Embryogenesis

“ 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. ”

“ This germ-plasm is very often subjected to all sorts of unfavourable conditions, especially those of alcohol and toxins, and such conditions have been acting upon it more or less for an immense number of generations, and yet the resistance to modification at the hands of these internal factors is so great that all the processes which follow upon the fertilisation of the ovum, all the thousand complications which thereafter ensue in the building up of the young embryo are hardly ever interfered with. ”

“ 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. ”

“ When a male germ-cell or sperm unites with a female germ-cell or ovum, within the female body, fertilisation of the ovum takes place, and this gives rise to the fertilised germ-cell from which is to arise first the germ-cells or direct descendants of itself, and secondly the embryo in which these germ-cells will come to lie. This happens by the repeated and continued division of the fertilised germ-cell, a division which constitutes growth, and which under suitable conditions of nourishment and protection and exercise will ultimately produce a human being. ”

“ Vast as is the range of organisation presented by the great modern sub-kingdom, embracing forms adapted to every environment, there is yet a marked uniformity in certain points of structure, as in the development of the embryo-sac and its contents, the pollination through the intervention of a stigma, the strange phenomenon of double fertilisation (One sperm fertilising the egg, while the other unites with the embryo-sac nucleus, itself the product of a nuclear fusion, to give rise to a nutritive tissue, the endosperm.) ”

“ As the cicatricula of these eggs is given by the cock, and is evidently the rudiment of the new animal; we may conclude, that the embryon is produced by the male, and the proper food and nidus by the female. For if the female be supposed to form an equal part of the embryon, why should she form the whole of the apparatus for nutriment and for oxygenation? the male in many animals is larger, stronger, and digests more food than the female, and therefore should contribute as much or more towards the reproduction of the species ”

“ Now, egg and sperm are the two germinal elements which unite in development. Fertilization takes place by the union of sperm and egg, and inasmuch as the nuclei of each of these structures contain one-half of the number of chromosomes characteristic of the species, [Pg 313] their union in fertilization results in the restoration of the original number of chromosomes. The fertilized ovum is the starting-point of a new organism, and from the method of its fertilization it appears that the parental qualities are passed along to the cells of every tissue. ”

“ Embryology has shown us that, during gestation, each human embryo is a replica of the past; it passes through the different Imago stages from protoplasm to man, being unrecognisable at certain stages from a monad, an amœba, a fish with gills, a lizard, and a monkey with a tail and dense clothing of hair over the whole body. The human embryo has also, at an early stage, the thirteenth pair of ribs, which is found in lower animals and is still seen in a rudimentary form in anthropoid apes, but which disappears from the human embryo before birth. ”

“ Before fertilization a neck-canal cell is formed by the division of the ovum-nucleus. After the body of a spermatozoid has coalesced with the egg-nucleus the latter divides repeatedly and forms a mass of tissue which grows more vigorously in the lower part of the fertilized ovum, and extends upwards towards the apex of the ovum as a peripheral layer of parenchyma surrounding a central space. By further growth this tissue gives rise to a proembryo, which consists, at the micropylar end, of a sac ”

“ And yet in all cases the egg or embryo is always distinguishable from the body of the mother and there is no protoplasmic connection between the two. In mammals generally, including also the human species, not a strand of protoplasm, not a nerve fiber, not a blood vessel passes over from the mother to the embryo; the latter is from the moment of fertilization of the egg a distinct individual with particular individual characteristics, and this is just as true of viviparous animals ”

“ For the rest, in all that respects the development, the embryo is produced from the conception in the same manner and order as the chick from the egg, with this single difference, that whatever is required for the formation and growth of the chick is present in the egg, whilst the conception, after the formation of the embryo, derives from the uterus of the mother whatever more is requisite to its increase, by which it continues to grow in common with the fœtus. The egg, on the contrary, becomes more and more empty as the chick increases ”

“ They stand for the most important phenomenon in the living world. Without it there would be no plants and no animals, excepting a few very low forms of no importance, and of course no human beings.
Fecundation or fertilization is the process of union of the female germ cell with the male germ cell; speaking of animals, it is the process of union of the egg or ovum of the female with the spermatozoön of the male. When a successful union of these two cells takes place a new being is started. The process of fertilization or fecundation is also [83] known as impregnation and conception. ”

“ This process of fusion between the two kinds of reproductive cells, which are termed gametes, is called conjugation: it is the process which is sometimes spoken of as the fertilization of the ovum, and its result is the establishment of a new individual. This new individual at first is simply a uninucleated mass of living matter, which always contains a certain amount of food-yolk, and is generally bounded by a delicate cuticular membrane called the vitelline membrane. ”

“ During embryological development the fertilized ovum—consisting of two fused fragments of this germinal substance—gives rise to a host of ordered and marshalled cells, which are divisible into two groups: the one forms the body with its muscles, bones, glands, digestive system, skin, sense-organs, nerve-centres, and so forth; the other forms a reserve store of germinal substance, from which are derived the ova and sperms. The former take no direct share in reproduction; they are off the line of continuous descent; they die without issue. ”

“ 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. ”

“ 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. ”

“ For instance, in the wall of the medullary canal in the Vertebrate embryo, in the splanchnic and somatic layers of mesoderm of the same embryo, and in the developing nerve cords of the Peripatus embryo, the nuclei are at first so densely crowded together that it is almost impossible to see the protoplasmic framework in which they rest, but as development proceeds this extra-nuclear tissue becomes more largely developed, and the nuclei are forced apart, so that it becomes visible and receives various names according to its position. ”

“ With this conception of the blending of the parental qualities within the germinal elements we can conceive how inheritance would be possible and how there might be included in the egg and the sperm a representative in material substance of all the qualities of the parents. Since development begins in a fertilized ovum, this complex would contain minute particles derived from every part of the bodies of both parents, which by growth would give rise to new tissues, all of them containing representatives of the tissues of the parent form. ”

“ Artificial parthenogenesis designates the development of unfertilized ova which normally would not develop at all and which are stimulated to development by placing them under artificial chemical conditions. Doubtless many of you have seen in the newspapers these experiments referred to as if they gave the actual creation of life. Of course that is nonsense. The life is there in the ovum. What artificial parthenogenesis accomplishes is to supply a stimulus, chemical in nature and capable of replacing the fertilization by the spermatozoon, which would otherwise be necessary. ”

“ As soon as a spermatozoon enters an ovum, it disappears and is completely absorbed, and, as the ovum in turn is instantly possessed of new powers, the result of this union is a cell which was previously non-existent.
This new cell is not only capable of reproduction by means of segmentation or cell division, but in the course of its sub-division and proliferation, it forms groups of cells which develop into tissues and structures widely different from each other. The entire complex human body, in addition to the placenta, cord, and membranes, arises from the single, extraordinary cell. ”

“ In such cells an accumulation of food to any large extent would hinder rather than help them, because it would seriously interfere with their activity.
Fertilization.—In fertilization (Fig. 12, p. 48) the spermatozoon penetrates the wall of the ovum and after undergoing considerable alteration its nucleus fuses with the nucleus of the egg. In some forms only the head (nucleus) and middle-piece enter, the tail being cut off by a so-called fertilization membrane which forms at the surface of the egg and effectually blocks the entrance of other spermatozoa. ”

“ A spot on the external surface of an organism grows more rapidly than contiguous spots, and forms a tubercle which grows into a bud, assumes the form and structure of the parent, and finally separates. In the next grade the budding is internal, from a special organ simulating an ovary, though not a true ovary, as in aphides. Finally, in parthenogenesis we have a perfect ovary forming true ova and perfect embryo without fertilization or coöperation of the sperm-cell. ”

“ The effect of impregnation appears in all cases to be that the impregnated protoplasm tends to divide into portions (blastomeres) , which may remain united as a single cell-aggregate, or some or all of which may become separate organisms. A larger or shorter period of rest, in many cases, intervenes between the act of impregnation and the commencement of the process of division.
As a general rule, the female cell which directly receives the influence of the male is that which undergoes division and eventual development into independent germs ”

“ The clue which Miescher furnished has been followed by others, until to-day we seem to be approaching a determination of the structure of the proteid molecule—the first step necessary to solving the problem of the chemistry of living matter. Modern histology has shown that the fertilization of the ovum, from which the animal body develops, consists essentially in the entrance of the nucleus of the spermatozoon or male cell, and its fusion with the nucleus of the ovum. ”

“ The latter succeeds immediately to the former; beginning at the moment when the new being is grafted on the maternal womb, and continuing until its expulsion from thence at the full period of gestation. It follows, therefore, that the Embryonic or former state is that in which the new being is as yet, without any direct or indirect communication with the mother, and still less so with external objects. This state persists for about two weeks after fecundation, during which the Embryo continues to derive its nourishment from the cortical membrane of the Ovum. (Boer, Soemmering, Plagge, myself.) ”

“ We are now ready to go back to the germinal tissue and trace the process of heredity as it actually works out in the developing offspring. In the ordinary cells of the germinal tissue, as we have already seen, the chromosomes are exactly like those in the other cells of the body. Cell multiplication goes on actively in the germinal tissue of both parents; in the mother this leads to the production of germ cells which are called eggs, and in the father to the production of cells that are called sperm. ”