Embryology

“ The science of embryology, then, is the science of individual development, and includes within its purview all those changes of form and structure, whether embryonic, larval or post-larval, which characterize the life of the individual. The beginning of this period is precise and definite—it is the completion of the fertilization of the ovum, in which the life of the individual has its start. ”

“ Having in this brief preliminary consideration of the fundamental facts upon which the science of Embryology is based cleared the ground as far as possible, we may now summarise, in a few simple statements, the point at which we have arrived in order that we may proceed at once to the more detailed study of the actual development of the embryo itself. ”

“ And it is to be recognized without question that discussions regarding heredity, regeneration, the nature of the developmental process, the question of inherited organization within the egg, of germinal continuity, etc., have done much to advance the subject of embryology.
Embryology is one of the three great departments of biology which, taken in combination, supply us with a knowledge of living forms along lines of structure, function, and development. The embryological method of study is of increasing importance to comparative anatomy and physiology. ”

“ Or what pure anatomist could ever have discovered that the spermiduct is the modified duct of a kidney present in the embryo, but which in the adult has as such totally vanished? If we pass from mere human anatomy to the larger and more scientific subject of comparative anatomy, we feel again the value of embryology, which establishes the real homologies of structure, proving exact homologies from the study of the early stages of parts, which in different types become so unlike that their fundamental identity of origin is completely hidden. ”

“ If, then, embryology is to be regarded as including not only the study of embryos, but also that of larvae, i.e. if it includes the study of the whole developmental history of the individual—and it is impossible to treat the subject rationally unless it is so regarded—it becomes exceeding difficult to fix any definite limit to the period of life with which embryology concerns itself. ”

“ The study of embryology has shown us that many of the deviations from the normal development of the human body are easily explained by embryology. One of the most important changes in our idea, for example, of tumors is due to the fact that the study of embryology and of the tissues of the embryo have shown us that diseased structures, which lack explanation entirely, when compared with the adult human tissues, readily find their explanation and fall into an unexpected order when compared with the tissues of the embryo. ”

“ Embryology has been compared by one of the greatest of modern embryologists to "an ancient manuscript with many of the sheets lost, others displaced, and with spurious passages interpolated by a later hand." ”

“ In its strict sense, therefore, embryology is the study of the intrauterine young or embryo, and can only be pursued in those animals in which the offspring are retained in the uterus of the mother until they have acquired, or nearly acquired, the form of the parent. ”

“ How, then, can we explain these several facts in embryology,— namely, the very general, though not universal, difference in structure between the embryo and the adult;— the various parts in the same individual embryo, which ultimately become very unlike, and serve for diverse purposes, being at an early period of growth alike;— the common, but not invariable, resemblance between the embryos or larvæ of the most distinct species in the same class; the embryo often retaining, while within the egg or womb, structures which are of no service to it, either at that or at a later period of life ”

“ In embryology the different changes of the embryo are minutely described, classified into types, into a certain number of definite stages, and then all the changes, in the infinite wealth of their variety, are expressed in the general proposition that the embryo in the short period of its development traverses in an abbreviated form all the stages that the species has passed through in the many ages of its existence ”

“ THE OVUM AND THE AMOEBA. In order to understand clearly the course of human embryology, we must select the more important of its wonderful and manifold processes for fuller explanation, and then proceed from these to the innumerable features of less importance. The most important feature in this sense, and the best starting-point for ontogenetic study, is the fact that man is developed from an ovum, and that this ovum is a simple cell. ”

“ As many are interested in the problems who have little acquaintance with the technical facts of embryology, a simple account of the early stages in the development of an animal may be useful for reference. I shall choose back-boned animals, as, from the inclusion of man among them, they are of more general interest. The process begins with the fertilisation of the egg-cell by the fusion with its nucleus of the nucleus or head of a male-cell or spermatozoon. ”

“ Embryology gives a vast amount of additional independent testimony. For like all embryo mammals and birds and reptiles, the human embryo possesses gill-slits, and fish-like heart and brain. Above all it begins life as a single cell. Zoology asks: What can these things mean, if they do not mean evolution and a common ancestry with other forms? ”

“ While it has a strictly technical side, the science of embryology must always remain of interest to intelligent people as embracing one of the most wonderful processes in nature—the development of a complex organism from the single-celled condition, with a panoramic representation of all the intermediate stages. ”

“ Each and every man today has passed through all the forms of the ascent within a few months, from single cell to the new-born, fully formed infant.
Embryology teaches us that the eggs from which all animal forms evolve are all practically alike so far as one can ascertain by microscopic examination, no matter how diverse may be the forms which will evolve from them, and this resemblance is maintained even when the embryo of the higher forms begins to manifest traces of its future form. ”

“ The domain of embryology ceases in the case of viviparous animals at birth, in the case of oviparous animals at hatching; it ceases as soon as the young form acquires the power of existing when separated from the parent, or when removed from the protection of the egg membranes. ”

“ The importance of thoroughly realising these simple facts of embryology should at this stage be obvious. They constitute possibly the most important lesson which is demanding attention at the hands of modern teachers, parents, and sociologists. ”

“ Such a view of the processes of embryology brings embryology into harmony with comparative anatomy and phylogeny, for it makes the central nervous system and not the alimentary canal the most important factor in the development of the host. ”

“ Embryology emerged as a distinct scientific discipline during the last half of the sixteenth century and early years of the seventeenth century as a result of the aforementioned investigations of Aldrovandus, Coiter, and Fabricius. ”

“ Now, since the embryo follows the same developmental track as did the parent, passing through the successive stages of germ-cells, fertilised ovum, embryo, fœtus, infant, child, youth, and adult, it follows that should it exhibit any additional peculiarity, unnoted in the parent, the embryo has obviously varied progressively. ”

“ It is only by their study that we realise that the human embryo, which is to become the human individual, consists, to a very large extent, of characters and features which are unalterably settled for it beforehand, to which nothing can be added, and from which nothing can be taken away. In other words, the possibilities for any individual are those which are pre-existent in the germ-plasm from which he or she originates. These possibilities, however, depend upon the environment in which the embryo, infant, child, and adult is subsequently placed for their full expansion. ”

“ The millions of cells so produced include, as we have already seen, cells which have all the various functions which are necessary for the continuation of the life of a human individual; that is to say, that as the result of this division of the fertilised ovum there are produced first of all germ-cells to secure the still further continuance of the race, and then multitudes of all the other kinds of cells which gradually assume the shape of an embryo or young individual, and ultimately grow into a human being. ”

“ We may next turn our attention to the developing embryo at a very early stage, and note from which parts of its growing cells the different structures are ultimately developed, remembering all the while that all the subsequent division into specialised tissues is the result of the inherent possibilities in one single fertilised germ-cell. ”

“ He had also the merit of fixing the attention of physiologists upon general laws of development as deduced from actual observation of the phenomena, by the enunciation of two important propositions, viz.— (1) that all animals are produced out of ova, and (2) that the organs of the embryo arise by new formation, or epigenesis, and not by mere enlargement out of a pre-existing invisible condition (Exercitationes de generatione animalium, Amstelodami, 1651) . ”

“ All that can be done under the best conceivable [Pg 41] circumstances is so to arrange the environment and surroundings of the embryo, and the subsequent individual, so that these tendencies are acted upon in such a way that the best are developed, and the worst eliminated. It must be remembered that it is under the constant action of everything that constitutes the environment of an embryo that the mass of body-cells gradually grows into a recognisable human personality. ”

“ It is not necessary to describe in detail the beginnings of all the various structures which arise from that important layer of cells in the embryo which is termed the ectoderm; but since it gives rise to that part of the embryo, which eminently places man in the first place in the world of animals, we may select it for a little further description. We may leave out of account the beginnings of the skin and the glands, and some other parts, and look for the moment at the origin of the nervous system, which includes the brain, the spinal cord, and the whole nervous mechanism of the individual. ”

“ Each successive step from the very beginning of the fertilisation of the ovum repeats a stage through which previous generations have passed. If from any accident a step in this recapitulation is omitted, the embryo is to that extent deprived of some feature possessed by a parent or ancestor; and if this be a sufficiently important omission, it is impossible for such an embryo to survive. That is one way in which an embryo may differ from its parents. ”

“ The Transient embryonic organs. remarkable point is that while in most cases this embryonic growth is a direct and simple process—e.g. animal and plant buds, embryonic development of plant seeds—in many cases of sexual reproduction of animals it is not direct, and the embryonic phase shows stages of structure which seem to possess a meaning other than that of being merely phases of growth. ”

“ How do the different organs of the partly developed embryo stand with regard to their future fate? What are the stimuli (Reize) effecting differentiation? What is to be said about the specific character of the different formative effects? And as the most important question of all: Are all the problems offered to us in the physiology of development to be solved with the aid of the laws known hitherto in science, or do we want specifically new “vitalistic” factors?
Energy in different forms is required for development, and is provided by the surrounding medium. ”

“ This is notably the case, for example, with the reproductive organs which, though developed during the life of the embryo, remain functionless until the period of adolescence. The development of the human mind and intellect too, although depending, of course, upon the embryonic growth of the brain and the nervous system generally, is a matter of time and the environment subsequent to birth. ”

“ In all the highly complicated animals fertilisation by union of germ-cells from male and female must precede reproduction. The result eventually is this multicellular individual composed of a number of different kinds of cells each set apart for its own work. ”

“ If the experiment be made at a sufficiently early stage, each part, if not too small, will develop into a normal, though small, embryo. In some cases the embryo remained imperfect for a certain time after the experiment, but the loss is eventually made good by regeneration. ”

“ Of late years a great number of experiments have been made to discover the effects of dividing the embryo during its cleavage, and of destroying certain portions of it. These experiments have been made with the object of testing the view, held by some authorities, that certain segments Division of embryo. are already set apart in cleavage to give rise to certain adult organs, so that if they were destroyed the organs in question could not be developed. ”

“ In the period under consideration the output of embryological work has been enormous. No group of the animal kingdom has escaped exhaustive examination, and no effort has been spared to obtain the embryos of isolated and out of the way forms, the development of which might have a bearing upon important questions of phylogeny and classification. ”