the value of his work is greatly diminished by the imperfections of hia observations, and still more by his errors of interpretation. Pernaps his greatest importance was in his influence upon Remak, whose masterly investigations upon the differentiation of the imiform embryonic cells into the tissues of the adult at once converted embryology into a science closely allied to histology ; to Remak we owe also the recognition of the mesoderm as a unit, he having discovered that Von Baer's Fleischschicht and Oefdsaschicht are really parts of the same layer. There followed next a series of minor investigations by sundry authors, which, though not very numerous, nevermeless by their gradual accmnulation afforded much knowledge. It is not until 1868, when His published his monograph on the chick, that anything fundamentally new was added to our notion of the germ-layers; in that work His draws the distinction between the archiblast and parablast, see p. 153.

From another side progress was being made by gathering materials by the comparative study of the germ-layers throughout the animal kingdom; here Huxley led the way by discovering the two layers which compose the body of ccBlenterates — a discovery- which he announced m 1849, adding at the same time the fortunate suggestion that the two layers are homologous with the two primary germlayers of vertebrates. Four years later (1853) Allman proposed for the two layers of coelenterates the terms ectoderm and entoderm^ which have since come into general use, not only for these layers, but for the corresponding germ-layers throughout the animal kingdom. Beginning about 1845 we have a series of researches on the embryology of invertebrates, especially of marine forms. The leader in these studies was Johannes Miiller, whose memoirs are classic and were published for the most part by the Berlin Academy, 1846-1854. He had numerous followers, among whom Alexander Agassiz and Metschnikoff may be mentioned. The naturalist, to whose work in this field we owe most as far as the development of the theory of the germ-layers is concerned, is Anton Kowalewsky, who, b}' a long series of well-known investigations accumulated a vast amount of evidence in favor of the homology of the germ-layers throughout the animal kingdom. Kowalewsky's investigations culminated in the theory that the planula, or, as it is now called, the gastrula, is the primitive embryonic type; he is the originator of the gastrula theory, an account of which has already been given, p. 112. Ernst HaeckePs two essays, 74.2, 75.1, contain, asalread\' stated, exceedingly little that is really original and valuable. Lankester's two essays, 73. 1, 77. 1, are more scientific, and are also noteworth}^ from having furnished us ^vith a considerable number of terms, which have since become current in embryology. Lankester's essays are further remarkable for containing the first enunciation of the coelom theory. It will be remembered that Von Baer conceived the body cavity to be bounded by two distinct layers, the Fleischschirht and Gefdssschicht; Remak showed that the coelom is bounded by one layer only, the mesoderm; Huxley, 75.1, p. 54, attempted to make clear the morphology of the body cavity by distinguishing three types thereof — 1, tiie enterocoele or body cavity, arising as a diverticulum of the alimentary canal, such as was then shown to be the case in the echinoderms and Sagitta; 2, schizoccele, formed by simple splitting of the mesoderm ; 3, epiccele^ formed by invagination of the outer wall of the body like the atrial chamber of Tunicata. Huxley suggests, p. 56, that the coelom of vertebrates might be an epicoele. Lankester, 77. 1, maintained the opposite view, that the vertebrate coelom is an enterocoele; for the subsequent history of Lankester's theory, especially as modified by the Hertwigs, 81.1, see Chapter VI,, p, 155.



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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)


Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History



Cite this page: Hill, M.A. 2017 Embryology 1897 Human Embryology 7. Retrieved December 18, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/1897_Human_Embryology_7

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© Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G