Germplasm

“ A living being takes its individual origin only where there is separated from the stock of the parent a little piece of the peculiar reproductive plasm, the so-called germplasm. In sexless reproduction one parent is enough; in sexual reproduction equal masses of germplasm from each parent combine to form the new individual. The germplasm resides in the nucleus of cells, and Weismann identifies it with the nuclear material which microscopists have named chromatin, on account of the avidity with which it absorbs certain dyes. ”

“ The distinction between these is that where the germ cells are different, the difference will be transmitted by heredity to future generations; if it is a modification that comes on in connection with development and is not due to a difference in the germ cell, it will not show itself in future generations. What the animal and plant breeders have to do is to watch for changes in their stock, and when the kinds that they are looking for appear, they breed them in the hope that the desired features will prove to be hereditary, so that a race can be established showing them. ”

“ And lastly, the problem of hereditary transmission has been rendered more definite and approachable by the discovery that the male germ is likewise a single cell, that fertilization of the egg is therefore the union of two cells, and that in consequence the mechanism of inheritance must be stowed away somehow in these two cells.
Heredity in Unicellular Forms.—In unicellular animals one can readily see how it is possible for an [Pg 23] individual always to give rise to its own kind. ”

“ If that germ-plasm is liable to certain peculiar modifications in the father or grandfather, it is liable to the same or similar modifications in the offspring, that is, if the father could become a drunkard, so could the son, only we must not think that the post hoc is here the same as the propter hoc. If we compare the germ-plasm to the molecules constituting the stem or branches of a vine, its grapes and leaves in their similarity and their variety would be comparable to the individuals belonging to the same family, and springing from the same family tree. ”

“ In other words the offspring of a human being, though obviously and necessarily, from the continuity of germ-plasm, it must be another human being, is never exactly similar to any other. Now these variations are many of them present from the very beginning, they take their origin in the germ-plasm of the two germ-cells which form the fertilised ovum. They are, that is to say, many of them germinal in origin. These must be carefully distinguished from such characteristics as are afterwards acquired by the child as the result of its adaptation to the environment in which it passes its existence. ”

“ ESSENTIAL PREMISES The mechanism of heredity lies in a single germ cell carrying the character-determining chromosomes which consist of organized chains of genes. In the character and organization of these genes in the fertilized cell we find the complete "blueprint" for the future individual in so far as it is to be determined by heredity. In the twenty-four pairs of chromosomes in the fertilized human germ cell we find the long and diversified heritage of each parent represented through the union of the sperm and the ovum. ”

“ In order better to understand the processes of heredity we should be familiar with what takes place when a new individual is formed. The new individual, whether an animal or a plant, has its beginning in the union of two bits of germ-plasm, an egg cell furnished by the mother and a sperm cell furnished by the father. ”

“ The inheritance of any character means the transmission in the germinal material of matter which, brought under the necessary external conditions, develops into the character of the parent. There is necessarily an acquired or epigenetic side to every character; but there is nothing in our knowledge of the actual processes to make necessary or even probable the supposition that the result of that factor in one generation appears in the germ-plasm of the subsequent generations, in those cases where an embryological development separates parent and offspring. ”

“ But when we proceed to ask how the germ-cells will be constituted in the case of an individual who is cross-bred in some respect, containing that is to say, an ingredient from the one side of his parentage and not from the other, the answer is entirely contrary to all the preconceptions which either science or common sense had formed about heredity. ”

“ This hereditary substance is the germ-plasm. It is the living, vital substance of organisms that takes part in the development of new generations.
Naturalists are agreed on this point, that the more complex animals and plants have been derived from the simpler ones; and, this being accepted, the attention should be fixed on the nature of the connection between generations during their long line of descent. In the reproduction of single-celled organisms, the substance of the entire body is divided during the transmission of life, and the problem both of heredity and origin is relatively simple. ”

“ For the same reason we may understand why the species of the same genus, or genera of the same family, resemble each other more nearly in their embryonic than in their more fully developed state, or how it is that in the eyes of most naturalists the structure of the embryo is even more important in classification than that of the adult, 'for the embryo is the animal in its less modified state, and in so far it reveals the structure of its progenitor. ”

“ We know that the germ-plasm of each individual is not a simple substance, but possesses a very complex composition, for it consists of a number of ancestral germ-plasms represented in very different proportions. Now, although we cannot learn anything directly about the processes of growth of the germ-plasm, and its resulting ontogenetic stages, yet we do know, chiefly from observations upon man, that the characters of ancestors appear in the offspring in very different combinations and in very different degrees of strength. ”

“ In all higher organisms the sexes are separate, and the reproduction of their kind is a sexual process. The germinal elements involved are seeds and pollen, eggs and sperms. In animals the egg bears all the hereditary qualities from the maternal side, and the sperm those from the paternal side. The intimate mixture of these in fertilization gives great possibilities of variations arising from the different combinations and permutations of the vital units within the germ-plasm. ”

“ 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. ”

“ I believe that heredity depends upon the fact that a small portion of the effective substance of the germ, the germ-plasm, remains unchanged during the development of the ovum into an organism, and that this part of the germ-plasm serves as a foundation from which the germ-cells of the new organism are produced [181] . There is therefore continuity of the germ-plasm from one generation to another. One might represent the germ-plasm by the metaphor of a long creeping root-stock from which plants arise at intervals, these latter representing the individuals of successive generations. ”

“ But is it less certain that the egg of an eagle developes into an eagle, or that the peculiarities of the father and mother are transmitted to the child, than that a stone falls to the ground when its support is taken away? Again, is it not possible to draw a perfectly distinct and certain conclusion as to the relative quantity of the material basis of heredity, present in the germ-cells of either parent, from the fact that the father and mother possess an equal or nearly equal share in heredity? ”

“ But it differs from gemmules and further resembles germ-plasm in all the following particulars. It is derived from the stirp of proceeding generations, and constitutes the sole basis of heredity. Only a part of it, however, is consumed in each ontogeny—the residue being handed over to “contribute to form the stirps of the offspring,” where it undergoes self-multiplication [59] at the expense of the nutriment supplied to it from the somatic system of the offspring, and so on through successive generations. ”

“ Suppose then the offspring of any parent to vary in many directions. By cross-breeding among themselves and with the parent stock, these are usually merged in a common type, their differences pooled, and the species remains fixed or else advances slowly by natural selection, along one line, as physical conditions change in geological time. But from time to time there arises a variation in the reproductive organs of some individuals, of such kind that these individuals are fertile among themselves, but sterile or less fertile with other varieties and with the parent stock. ”

“ Certain animals and plants transmit male characteristics to the female descendants, while the female transmits her characteristics to the male descendants. There are many variations of this kind. These strange movements in heredity are explained by the laws governing chromosomes and idio-chromosomes and elementary cells.
According to the germ plasm theory of inheritance, the separate parts of living organisms are assumed to be represented by separate material particles in the germ cells. ”

“ In such an observation two things are strikingly exemplified, (1) the fact of the permanence of the unit, and (2) the fact that a mixture of types in the family means that one or other parent is cross-bred in some respect, and is giving off gametes of more than one type.
The problem of heredity is thus a problem primarily analytical. We have to detect and enumerate the factors out of which the bodies of animals and plants are built up, and the laws of their distribution among the germ-cells. ”

“ But the egg, before entering on the process of development, must be stimulated by the union of the sperm with the nucleus of the egg, and thus the starting-point of every animal and plant, above the lowest group, proves to be a single cell with protoplasm derived from two parents. While questions regarding the origin of cells in the body were being answered, the foundation for the embryological study of heredity was also laid. ”

“ It is a well-established law in biology that the union of germ cells of very closely related individuals, that is, of consanguineous individuals (in and in breeding) leads to less vigorous and variable offspring, and the parents are less prolific; while the commingling of diverse heritages by the union of germ cells from individuals belonging to strong but different varieties leads to vigorous and quite variable offspring. The union of such individuals is also most prolific. ”

“ 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. ”

“ First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not quite universally, fertile. Nor is this nearly general and perfect fertility surprising, when we remember how liable we are to argue in a circle with respect to varieties in a state of Nature; and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences, and not of differences in the reproductive system. ”

“ Distinction between the sterility of first crosses and of hybrids—Sterility various in degree, not universal, affected by close interbreeding, removed by domestication—Laws governing the sterility of hybrids — Sterility not a special endowment, but incidental on other differences—Causes of the sterility of first crosses and of hybrids—Parallelism between the effects of changed conditions of life and crossing—Fertility of varieties when crossed and of their mongrel offspring not universal—Hybrids and mongrels compared independently of their fertility—Summary. ”

“ These reproductive cells contain the germinal material or germ-plasm, and this again, in its specific molecular structure, is the bearer of the hereditary tendencies of the organisms from which the reproductive cells originate. Thus in sexual reproduction two hereditary tendencies are in a sense intermingled. In this mingling, I see the cause of the hereditary individual characteristics; and in the production of these characters, the task of sexual reproduction. It has to supply the material for the individual differences from which selection produces new species. ”

“ Secondly, the sterility of distinct species when first united, and that of their hybrid offspring, graduates, by an almost infinite number of steps, from zero, when the ovule is never impregnated and a seed-capsule is never formed, up to complete fertility. We can only escape the conclusion that some species are fully fertile when crossed, by determining to designate as varieties all the forms which are quite fertile. ”

“ We may at least maintain that it has been rendered intelligible, for we can thus trace heredity back to growth; we can thus look upon reproduction as an overgrowth of the individual, and can thus distinguish between a succession of species and a succession of individuals, because in the latter succession the germ-plasm remains similar, while in the succession of the former it becomes different. ”