Embryo

“ The term "the embryo" is employed to designate the developing young creature in the earlier stages of its development, particularly before the end of the third month of its existence. After the end of the third month the embryo is called "the fetus." In the short space of 280 days the young creature evolves and develops from a single simple cell into a complex organism—a perfect miniature human being. Nature works a wonderful miracle here, and yet so common is it that we take it all as a matter of course, and lose sight of the miracle. ”

“ This is the first attempt of the fertilised ovum to form itself into the different layers, which are ultimately going to give rise to all the different tissues of the embryo. But it is interesting to know at this stage that the outer layer of cells, those representing a margin in the figure, has nothing to do with the forming of the embryo at all, but gives rise to a structure whose function afterwards is to be that of nourishing the growing embryo. ”

“ 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. ”

“ 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. ”

“ CHAPTER III THE EMBRYO The Development of Form—The Determination of Sex—Twins—The Rate of Growth—The Newborn Infant—Heredity—Maternal Impressions.
The new human being begins existence, as I have shown, as soon as the ovum is fertilized, though at that moment it consists merely of a solitary cell formed by the union of the two parental cells. From a beginning relatively simple the human body develops into the most complex of living structures ”

“ No embryo ever takes or uses any organ or part of the body of its father, or mother, except the spermatozoön and the ovum, which form the germ-cell and serve as a germ to start the growth of the body. Each embryo is a new combination and a new arrangement of the atoms and cells of which it is composed. It is produced by new forces and motions peculiar to itself, which are similar to, but different from, those which produce every other embryo body. It is admitted on all sides, that every human being begins life as a fertilized ovum. ”

“ By the thirtieth day the tiny embryo is already one-third of an inch big, and were it practicable, which, of course, it is not, to remove it living from its bed of tissue in the mother’s womb and examine it, even with the naked eye, and still more with a magnifying glass, it would be possible to see the rudiments of the legs, head and arms which are to be. ”

“ The one outstanding discovery which has placed the science of Embryology on an absolutely firm basis, and which has made clear so many of the facts, which were previously puzzling, is this: that the germ-cells which give rise to new individuals are themselves produced from pre-existing germ-cells. The entire embryo, or young infant, is derived from one single cell which we have called the fertilised ovum, and that in its turn was derived from the union of two germ-cells, one from the male parent, and one from the female. ”

“ In Darwin's words: "Embryology rises greatly in interest, when we look at the embryo as a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class." ”

“ From this stage to the stage of complete development a satisfactory series of embryos has now been collected, but it is impossible to give here, even in outline, a description of the evolution of the human embryo. ”

“ This inner mass of cells forms the embryo.
It has proved a difficult task to secure very young human embryos, and many of the ideas we hold relative to the initial stages in the development of man are based upon what has been found true in certain mammals, the class of animals to which we belong. The youngest human ovum known at present has already undergone about two weeks' development, and there the embryo is represented by a flat disk. ”

“ In a more technical sense, however, the use of the term embryo is limited to the first six weeks of pregnancy and designates the condition of the young creature before it has acquired the form and the organs of the infant; after that time the unborn child is called a fetus. Embryonic development, therefore, in the strictest sense of the term, chiefly involves the shifting of various groups of cells and the bestowal upon them of different kinds of activity. ”

“ By the end of thirty days the little embryo, about as large as one inch of the end of a lead pencil, would be recognized as the embryo of some mammalian animal, but it would be quite impossible to say whether it would develop into a human being or some other animal, if it were seen quite apart from its immediate surroundings. ”

“ It is obvious that the materials of which the embryo body is built up, except the fertilized ovum, are derived from the food eaten by the mother; that her heart and arteries generate the forces and produce the motions which carry the materials to the building site of the embryo, just as the builder assembles the bricks, stones, sand, lime, lumber, nails and other materials to build a house. ”

“ 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. ”

“ They prove that while the reproductive cells multiply and arrange themselves during the evolution of the embryo, some of their germ-plasm passes into the mass of somatic cells constituting the parental body, and becomes a permanent component of it. ”

“ 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. ”

“ 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. ”

“ This germ cell under these conditions, within the female, and in connection with her body, develops very rapidly into the embryo. It is true that the relation of the embryo to the mother continues to be almost parasitic in its nature during its development up to the time of birth, as it also remains for a considerable time after birth. ”

“ The fertilized egg becomes an embryo or developing animal. Many cell divisions take place. In time some cells become muscle cells, others nerve cells or blood cells, and organs such as eyes, stomach, and intestine are formed.
'The body of a human being grows in the same way, but it is much more remarkable than that of any animal, for the embryo has a human soul infused into the body by God. Human parents are partners with God in creation. ”

“ The hypothesis explains also the way in which the orderly succession of stages in embryogeny is brought about, for we can readily understand that the embryo will not remember any stage until it has passed through the stage immediately preceding it. "Each step of normal development will lead the impregnated ovum up to, and remind it of, its next ordinary course of action, in the same way as we, when we recite a well-known passage, are led up to each successive sentence by the sentence which has immediately preceded it.... ”

“ The mother fish, insect and bird store up food in their eggs for the little embryos to live upon until they are hatched. Among the higher animals and man, the embryo begins with the tiny fertilized egg in the mother’s womb and receives nourishment and life from the mother’s blood through a duct, called the placenta, which is connected with the mother’s womb at one end while the other end connects with the body of the embryo at a point called the navel. ”

“ This embryo, as it is called, then passes into other conditions. I should tell you that there is a time when the embryos of neither dog, nor horse, nor porpoise, nor monkey, nor man, can be distinguished by any essential feature one from the other ”

“ 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. ”

“ The embryo or fetus in its development seems to recapitulate the various stages through which the species has passed. Thus the human embryo at one stage of its development resembles the fish; at another stage, the embryo of a dog; and for a long time it is impossible to distinguish between the human embryo and that of one of the larger apes. These embryological facts, biologists say, indicate genetic relation between the various animal forms which the embryo in its different stages simulates. ”

“ The same is true of each ovum and fertilized ovum, and of the atoms in them, respectively, vice versa. So each embryo is composed of myriads of cells, the cells in each having identically the same chemical composition and mechanical arrangement that those in every other embryo of the same age and sex have; all embryos of the same age and sex having substantially the same chemical combinations and mechanical arrangements of their cells, organs and parts. ”

“ I. fig. 45 a) ; and it will be noticed that the new cells are formed at the end of the embryonal vesicle, opposite to that situated at the apex of the embryo-sac. As the cell-division and formation proceed further, a mass of new cells is produced (Pl. I. figs. 46 c{47} & 47) , forming the rudimentary embryo; and from this, by further growth, the perfected embryo (fig. 55) results; or, to use a fashionable technical term, the simply cellular embryonic mass becomes differentiated into the radicle, cotyledons, and plumule, forming the embryo. ”

“ All the changes in the evolution of the animal kingdom, that man had to pass through to arrive at his present shape, the human embryo goes through step by step within the uterus in a very short period. Immediately after fertilization the ovum begins to divide into sections or lobes, into 2, 4, 8, 16, 32, etc. cells until they are almost countless. Each cell splits in the middle of the nucleus, forming two complete new cells and so on. ”

“ At the period of fertilization the embryo-sac lies in close proximity Fertilization. to the opening of the micropyle, into which the pollen-tube has penetrated, the separating cell-wall becomes absorbed, and the male or sperm-cells are ejected into the embryo-sac. Guided by the synergidae one male-cell passes into the oosphere with which it fuses, the two nuclei uniting, while the other fuses with the definitive nucleus, or, as it is also called, the endosperm nucleus. ”

“ In other words, the genetic power of the fertilized ovum weakens, the more it is spread over the growing mass of the tissues of the embryo; but, while it is being thus diluted, it is concentrating anew something of itself on a certain special point, to wit, the cells, from which the ova or spermatozoa will develop. ”

“ The embryo-swelling. (3) The embryo-keel. (4) The spreading of the blastoderm over the yolk-sac from a point corresponding with the position of the embryo, and not with the centre of the germ. The growth is almost nothing at that point, and most rapid at the opposite pole of the blastoderm, being less and less rapid along points of the circumference in proportion to their proximity to the embryonic swelling. (5) The medullary groove.
In external appearance the early embryos of Dog-fish and Teleostei are very similar; some of my drawings could almost be substituted for those given by Oellacher. ”

“ 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) . ”

“ The internal cavity is laid open so as to exhibit an embryo in each half of the Ovum, the one being double the size of the other. The smaller embryo is about as large as a good-sized house-fly. The cord of the largest embryo is nearly an inch and a half long, sacculent, and expanding towards the abdomen so as to form the only covering of that cavity. The cord of the smaller embryo is half an inch in length and filiform. ”