Book - A Laboratory Text-Book of Embryology 1 (1903)
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Chapter I. General Conceptions
The student of embryology should start with as clear and definite a conception as possible of what he is to gain from his pursuit of that science. If he is a student of biology or of zoology, he must appreciate that knowledge of the laws of development is an indispensable part of what he must master in order to understand those sciences. He must appreciate that it is from the studies of the embryologist that are derived our conceptions of the nature of sex, of heredity, of variation, of differentiation, and many of our most important notions concerning evolution, both of the individual and of the race. He will learn, further, that the embryo illustrates to him with particular clearness the fundamental principles of morphology. If he be a medical student, he will find in embryology first of all the clue to the intelligent comprehension of the anatomy of the adult, a comprehension which he can obtain in no other way, but he will also gain much knowledge of direct practical value as to the embryo and as to the conditions in the adult, acquaintance with which is invaluable in medical practice. And, finally, he will find that it throws a vast light on pathology, both upon the problems of malformations and monstrosities, and also upon the whole question of pathological change in the tissues.
The best study of embryology, therefore, is that which continually passes beyond the direct observations to the conceptions which they justify and which underlie many important branches of science which are related to, and in a large part dependent upon, embryology.
The student ought to strive, accordingly, to pass from the direct observation of the specimen to the generalizations, and accustom himself to regard always each special preparation, which may be submitted to his observation, as an illustration of some general principle. To facilitate his reaching this result the following pages are arranged so as to offer a digest of some of the more important generalizations and fundamental laws of embryology.
The Vertebrate Type of Structure
When one traces the course of development of any vertebrate, one finds, speaking in general terms, that the fundamental characteristics, which are more or less common to all vertebrates, are those which first appear. Later, there come in the secondary characteristics, which distinguish one class from another*, and still later the subordinate characteristics by which the smaller subdivisions of the vertebrate type become differentiated one from another. This statement, however, is correct only if we add to it certain indispensable limitations. Every embryo at every stage of its development is an individual of the particular genus and species to which it belongs. It has at every stage peculiarities which distinguish it from every other species. The embryos of allied forms resemble one another more closely than do the embryos of forms which art only distantly related to one another. The specific qualities of an embryo are, however, far more difficult to recognize than those of the adult, and the student will be far more impressed by the resemblances between embryos than by their differences. It is owing to this very fact that the distinctive peculiarities of the species are not accentuated in the embryo. We are able to derive from the embryos themselves a series of conceptions which render it comparatively easy to perceive the dominant morphological features of the vertebrate type.
It will be convenient to put down six fundamental characteristics of the vertebrate type as the most important, and to add to these six others which are also fundamental, but perhaps less distinctive. This enumeration is necessarily arbitrary, and can serve only to facilitate the work of the student. When his knowledge deepens, he will be able to free himself from the limitations which such a numerical classification has put on his understanding of the matter.
A. The six most important characteristics are :
- The pharynx and pharyngeal structures (gill clefts, nerves, aortic arches, heart).
- The notochord or structural axis.
- Hollow nervous system.
- Position of mouth.
- Division of the coelom into :
- segmented part (mesomeres) ;
- unsegmented part (splanchnocele), which is subdivided by the septum transversum.
- Stomach, intestine, and mesentery.
- Position of liver, and its relation to veins.
- Wolffian tubules and ovotestis (= urogenital ridge).
- Urogenital ducts (Wolffian and Mullerian).
- Special sense-organs (nose, eye, and ear).
The pig embryo illustrates all these characteristics, and we shall study the ways in which the typical mammalian modifications of the type are gradually evolved.
Let us now pass in review these twelve characteristics :
- The pharynx is the cephalic portion of the digestive canal, and it acquires in all vertebrates a somewhat complicated structure. This complication depends primarily upon a series of lateral outgrowths from the pharynx which are known by the name of gill pouches. They are symmetrically arranged and therefore form pairs. They are designated by numbers, the pouch which lies nearest to the mouth being called the first, the next the second, and so on. In many of the lower vertebrates the number of these gill pouches varies from five to perhaps nine. In mammals there are always four^ pairs on each si de. In aquatic vertebrates the pouches acquire each an opening to the exterior at the side of the neck, and are then designated as gill clefts or branchial clefts. We find that the position of the clefts determines the distribution of a series of the most important of the cephalic nerves and the primitive distribution of the branches of the aorta and of certain important muscles, hence the morphological features of the pharynx have a profound influence upon the entire anatomy of the body in that region. No similar pouches are formed from any other part of the digestive canal.
- The notochord is a rod of cells which extends nearly the entire length of the embryo. It lies in the median plane, a little below the ventral edge of the central nervous system. Its cephalic termination is always in the neighborhood of the pituitary body. It may be considered the primitive structural axis of the vertebrates. There are vertebrates in which it is the only structural axis ever produced, but in the great majority of vertebrates there is developed around the notochord a series of skeletal elements which we know as vertebra?, and which make a new structural axis in these forms. The notochord in these animals is found to run through the bodies of the vertebrae. The notochord diminishes in size as we ascend the vertebrate series. It is of very considerable diameter in the lowest fishes, smaller in amphibia and reptiles, and smallest of all in mammals. Its duration through the life-history of the individual also diminishes as we ascend the series, for we find that in the lowest fishes it persists well developed throughout life; in other fishes it disappears in part, in amphibia it disappears almost completely, and in mammals it aborts entirely, and, so far as known, no remnant of it normally persists in the adult.
- The hollow nervous system. This is found in vertebrates only, or in animals which are closely related to vertebrates, so closely that to many naturalists they are included in the same sub-kingdom. The hollow nervous system is enlarged in the region of the head, the enlargement constituting the brain. The rest of it is of smaller size and constitutes the spinal cord.
- The limbs. There are two pairs, which are lateral extensions of the surface of the body and acquire in their interior a skeleton by which they are supported and muscles by which they are moved. No homologous structures are known in any invertebrate animal.
- The position of the mouth. The typical invertebrate mouth is surrounded bv the nervous system. For instance, in insects or in the jointed worms (annelids) there is a brain, so called, above the mouth, and a strand of nervous tissue running down on either side of the body past the mouth to join the ganglion on the lower side, thus completing a ring of nervous material through which the oesophagus passes. In vertebrates, on the other hand, the mouth is not enclosed by any oesophageal ring, and the entire nervous system is on one side of the body and dorsal to the mouth.
- The division of the primitive body-cavity. The body-cavity in the embryo is known by the comprehensive name of the coelom. It will not be possible to acquire a clear idea of its division until the embryos are actually studied. It forms many parts. Of these, there are two series, one on each side of the central nervous system, which form cavities of what we designate as the primitive segments of the body. There are also two large divisions which extend from the region of the head to that of the future pelvis, one division for each side of the body. These two large parts are not divided into segments at all, though the cavities of all of the segments are primitively connected with these two main divisions. Comparatively early in the development the two main cavities become connected with one another so as to constitute a single cavity to which we apply the name of splanchnocele. The splanchnocele surrounds the heart of the embryo, where we recognize it as the pericardial cavity, and it extends through the future abdominal region, where we recognize it as the abdominal cavity. The pericardial and abdominal regions of the cavity are separated from one another in the embryo by a broad transverse partition which bears the name of septum transversum, This septum in mammals becomes in the adult the diaphragm. It is one of the most striking of all the morphological peculiarities by which vertebrates are distinguished from invertebrates.
- The stomach, intestine, and mesentery. The division of the digestive tract of vertebrates into these two fundamental parts is very characteristic. The stomach is not only an enlargement of the digestive canal, but also may be distinguished from the intestine by its developing glands, which are specific to it and unlike those of the intestine proper. The mesentery by which the intestine is suspended to the dorsal wall of the abdomen is the survival of the original partition by which the two halves of the splanchnocele cavities were separated from one another. The cavities in the abdominal region come into communication with one another by the veYy early disappearance of the partition on the ventral side of the intestine. But it should be noted at once that a portion of this primitive ventral partition, or, as we may call it, ventral mesentery, persists permanently in relation to the position of the liver.
- The position of the liver. The primitive large veins of the embryo pass through the septum transversum, and it is in connection with these veins, and as an appendage to the septum itself, that the liver is developed.
- The urogenital ridge. Out of a part of the primitive segments there are developed excretory organs, and these, as they increase in size, form two protuberances on the dorsal side of the abdominal cavity. Each protuberance is what we know as the urogenital ridge, so named, first, on account of its form; and, secondly, on account of its producing not only the excretory organs proper, but also the genital glands.
- The urogenital ducts. There is primitively a single duct for each urogenital ridge. This duct is commonly known as the Wolffian duct. A little later in the history of the embryo there appears a second duct which is closely parallel to the first, but which has no connection with any of the excretory apparatus, and is destined to serve later as the female genital duct. In no invertebrate have we found anything homologous with these two ducts.
- Special sense-organs. These are the olfactory, the visual, and the socalled auditory organs. We have to use the term " so-called " in speaking of the auditory organ because we now know that the ear in the lower vertebrates is not an organ of hearing, but an organ of balancing or orientation, and it is only in the higher vertebrates that there is added to this primitive function that of audition proper. It seems not improbable that many invertebrate animals have senseorgans which are homologous with those of vertebrates. Nevertheless, in the vertebrate type there are many peculiarities which are distinctive, and these we shall best learn from a study of the actual development.
- The hypophysis. The hypophysis is the embryological name applied to the structure which we know in the adult as the anterior lobe of the pituitary body. The posterior or infundibular lobe is a portion of the brain, but the anterior lobe is an outgrowth from the cavity of the mouth of the embryo. Comparatively early in the development of the individual this outgrowth becomes entirely separated from the mouth-cavity (from the walls of which it arose), and forms a closed vesicle. It exists in every known vertebrate animal, has been much studied, but still remains an organ the significance of which we cannot explain. Its absolute persistency and the uniformity of its development indicate that it is an organ of importance, but beyond that we can hardly go.
To these conceptions, the student should add the following comprehensive morphological notions : The mammalian body may be defined as two tubes of epithelium, 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 tubes is the very 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 splancknopleure of embryologists. The mesoderm is permeated by two sets of cavities : (i) 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.
The Principal Modifications of the Vertebrate Type
Our knowledge of human development being at the present time very incomplete, it is often necessary to supplement that knowledge by reference to facts of observation on the development of various vertebrates. Indeed, the best stud}- of human embryology includes more or less comparative work. We shall, therefore, find frequent occasion to refer to the development of many vertebrate tvpes. Accordingly, in this section there are given definitions of the principal subdivisions of the vertebrates to which we shall have occasion to refer.
From an embryological standpoint, vertebrates may be separated into two main divisions, which are commonly designated as the Amniota and Anamniota, distinguished by the presence or absence of the amnion, the amnion being a thin membrane, which immediately surrounds the embryo in the higher forms. It occurs in reptiles, birds, and mammals, which together constitute the Amniota. It is absent in the fishes and amphibians, which therefore constitute the Anamniota. These two divisions are also distinguished by other peculiarities. The higher forms referred to all have the organ known as the allantois, an appendage of the embryo, which is lacking in the lower forms. The comparative anatomist finds many points of resemblance between the various classes of fishes, on the one hand, and the amphibia, on the other, and indicates this relationship by the use of the term Ichthyopsida, which means "fish-like." In our present classification the term Ichthyopsida is synonymous with Anamniota. The comparative anatomist further recognizes a close relationship between birds and reptiles, and puts these together under the common designation of Sauropsida, or " reptile-like." As regards the fishes, many classifications are more or less in vogue at the present time. For the purposes of this book, the following names for the classes have been adopted as names generally understood and sufficiently exact to meet our needs : The lowest fishes are the hag-fishes and lampreys, constituting the group of Marsipobranchs. Next comes the group comprising the sturgeon and its allies, for which we have retained the old term of Ganoids. To these fishes the central position in the system must be assigned, and it is probable that the higher fishes are more or less directly descended from Ganoid-like forms. They fall into three further classes, of which the largest and most varied is that of the bony fishes, or Teleosts. Another class, known as the Elasmobranchs, comprises the sharks, skates, rays, and electric fishes. The last class is known as the Dipnoi, or lung fishes, which comprise only three living forms, the Ceratodus, living in Australia, the Protopterus in Africa, and the Lepidosiren in South America.
The amphibia are divided into two classes, the Urodela, of which the newts and salamanders are familiar examples, and the Anura, of which the frogs and the toads are the best known representatives.
As to the reptiles, it is unnecessary to consider their classification, as we shall not have much occasion to refer to them, our knowledge of their embryology being very fragmentary at the present time, save for a rather extended series of observations on the development of lizards. As regards birds, it may be noted that embryologists have worked chiefly upon the chick, which has been for a century the classic object of embryological study. There are comparatively few observations on the development in other species of birds.
Mammals are divided into three principal classes. Of these, the lowest is that of the Monotremes, of which the only living representatives are found in Australia and neighboring islands, a very few species concerning the development of which very little is as yet known, but which are of importance, as they resemble in certain respects the reptiles and assist us in drawing comparisons between the reptilian and the mammalian types. Of this class, the Australian duck-bill may be mentioned as typical. The second class is that of the Marsupials, familiar to us in America through the common opossum. In Australia there are many genera and species of marsupials.
- Tunicata (Ascidia)
- Anamniota (Anallantoidea)
- Marsipobranchia (lampreys, etc.)
- Ganoidea (sturgeon, etc.)
- Teleostea (bony fishes)
- Elasmobranchia (sharks, skates, etc.)
- Dipnoi (lung-fishes)
- Urodela (newts, salamanders, etc.)
- Anoura (frogs, toads)
- Amniota (Allantoidea)
- Reptilia (lizards, crocodiles, snakes, turtles, etc.)
- Montotremata (duck-bill, etc.)
- Marsupialia (opossum, kangaroo, etc.)
- Unguiculate series
- Insectivora (moles, etc.)
- Cheiroptera (bats)
- Rodentra (rats, rabbits, guinea-pigs, etc.)
- Carnivora (cats, dogs, etc.)
- Primata (lemurs, monkeys, apes, man)
- Unguiculate series
- Ungulate series
- Ungulata (horse, sheep, pigs, etc.)
The third class comprises the majority of well-known mammals, and may be termed the Placentalia, and, for embryologieal purposes, it is convenient to consider the Placentalia as forming two principal subclasses, the animals with claws and the animals witli hoofs, the Unguiculates and the Ungulates. Of the Unguiculates, we shall have occasion to refer to the Insectivora, of which the mole may serve as a type; the Cheiroptera, or bats; the Rodents, including the rats, guinea-pigs, rabbits, etc.; the Camivora, cats, dogs, and allied animals; and, finally, the Primates, which include the lemurs, monkeys, apes, and man.
Of the Ungulates, we shall have occasion to refer chiefly to the pig and the sheep. The following table presents these animals which we shall have occasion to consider in their proper order.
Of the invertebrate animals there will be little to be said. There are two types of invertebrates which show relationship in their structure to true vertebrates. One of these is the class of jointed worms, or Annelids; the other is the class of Atriozoa, which comprises the subdivisions of Tunicata and of the Cephalochorda.- All of our observations on the development of this last type are based on the one genus, Amphioxus, which will therefore be the name which we shall use whenever we have to refer to these animals.
Definition of Anlage
There will be frequent occasion to use this word in a strictly technical sense it has been adopted from the German, as there is no satisfactory English equivalent for it. The French use the word "ebauche," and the Italians " abozzo." Attempts have been made to introduce Greek derivatives or other terms, but they have not met with success, so that "Anlage" is now used very widely both in America and in England. It may be defined as follows: The first accumulation of cells in the developing embryo recognizable as the commencement of a structure, organ, or part.
A Summary of Embryological Development
The following summary applies to what is known of vertebrates only. It would require some modifications to be applicable to the whole animal kingdom. Each individual arises from a single cell which is termed the impregnated or fertilized ovum. From this all embryological study starts. The fertilized ovum has its earlier history, since it is the product of the fusion of two sexual elements. It is a living cell, and therefore contains protoplasm and nucleus. It is also furnished with a certain amount of material known as yolk, which exists in the form of separate granules imbedded in the protoplasm. This yolk is the reserve food material, and by the assimilation thereof the protoplasm of the ovum can grow.
The first step in development is the repeated division of the original cell so that there is produced an increasing number of cells. The earlier stages of this cell multiplication are designated as the segmentation of the ovum. This name is due to the fact that the process was first observed in the eggs of amphibia in the early part of the last century, before the cell doctrine had been established. In default of a better name, the separate cells into which the ovum divided were called segments, for it was, of course, not known that they were cells. Although this term is no longer appropriate, it is still universally used because of its convenience. There are two principal types of ovum known: in one type we find only a small amount of yolk material; in the other a very large amount. There are ova known intermediate between these two types. When the ovum is of the first type, the whole of it undergoes segmentation at once, and to such an ovum the term holoblastic is applied. In the second type, on the contrary, we find that the protoplasm tends to accumulate at one pole of the cell and the yolk granules at the other. The protoplasmic portion exhibits a far more active cell division than the yolk-bearing portion, so that the segmentation seems to take place exclusively around one pole or part of the ovum, which is, therefore, designated as meroblastic. After the segmentation of the ovum the multiplication of the cells continues, and they gradually arrange themselves in such a manner as to form three distinct sheets or laminae, which are named "germ-layers." These layers are designated: the outermost as Ectoderm, the innermost as Entoderm, and the middle as Mesoderm From an embryological point of view the importance of these three primitive germ-layers cannot be over-emphasized. The principal occupation of the student will be to familiarize himself with the appearance of these layers and the modifications which they undergo, and the adult tissues which are produced from them. They dominate every phase of development, the form of every organ, the production of every tissue. Their importance is so great that embryology might almost be defined as the science of germ-layers.
The primitive germ-layers consist of very simple cells, and are themselves at first extremely simple in their organization. The majority of the cells which they contain undergo a greater or less degree of modification as development progresses. This modification is termed differentiation, and is more fully considered in our next section, on Cytomorphosis. It is probable, however, that a certain number of the cells very early in the development are set apart, preserving the primitive character of their protoplasm and taking no share in the formation of the tissues of the body. These cells, comparatively unmodified, are known as the qerm-cclls. Their significance is more fully explained in the section on Heredity. As the remaining cells form part of the body of the individual, they may be designated as somatic cells. Besides the process of differentiation of the cells, we find that the production of organs is largely dependent upon the unequal growth of the germ-layers, one part growing rapidly, another more slowly, so that the layers acquire, as the embryo develops, a more or less complicated form, owing to the folding of the layers. The general principles which govern these important developments are considered in the section upon the Relations of Surface to Mass.
This term is used to designate comprehensively all the structural modifications which cells or successive generations of cells may undergo, from the earliest undifferentiated stage to their final destruction. It will be convenient, though somewhat arbitrary, to distinguish four fundamental successive stages of cytomorphosis. These stages are (i) the undifferentiated stage; (2) the stage of progressive differentiation, which itself often comprises many successive stages; (3) the regressive stage or that during which degeneration or necrobiosis occurs; (4) the stage of the removal of^the dead material.
In the various parts of the body we find these stages to succeed one another at varying rates, and there are always to be found in every living body a considerable number of cells which have only passed through a certain differentiation and do not present any of the phenomena of degeneration or of death. On the other hand, there are cells at every epoch of life after the embryonic period which degenerate and die off, although the life of the individual is uninterrupted. At any given moment the body consists of cells, which have made unequal progress through the cytomorphic cycle.
1. The Undifferentiated Stage. — A fertilized ovum is an undifferentiated being, although it has a very complex organization. As it has only one nucleus, there can be no variety of nuclei. The term "undifferentiated " therefore applies especially to the protoplasm which seems to have a uniform essential structure throughout, although the masses and strands of protoplasm may exhibit characteristic peculiarities, especially in relation to the distribution of the yolk. In the adult tissues, on the contrary, we see that the protoplasm of the cells of different kinds offers many varieties of structure visible with the microscope. We may legitimately conclude that the absence of similar visible peculiarities in the egg, by which one part may be distinguished from another, is evidence of uniformity of structure throughout the egg. We have also direct experimental proof that the egg is uniform throughout, or, to use a better phrase, that the egg is isotropic. Pflviger, in 1884, proved that the side of the frog's egg which normally develops into the ventral surface of the embryo can be made to develop into a perfectly typical dorsal surface. The frog's egg has a small white area, which normally lies underneath, the larger, darkly pigmented area of the egg alone showing from above. Out of the dark area the back, with the nervous system and other parts, takes its origin. If the eggs, freshly fertilized, are fastened with the white side up, then the white side produces an absolutely normal back and nervous system, normal as to form and function, though lacking the typical pigmentation. These observations were confirmed by Born, who further discovered that the segmentation nucleus always rises toward the upper side of the egg, and that the position of the nucleus determines which part of the ovum shall become the dorsal side of the embryo. Another set of experiments by Oskar Schultze demonstrated that both the unpigmented and the pigmented sides of the same egg could be made to produce dorsal structures. Another class of experiments, which were first made by Hans Driesch, have revealed that the earliest cells (segmentation spheres, blastomeres, or cleavage cells, as they are variously called) produced by the ovum preserve the undifferentiated qualities of the parent egg, and may develop in one way or another according to circumstances. The egg of a sea-urchin divides into two cells, each of which multiplies and normally gives rise to half of the body of the animal. By somewhat violent shaking the two cells may be artificially separated; each cell may then develop into a complete larval sea-urchin, but of half the normal size only. Similar experiments have since been made by several investigators, who have obtained like results with other animals, vertebrate as well as invertebrate. Even more remarkable larvse have been raised from blastomeres of the four-cell and eight-cell stages of segmentation, producing larvae of one-fourth and one-eighth the normal size. Zoja claims to have repeated the experiment successfully on the eggs of Clytia, and to have obtained one-sixteenth larvae.
The facts offered suffice to illustrate the two aspects of our conception of the undifferentiated condition of living matter. The first aspect is morphological and presents to us the apparent uniformity of the visible minute structure of protoplasm. While we readily admit that the uniformity may be only apparent in the sense that we fail to observe fine differences, yet we none the less maintain that the uniformity is real, because there is an absence of variations of structure comparable to the variations which we can observe in the cells of adult tissues. The second aspect is physiological, and offers to our view the wide range of possibilities in the future developmental history and growth of the protoplasm. The fate of the protoplasm of any given part of the ovum is not fixed ; but if its conditions of development are changed, its fate is changed. A few years ago the mosaic hypothesis was advanced by W. Roux, and it has been vigorously defended by him. According to the mosaic theory, the egg is a mosaic pattern, each member of which has its predestined history. It is fortunate for our comprehension of embryological processes that we are already able to say that Roux's hypothesis is erroneous.
We must start, then, with the right conception of the ovum, every part of the protoplasm of which is to be regarded as potentially capable of producing any or all of the tissues of the adult.
2. Differentiation. — This may be defined as a process by which the structure of the cells is modified, so that cells become dissimilar in structure by acquiring an organization which adapts them to special functions. The cells which arise during the segmentation of the ovum differ but slightly from one another. As development progresses we find the cells change, some in one way, some in another, so that many kinds of cells are produced, but of each kind we find a large number of cells. Each kind of cell may be said, roughly speaking, to form a tissue for itself. Cells of each tissue offer visible peculiarities by which they may be readily distinguished from one another under the microscope. It thus appears that the production of tissues is the main result of differentiation, so that this process of development may be fairly accurately defined as equivalent to histogenesis. As to the factors which cause differentiation, we have no satisfactory knowledge. We can, at present, only note the changes, when they acquire such magnitude as to become microscopically visible. As to the physiological conditions which cause these changes we have almost no conceptions. It' is probable that the nucleus has a leading role to play, but our knowledge of this role is too little advanced to permit a profitable discussion of the subjecthere.
The actual process of differentiation shows itself both in the protoplasm and in the nucleus of the cell. The changes in the latter are the more conspicuous, and therefore the better known. The changes in the nucleus have still to be adequately studied. The changes in the protoplasm are twofold: First, in the intimate structure of the protoplasm itself and in the size and disposition of its strands and filaments ; secondly, in the character of the various substances to be found imbedded in the protoplasm. These two kinds of change are well illustrated, the first, by the nerve-cells; the second, by the gland cells, for instance, in the pancreas. The student can easily see that the character of the protoplasm in the adult nerve-cell differs profoundly from that of a cell from one of the embryonic germ-layers, and that the body of the nerve-cell consists of protoplasm with little, if any, of other substances imbedded in it. In the secretory cells of the pancreas the zymogen granules are conspicuous; their distribution, uniform size, and refractile qualities demonstrate immediately their unlikeness to anything found in the embryonic cells. These granules are not protoplasm, but particles imbedded in the protoplasm or, as they may be called, enclosures.
The Law of Genetic Restriction. — Another fundamental idea, which it is most important for the student to grasp, is that differentiation acts as a progressive restriction upon the further development. Each successive stage of differentiation puts a narrower limitation upon the possibility of further advance.
The range of possible changes at any given time is determined not merely by the nature or kind, but also by the stage or degree of the previous differentiation. The law of genetic restriction dominates the entire ontogeny. In order to illustrate it and to emphasize it, it will be profitable to consider a few illustrations from each of the germ-layers. First, then, the ectoderm. This layer early separates into two parts, one to form the nervous system, the second the epidermis; the nervous part thereafter never forms epidermal structures, the epidermal part never forms a medullary canal. The central nervous system retains in part a simple epithelial character (ependyma proper), but most of its walls become nervous tissue; its cells pass from the indifferent stage and become neuroglia cells or young nerve-cells (neuroblasts). Neuroglia cells never become anything else, and the nerve-cells are always nerve-cells to the end. Next, as to the entoderm. Wherever in it specialization takes place, as in the tonsil, thymus, thyroid, oesophagus, liver, or pancreas, each territory of cells keeps its characteristics and never assumes those of another territory. Finally, as to the mesoderm. It is found very early to include in vertebrate embryos four kinds of cells, of which the most numerous are the undifferentiated cells, the other three kinds being the endothelium of blood-vessels, red blood-cells, and germ-cells. All of these are precociously specialized; they are few in number, yet they are probably the parents of all the cells which are produced of their kind each throughout life. Passing on to a later stage, we note that when a striated muscle-fiber is produced a striated muscle-fiber it always remains, and it never becomes anything else.
Two Types of Differentiation. — There are two distinct types of cell differentiation which I think have not hitherto been clearly recognized or defined. For both types the starting-point is the same — the undifferentiated embryonic cell. In one type we find that, as the cells proliferate, a portion of them only undergoes differentiation, and another portion remains more or less undifferentiated and retains more or less fully the power of continued proliferation. The epidermis is a good representative of this type. Its basal layer consists of embryonic cells, which multiply; some of these cells move into the upper layers, enlarge, and differentiate themselves into horny cells; others remain in the basal layer and continue to multiply. The progeny of a given basal epidermal cell do not all have the same fate, but divide themselves into two kinds of cells, one kind retaining the ancestral character, the other becoming something new and unlike the parent cell. Differentiation according to the second type is characterized by its inclusion of all the cells. This type has its culminating and most perfect illustration in the central nervous system, where comparatively early in embryonic life all the cells become specialized, and with the acquisition of specialization they forfeit their power of multiplication — the neuroglia cells partly, the nerve-cells wholly.* The growth of the brain after early stages depends not on the proliferation of cells, but chiefly upon the increase in size of the individual cell. The correctness of this statement is not affected, in my belief, by the fact that epithelial portions of the medullary tube in comparatively late stages may be added to the nervous portion, the cells multiplying rapidly, as we see at the growing edge of the young cerebellum. The brain here grows by the addition of cells in the indifferent stage, but as soon as these cells are differentiated they conform to the general law and divide no more (neurones) or slowly (glia cells).
The importance to pathologists of a thorough knowledge of the genesis of ' the tissues from their germ-layers can hardly be emphasized too strongly, for it is more than probable that all pathological tissues are as strictly governed by the law of genetic restriction as are the normal tissues.
3. Regression. — The use of this term does not imply that a cell can move backward after differentiation into a stage of lower differentiation or into an undifferentiated condition. So far as we know at present, such a change does not occur, and we therefore look upon it as impossible. Regressive changes are very unlike the constructive changes which appear in differentiation, for they are destructive. They fall into three main groups : first, changes of direct cell death; second, necrobiosis or indirect cell death preceded by changes in cell structure; third, hypertrophic degeneration or indirect cell death preceded by growth and structural change of the cell, often with nuclear proliferation. Direct cell death implies that the cell loses its vitality, and, being dead, disintegrates; or, may be, is removed by some means, chemical or phagocytic, before disintegration occurs. Necrobiosis and hypertrophic degeneration are normal processes, which invariably occur in the normal body and play an important role in its development. Without their occurrence on a large scale the normal round of human life would be impossible. The student should free himself from the unfortunate tradition that these processes are exclusively pathological.
- With possibly very rare exceptions.
Correct notions on this subject are so important that a few illustrations may be mentioned. Let us begin with necrobiosis. There are organs whose existence is limited in time, such as the thymus and foetal kidney. These organs attain their full differentiation, their elements during the next stage die off and finally are resorbed, most of the organ disappearing. Another familiar illustration is offered by the notoehord, which in mammals totally disappears. Cell death on a large scale is a common phenomenon of the tissues. It occurs in the cartilage both when the cartilage is permanent and, even more conspicuously, when the cartilage gives way to bone, the disintegration of the cartilage cells preceding the irruption of the bone-forming tissues. It occurs among the gland cells of the intestine, in the pregnant uterus, and in all the tissues of human decidua reflexa. Degeneration in the stricter sense of the ante-mortem and hvpertrophic change of cell structure is also of wide-spread occurrence in the healthy bodv. Perhaps no instance of this is more familiar than the production of horny tissue in the epidermis or elsewhere. That fatty degeneration takes place normally has long been taught, while mucoid and colloid degeneration are so obviously normal that we commonly think of their pathological occurrence as merely an exaggeration of a normal state. Hypertrophic degeneration is an extremely common pathological process, but it also occurs as a normal process, as, for example, in epidermal cornification, as just mentioned, and very strikingly in the production of giant-cells (myeloplaxes, etc.), and on an astounding scale in the uterine tissues during pregnancy in many, perhaps all, mammals.
4. The Removal of Cells. — The sloughing off of cells is one of the most familiar phenomena, since it occurs incessantly over the epidermis and with hair--. Its part in menstruation and its colossal role in the after-birth are known to all, and every practitioner is accustomed to look for shed cells in urinary sediment. Large numbers of cells are lost by the intestinal epithelium. The destruction of blood-corpuscles is incessant, and we might greatly extend the list of these illustrations. Owing to the enormous loss of cells to which the body is subject, there is provision to make good this loss. This provision is called "regeneration," and has been dealt with in an enormous number of investigations. During embryonic life regeneration plays a comparatively insignificant part, and we shall not have to deal with it further.
Of the four stages of cytomorphosis, the second, or stage of differentiation, is that which will principally claim our attention. But we cannot fully understand the developmental processes unless we also have constantly in mind the normal defeneration and death of cells, even in the embryo.
Comparison of Larval and Embryonic Types of Development
We have seen in the preceding section that the first cells produced in development from the ovum are undifferentiated, and are capable of development in many and varied directions. The more they become specialized, the more their possibilities of further varied development are decreased. It is thus obvious that the greater the number of cells of the undifferentiated type that can be produced, the greater will be the number of elements which can be later differentiated. Hence, the more the period for the production of undifferentiated cells is prolonged and the commencement of differentiation postponed, the more complex may be the degree of organization ultimately attainable.
It is convenient to designate the undifferentiated cells as they asire from the segmentation of the ovum by the term "embryonic cells.' 1 The object of this section is to point out that the larval type of development is less favorable for the multiplication of embryonic cells than is the embryonic type; and, further, that the embryonic type becomes more and more marked as we ascend in the animal kingdom.
The Larval Type. — In the lower multicellular animals we encounter only larvae; sponges, jellyfish, starfish, and worms all pass through their early stages as larvae. Now, larvae are animal forms which have to obtain their own food and to protect themselves against enemies. They are, therefore, provided with a variety of organs, or, as we may say, with differentiated tissues which enable them to perform the various physiological functions which are necessarv for the maintenance of their existence. The differentiation of tissues comes in very early.
The Embryonic Type. — True embryos arise from eggs which contain a more or less considerable amount of yolk or nutritive material, the presence of which renders unnecessary any activity on the part of the embryo to obtain its foodsupply; and we find, moreover, that these embryos are protected bv hard shells and other devices from their enemies. Their only task is to pursue their own development. Under these circumstances it is possible for the embryos to continue for a long time the production of embryonic cells, and we observe that the beginning of the differentiation proper is correspondingly postponed. The transition from the larval to the embryonic type is very gradual. The volk appears in the lower animals in small quantities, increasing in some of the higher types and attaining its maximum in some of the highest. Since the embryo is dependent on the yolk, and since the yolk exists only in the higher forms in sufficient quantities, it follows that fully typical embryos can occur exclusively in the higher animal types.
In the mammalia the ovum contains a rather small quantitv of volk, yet the mammals are the highest animals and develop most perfectlv according to the embryonic type. This peculiarity is due to the fact that two special phvsiological devices have been evolved in the mammals to supplv food to the developing embryo. First, there is a special relation established between the embryo and the uterus by means of a complicated adjustment of embryonic and uterine tissues, which supplies nutrition to the embryo from the blood of the mother. Second, there are the mammary glands, which also serve the same function. By these two devices the embryo is even more completely freed from the necessity of seeking its food and protecting itself than is the case with those forms, such as the birds or elasmobranchs, in which the supply of food material is very large.
The germ-layers are the first groups of cells to arise as the result of the segmentation of the ovum. They are three in number, and each forms a distinct sheet or lamina. As stated on page 26, these three primitive layers are termed "ectoderm," "mesoderm," and "entoderm." The ectoderm is the most external of the three, and upon the outside of the body parts of the ectoderm remain permanently to constitute the outside skin or epidermis. From its very position it necessarily is the part of the body to come into relation with the external world, and accordingly we find that its two great duties are to produce the protective covering of the body and the apparatus for receiving and utilizing sensations; in other words, the chief sensory organs and the nervous system. The entoderm, on the contrary, forms the internal cavity of the digestive canal and its appendages. It therefore is concerned chiefly with the production of the organs of digestion, and appears in the adult as the epithelium of the digestive and respiratory organs and of the glands appended to the digestive tract. The mesoderm, lying as it does between the other two layers, is shut off by them from direct relation with the external world or with food-matter, and is accordingly restricted to a series of internal functions, of which four are especially important: the function of movement, of supporting the body, especially the parts produced from the ectoderm and entoderm, of circulation, either of blood or of lymph through definite channels, and, finally, of excretion. It is from the middle germ-layer, therefore, that the muscular tissues arise, that the connective and skeletal tissues arise, that the blood, blood-vessels, and lymphatics arise, and that the excretory organs arise.
The inner and outer germ-layers are primarily simple epithelial structures, consisting each of a single layer of cells. This primitive characteristic is never wholly obliterated and really controls all of the modifications which these two layers undergo. The mesoderm, on the other hand, is primarily not epithelial, but mesenchymal. Mesenchyme consists of widely separated cells which form a continuous network of protoplasm, the meshes of which are originally filled by a homogeneous intercellular substance or matrix. The student will have frequent occasion in his practical work to study it in its embryonic stages.
The Coelom. — The coelom is the primitive body-cavity of the embryo. It arises as a space in the mesoderm. As soon as this space has appeared we find that the cells of the mesoderm, which bound it , assume an epithelial character, consequently the mesoderm, after the coelom has appeared, consists of mesenchyma and of an epithelial layer bounding the coelom. This epithelial layer is called the mesothelium. The mesoderm, therefore, differs fundamentally from the ectoderm and entoderm by this peculiarity, that it comprises both an epithelial and a non-epithelial portion. Both portions play very important roles in the production of the various tissues and organs of the body. There is another respect also in which the mesoderm differs from the other germ-layers, for we find that it increases in volume and in complexity as we ascend from the lower to the higher types of animals, or as we pass from the embryo toward the adult condition, more than does either the outer or inner germ-layer.
Classification of the Tissues
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|(A) Ectodermal||(B) Mesodermal||(C) Entodermal.|
The Specific Quality of the Germ-layers. — Each germ-layer has its specific and exclusive function in the production of tissues, giving rise only to the tissues which are proper to it, and never to the tissues which are proper to either of the other layers. We must, indeed, so far as our present knowledge goes, regard the cells in the germ-layers as originally wholly indifferent as individual cells. But we must, nevertheless, not forget that as members of a germ-layer, their potential fate is already restricted. It is probable, if we could successfully transplant an undifferentiated cell from one germ-layer to another, that it could take part in the production of the tissues proper to that layer. But it is further probable that .this would be impossible after the differentiation of the cells in any layer had fairly begun. The accompanying table presents the principal tissues classified according to the layers to which they belong. Or, as we may say, according to their layership, a word which is proposed to indicate the membership of a given cell or tissue in the germ-layer to which it belongs. The layership of a cell is never changed after the differentiation of the three primitive layers has been accomplished. There have been classifications of organs on the layership basis published before, but inasmuch as organs usually contain cells from two layers, we get a more correct presentation of the actual genetic relationship by confining our tabulation to the tissues. Leucocytes do not appear in the table for the reason that their first origin is uncertain. Blood-vessels arise very early, before the clear separation of the mesoderm and entoderm has occurred. It is possible that they are entodermal. With these two limitations the table presents our present knowledge.
The Constitution of Organs. — The layership of most organs is not simple, for as we find organs in the vertebrate body they usually consist of two parts, one of which may be regarded as the part proper of the organ, upon which the performance of its special function directly depends, and the accessory part, which supplies the necessary physiological conditions for the functioning of the organ. For example : in a salivary gland the actual work of secretion is performed by the epithelial cells of the gland, but these cells cannot act unless they are supported by connective tissue and supplied with blood and lymph, three conditions which depend upon the mesoderm, and also supplied with nerves, a condition which depends upon the ectoderm. By far the majority of organs have their functional part produced from epithelium, and this epithelium may come either from the original outer or inner germ-layer, as the case may be, or from the mesothelial portion of the middle layer. But the organ, as a whole, requires for its completion the addition of other elements, as indicated in the example given. We find, therefore, that there are no adult organs which are constituted solely by either the ectoderm or entoderm, although there are organs, the principal part of which may come from one or the other of these germ-layers, but to complete the organ the mesoderm must help. On the other hand, the mesoderm may form complete organs by itself, or at least with no other aid from the other germ-layers than is given by the supplying of nerve-fibers. Such purely mesodermal organs are illustrated by the spleen, the kidney, and the sexual glands.
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 geometrical 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 square, 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 bound twenty-seven units of mass, the proportion in the first cube is i : i, in the second 1:3. To maintain the proper proportion in the embryo, simple enlargement is insufficient, therefore the surface increases by becoming more and more irregular. The irregularities are characteristic of each organ and part, and may be either large or microscopic. They may be conveniently grouped under two main heads — projections and invaginations.
Projections are illustrated by the limbs, filaments of the gills in fishes, the villi of the intestine, folds of the stomach in ruminants, etc. In every case the projection is covered by an epithelium and has a core of mesodermic tissue.
Invaginations exist in much more varied form and play a principal part in the differentiation of the animal body. They may be classified under four principal heads: (1) Dilatations; (2) diverticula; (3) glands; (4) vesicles. Dilatations have considerable importance in embryology; the stomach, lungs, bladder, and uterus arise as gradual dilatations of canals or tubes of originally nearly uniform diameters. Diverticula, in the sense of relatively large blind pouches, also form important organs, such as the caecum and appendix vermiformis, or the gall-bladder; these structures arise, each as a blind outgrowth of a canal, the walls of which at a certain point rapidly grow to form the pouch. Glands are, as first shown by Johannes Midler's classic researches, only small diverticula, which end blindly and appear in an immense variety of modifications ; the manifold types of glands are discussed below in a separate paragraph ; they constitute the largest class of organs with which we have to deal. The glands are developed from epithelium and push their way into the mesoderm upon which the epithelium rests, while in dilatations, and in diverticula, the epithelium and 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 cavity. The closed sac separates from the epithelium from which it arose, and connective tissue grows between the two ; the sac may then undergo various modifications. The membranous 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 (cf. Chap. VIII) if we choose to consider it as a vesicle so much lengthened that it has become a tube.
The Law of Unequal Growth
The changing shapes of the embryo and the development of the irregularities — projections and invaginations — which preserve the proper proportion between the surface and the 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 : (i) 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 multiplication of the cells in all embryonic epithelia is the fundamental factor of development, and we see it shaping the embryo, its organs, and the parts of organs, before histological differentiation really begins. The distinct areas and centers of growth which are necessary to develop the human body out of the germ-layers are innumerable, and their distribution, limitations, and interactions make up a large part of the subject-matter of embryology. At every turn of our studies we encounter fresh illustrations. If in a limited area of a cellular membrane there occurs a growth of 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; thus when, at a certain point of the surface of the embryo, a steady and long-continued growth occurs, the limb appears, gradually lengthening 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 of the skin and for every gland of the intestine there is a separate center of growth.
The reason for the unequal growth is unknown. We have not even an hypothesis to offer as to why one group of cells multiplies or expands faster than another group of apparently similar cells close by in the same germ-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.
- By this limitation we exclude the mesenchyma, but not the mesothelium.
The conception that the development of an animal depends fundamentally upon the unequal expansion and consequent foldings and bendings of the germlavers was first suggested by the researches of C. F. Wolff on the development of the intestine, and was more clearly recognized by Pander, who definitely asserted that the formation of the embryo is effected by foldings of the germ-layers, and the truth of Pander's view was conclusively demonstrated by C. E. von Baer in 1828. In recent times His has studied the problem very intently, and in his memoir on the chick discussed it minutely. In this memoir is to be found most of what little we know of this aspect of embryological mechanics.
Recent investigations have made it probable that a few cells are set apart during the period of segmentation to form the germ-cells. Their number is small ; they preserve for some time the appearance of segmentation spheres, as the cells which are formed during the segmentation of the ovum are sometimes called. They multiply very slowly during the earliest stages of development. A great majoritv of the cells produced during segmentation lose the character of segmentation spheres, and divide rapidly and repeatedly. The cells belonging to the class of this majority form the various tissues of the body. The germcells, on the contrary, seem to multiply very slowly and never to become very numerous in the embryo. As they multiply they separate from one another and become more or less completely surrounded by tissue cells. They pursue their development, one is tempted to say, independently of tissue formation and somewhat like foreign members of the body. We put, accordingly, the germ-cells in a class by themselves in contrast to the body or somatic cells.
Our actual knowledge of the history of the germ-cells is very incomplete. The statements just made about them are based on observations on very few animals. Their exact origin has been traced only in three species of vertebrates, all fishes, the teleosts Cymatogaster and Micrometrus, and the elasmobranch Squalus acanthias. In these three forms the germ-cells arise during segmentation, remain more or less closely together, or segregated, during the earliest stages. They then separate from one another and gradually migrate into the epithelium, which covers the anlage of the genital gland, which thus becomes the so-called "germinal epithelium. The existence of the germinal epithelium has long been known, and its characteristics have been described in all recent text-books of embryology. The germ-cells in the germinal epithelium are commonly known by the name of the primitive ova. The transformation of these cells into true ova has been traced in a great many forms, so that the transformation may be considered as demonstrated conclusively for all vertebrate animals. It is further commonly assumed that the germ-cells or primitive ova also give rise to the male elements, playing in the formation of the testes a role similar to that which they play in the ovary. There is, unfortunately, up to the present time, no conclusive proof by direct observation that the primitive ova are the actual parents of the cells which give rise to the spermatozoa.
When a germ-cell is transformed into an ovum, it undergoes great enlargement, its nucleus is modified, the protoplasm is changed in appearance and becomes loaded with yolk granules, and over the surface of the cell appear two membranes, an inner very thin one, called the vitelline membrane, and an outer much thicker one, known as the zona pellucida. (For a fuller description see page 33.) We thus learn that the germ-cells preserve their resemblance to segmentation spheres only during embryonic life. When they become ova, they pass through a series of important changes in their organization. If it is true that these germ-cells also give rise to the male elements, then we must further say that in order to produce those elements the germ-cells pass through another series of profound changes.
It is further known that in order to evolve the sexual elements, both male and female, the cell which is to produce them divides twice, and in a special manner, which we designate by the term "reduction division." This process is described in all the recent text-books of cytology and histology. It does not fall within the scope of this work, which deals with embryology in the strict sense only.
The Theory of Heredity
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We owe to Moritz Nussbaum the theory of germinal continuity— the only theory of heredity which seems tenable at the present time. According to this theory, the germ-cells are set aside during the segmentation of the ovum and preserve the essentially undifferentiated qualities of the protoplasm and nucleus of the ovum, from the division of which they arise. Just as the cells formed during segmentation are capable of producing the various tissues of the body, so the germ-cells have and preserve this faculty. If we term the material of the original ovum germ-plasm, we may say that this germ-plasm gives rise to the various tissue-forming cells which make up the body. And by this very conversion into tissue cells, that germ-plasm is changed, and is no longer, as we- have learned before, capable of the full range of development. The germ-cells, on the contrary, do remain so capable, and it is precisely in order to preserve this capacity that they hold aloof from the formation of the bodyt.issues and pursue their own independent career. A portion of the germ-plasm of the parent ovum is, so to speak, short-circuited into the genital elements which produce the offspring.
If we accept this view, we are forced to make the supplementary hypothesis that the conspicuous complicated changes, by which the germ-cells are converted into sexual elements, do not involve the differentiation in the true sense — i. e., strictly comparable to that which we observe in the somatic cells. Although this hypothesis seems a logical necessity of the theory of germinal continuity, we cannot at present verify it by any observed facts. The only other theory of heredity which has ever been seriously considered is that of pangenesis, which was formulated by Darwin, whose words I quote: " But besides this means of increase I assume that cells, before their conversion into completely passive or 'form-material,' throw off minute granules or atoms, which circulate freely throughout the system, and when supplied with proper nutriment multiply by self-division, subsequently becoming developed into cells, like those from which they were derived. These granules, for the sake of distinctness, may be called ce'1-gemmules, or, as the cellular theory is not fully established, simply gemmules. They are supposed to be transmitted from the parents to the offspring, and are generally developed in the generation which immediately succeeds, but are often transmitted in a dormant state during many generations, and are then developed." Many modifications of this theory have been proposed by speculative writers, and many different names have been bestowed upon the gemmules of Darwin according to the fancy of each author and the particular set of qualities which he attributed to these imaginary particles. Such views attained their culmination in Weismann's complicated useless hypotheses. All of these speculations have only an historical interest, having proved themselves, from a scientific standpoint, to be absolutely barren.
The Law of Recapitulation
This law as commonly formulated, is that the development of the individual recapitulates the development of the race, or, in other words, the ontogeny recapitulates the phytogeny. This way of stating the law is in so far objectionable that it presents the theoretical interpretation of the law rather than the actual generalization of the facts. The essential datum upon which the law is based is that the embryo of a given animal has striking morphological resemblances to the adult forms of lower allied types. Since the theory of evolution was established by Darwin this resemblance has been interpreted as due to the inheritance of ancestral characters appearing in the embryo. The embryo is looked upon as the representative of the actual ancestor by modification of which the adult form was evolved. It is further assumed that the change of the embryo into the adult type follows the same general course as the development of the remote ancestor into the particular species under consideration. Speaking broadly, this interpretation is undoubtedly justifiable. If it were exactly true, it would be necessary only to know the embryology of an animal in order to establish the evolution of its species. Experience, however, very quickly demonstrates that this procedure is by no means possible, because the embryo is not a correct or adequate record of the ancestral type. It is inadequate chiefly for three reasons: First, because the embryo has necessities of its own, and in the course of evolution embryos acquire special peculiarities by which they become adapted to the conditions of their life. Such changes in organization do not correspond to, but on the contrary diverge from, the inherited ancestral traits, and in so far as they are present they mask or alter those structural features of the embryo which represent the ancestral record. Second, because the embryos consist of undifferentiated cells (q.v.). Now, the adult ancestors representing lower types of organization of course had differentiated tissues, which enabled them to perform the functions of adult life. One of the first things which will impress itself upon the student of vertebrate embryology is that though he may find at the proper stage in the embryo the organs of the body clearly developed, yet, owing to the fact that they consist of relatively undifferentiated cells, they are incapable, in large part, of performing the functions which they are ultimately to assume, and the performance of which is the very object of their development. This change in histological structure brings about a marked unlikeness of the embryo to the assumed ancestral type. Third, the embryo at each stage of its development must be regarded as the mechanical cause of the next and of all following stages. It must necessarily, therefore, have in itself peculiarities by which it is distinguished from all other embryos. It is impossible, accordingly, that all embryos should be alike. It is only necessary for the student to compare embryos of various vertebrates one with another to satisfy himself that they have conspicuous distinctive characteristics. When our knowledge shall have grown sufficiently, we shall be able to classify vertebrates by their embryos as perfectly, or perhaps even more perfectly, than we can by the consideration of the adult forms. Every embryo is modified from the very start away from the assumed ancestral organization, in order that its peculiarities may cause it mechanically to produce the new form which has been evolved.
In some of the invertebrate animals — as, for instance, among the hydroids and jellyfishes — the law of recapitulation can be much more easily verified than in the higher forms which have purely embryonic types of development. From what has been said, it will be recognized that the likeness of the embryo to the adult lower form is a general morphological resemblance only, not an exact one, and that therefore it is extremely difficult to infer from the embryonic organization what the ancestral type was. Hitherto all phylogenetic inferences drawn by embryologists have been largely speculative in character, and, it may be added, have been more remarkable for their number and variety than for their value.
The resemblance between embryos and lower adult forms has been known for a century past. It was first adequately asserted in 1811 byj. F. Meckel, and since then has been constantly discussed. More, perhaps, was done to emphasize it by Louis Agassiz than by any one else. Von Baer, the creator of modern scientific embryology, called attention in 1828 to the limitations which must necessarily be put upon Meckel's generalization. It is to be regretted that von Baer's wise thought on this subject has not been more appreciated. He put forth four generalizations : First, that which is common to a large group of animals develops in the embryo earlier than that which is special; second, from the most generalized stage structures less generalized are developed, and so on until finally the most special appears; third, the embryo of a given animal form, instead of passing through the other given forms, separates itself from them more and more; fourth, therefore, essentially the embryo of the higher forms is never like a lower form, but only like its embryo. The first to point out the possible phylogenetic significance of these facts with perfect clearness was Fritz Miiller, in a little book entitled "Fur Darwin," published in 1864. Ernst Haeckel took up this interpretation and secured wider attention for it. He termed the law of recapitulation the " biogenetic law."* The student will encounter in his practical study many illustrations of the resemblances which we have been discussing, so that it is unnecessary here to do more than mention a few for the purpose of illustration. In the embryos of birds and mammals the pharynx forms a series of lateral pouches which we know as the gill pouches, and which develop in the same way as, resemble strikingly, and are homologous with, the gill pouches of fishes, which in the fishes give rise to the so-called gill clefts. The heart of a young mammalian embryo is a simple tube with only a single continuous cavity resembling the heart of the lower fishes. The embryonic kidney or Wolffian body of man resembles, and is homologous with, the kidney of the frog, but it disappears almost completely before adult life. These few examples may suffice.
- " Biogenetisches Grundgesetz. "
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|A Laboratory Text-Book of Embryology: 1. General Conceptions | 2. Early Development of Mammals | 3. Human Embryo | 4. Pig Embryos | 5. Chick Embryos | 6. Blastodermic Vesicle and Ovum Segmentation | 7. Uterus and the Foetal Appendages in Man | 8. Methods | Figures | Second edition | Category:Charles Minot|
Cite this page: Hill, M.A. (2021, June 25) Embryology Book - A Laboratory Text-Book of Embryology 1 (1903). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_A_Laboratory_Text-Book_of_Embryology_1_(1903)
- © Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
- Some English and occasionally Continental authors use other terms for the germ-layers: namely, for ectoderm, eniblast ; for entoderm, hypoblast ; for mesoderm, mesoblast. I have preferred to maintain the older terms which have been in almost universal use for a century.