Germ cell

“ 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. ”

“ The sexual differentiation of the germ-cells is not material to the question we are now considering; the important point is to ascertain whether here, at the very origin of heteroplastid organisms, the germ-cells, sexually differentiated or not, arise from the somatic cells at the end of ontogeny, or whether the substance of the parent germ-cell, during embryonic development, is from the first separated into somatic and germ-cells. ”

“ As of course only the germ-cells of an adult organism pass on to form later generations, and as their content of chromatin is derived not from the sister-organs of the body but from the original fertilized egg, there is a direct stream of the germ-plasm which flows continuously from germ-cell to germ-cell through succeeding generations. ”

“ I may remind the reader that many of the embalmed bodies of the sacred Egyptian animals must be four thousand years old, and that the species are identical with those now existing in the same locality. Now, since the quantity of germ-plasm contained in a single germ-cell must be very minute, and since only a very small fraction can remain unchanged when the germ-cell developes into an organism, it follows that an enormous growth of this small fraction must take place in every individual, for it must be remembered that each individual produces thousands of germ-cells. ”

“ Germ cells give rise by division to somatic or tissue cells, but the converse is not true; for, once a cell has become differentiated and specialized into a tissue cell, it can never again give rise by division to germ cells, but only to other tissue cells of its own kind. Hence the possibility of a change in the tissue being transmitted to the germ has no antecedent probability in its favor. Neither is it grounded on the facts of observation. Bodily mutilations of the parent are not transmitted to the offspring. ”

“ To this it must be answered: (1) though sperm-cells and egg-cells, being living units, cannot be "fundamentally unlike" other living units, such as ordinary body-cells, yet they may be very unlike them; (2) that the germ-cells are very unlike ordinary body-cells is shown by the fact that they can do what no single body-cell can do, build up a whole organism; (3) so specific are germ-cells that in certain cases and in favourable conditions a small fraction of an egg, bereft of its own nucleus, may, if fertilised, develop into an entire and normal larva ”

“ The somatic or tissue cells, which are associated in the metists to form one organic whole, are of the same essential type as germ cells and unicellular organisms, although the parallelism is more close between the unicellular organism and the germ cell. The germ cell, like the protist, is equipped with all the potentialities of life, whereas tissue cells are specialized for one function rather than another. ”

“ Nearly always the containing cells, often called gametes, are unlike: the sperm-cell being the male product, and the germ-cell the female. But among some Protozoa and many of the lower Algæ and Fungi, the uniting cells show no differentiation. Sexuality is only nascent.
There are very many modes and modifications of modes in which these cells are produced; very many modes and modifications of modes by which they are brought into contact; and very many modes and modifications of modes by which the resulting fertilized germs have secured to them the fit conditions for their development. ”

“ If the appearance of the whole cell gives us any knowledge as to the structure of its nuclear idioplasm, we may be sure that the development of a primitive germ-cell proceeds without a break, from the moment of its first recognizable formation, to the ultimate production of distinct male or female sexual cells. ”

“ We should say, the germ-plasm of the ovum first doubles itself by growth, as the nuclear substance does at every nuclear division, and then divides into two similar halves, one of which, lying in the primordial somatic cell, becomes at once active and breaks up into smaller and smaller groups of determinants corresponding to the building up of the body, while the germ-plasm in the other remains in a more or less 'bound' or 'set' condition, and is only active to the extent of gradually stamping as germ-cells the cells which arise from the primordial germ-cell. ”

“ Throughout the animal kingdom, germ-cells present the same external aspect when carefully examined with the microscope. No difference can be observed between the cells of the flowers of the oak and those of the apple, but the cells of the one always produce oak trees, while those of the other always produce apple trees. The same is true of the germs of animals, there being not the slightest apparent difference. We are unable to perceive how one cell should give origin to a dog, while another exactly like it becomes a man. ”

“ To put the matter a little differently, one germ-cell differs from another in having alternatives present in place of some of the factors of the latter. A given germ-cell can never have more than one of the possible alternatives of each factor. ”

“ That the sperm-cell and germ-cell are thus contrasted in nature and function may further be suspected on considering the distinctive characteristics of the sexes. Of the two elements they respectively contribute to the formation of a fertile germ, it may be reasonably supposed that each furnishes that which it possesses in greatest abundance and can best spare. Well, in the greater size of the nervous centres in the male, as well as in the fact that during famines men succumb sooner than women, we see that in the male the co-ordinating system is relatively predominant. ”

“ Now we know that the child comes from the germ cells which are not made by the bodies of the parents but have arisen by the division of the antecedent germ cell. Every cell comes from a pre-existing cell by a process of division, and every germ cell comes from a pre-existing germ cell. ”

“ The germ-cell first divides into two, four, eight, sixteen, thirty-two, and sixty-four [58] daughter-cells, and so on into the millions; and this segmentation of the stem-cell and daughter-cells would surely annihilate any skeleton, model or other outline that might exist in the germ-cell; (3) the germ-cell first divides into two daughter-cells and so on, to infinity, as already stated; and these daughter-cells form what are called “germ-layers” or sheets of cells from which the human body and all of its organs and parts are built up. ”

“ According to this, the cells resulting from the subdivision of the germ-cell preserve their developmental force, unless employed in building up definite organic structures. In certain creatures, and in certain parts of other creatures, germ-cells unused are stored up, and by their agency lost limbs and other mutilations are repaired. ”

“ Further, we now understand at once that since one generation of germ-cells directly produces those of the next, there [Pg 26] is no reason in the world why an individual should not more nearly resemble a remote ancestor than his own immediate parents. As a simple matter of fact this frequently happens. He does so because the germ-cell from which he sprang is composed of protoplasm handed down in direct continuity by successive generations of germ-cells from time immemorial. ”

“ The blood corpuscles of man and woman are the same, and so are the cells of their nerves and muscles; and even the sexual cells, so different in size, appearance, and generally also in motile power, must contain the same fundamental substance, the same idioplasm. Otherwise the female germ-cell could not transmit the male characters of the ancestors of the female quite as readily as the female characters, nor could the male germ-cell transmit the female quite as readily as the male characters of the ancestors of the male. ”

“ In higher orders of animals, fecundation occurs internally, the conjunction of the sperm and germ cells requiring the conjugation of the male and female sexual organs. The sperm-cells of the male furnish the quickening principle, which sets in play all the generative energies, while the germ-cell, susceptible to its vivifying presence, responds with all the conditions necessary to evolution. The special laboratory which furnishes spermatic material is the testes, while the stroma of the ovaries contributes the germ-cell. ”

“ Early investigators tended naturally to look on the germ-cells as a product of the body. Being supposedly products of the body, it was natural to think that they would in some measure reproduce the character of the body which created them; and Darwin elaborated an ingenious hypothesis to explain how the various characters could be represented in the germ-cell. ”

“ In such cases the sex of the young individual can always be identified by the form of the first antenna and of the first thoracic appendage, both of which are always clawed in the male. But who can point to the direct causes which determine that the sexual cells shall become sperm-cells in this case, and not egg-cells? Does the determining cause depend on the conditions of nutrition? Or, again, in the females, can the state of nutrition determine that the third out of a group of four germ-cells shall become an egg-cell, and that the others shall break up to serve as its food? ”

“ It is much more difficult to understand in the formation of germ-buds, and more difficult still in the formation of germ-cells, how this very small, quite undeveloped portion of the body, this group of cells, or this single cell, not only directly takes with it certain parental qualities into its independent existence, but also after its195 separation from the parental individual develops into a many-celled body, and in this repeats the forms and vital phenomena of the original producing organism. ”

“ I am also far from asserting that the germ-plasm which, as I hold, is transmitted as the basis of heredity from one generation to another, is absolutely unchangeable or totally uninfluenced by forces residing in the organism within which it is transformed into germ-cells. I am also compelled to admit it as conceivable that organisms may exert a modifying influence upon their germ-cells, and even that such a process is to a certain extent inevitable. The nutrition and growth of the individual must exercise some influence upon its germ-cells . ”

“ In every real cell there is, besides protoplasm, and while still alive and independent, a second important constituent, the cellular germ, so called (nucleus or cytoblast) ; but even this germ consists of an albuminous chemical compound which is closely related to protoplasm, and was orginally produced from it by an exceedingly slight chemical alteration. The germ is usually a smaller and firmer formation within the protoplasm of the cell. ”

“ They have to form all the various elements, organs, limbs, and structures known and described by the anatomist. The germ-cells are a separate little group of themselves embedded in the male and female reproductive organs for the sake of nutrition, growth, and shelter, for many years, until they again take part in the process of fertilisation. Note carefully that no other cells in the body ever unite together to produce a new individual except germ-cells. ”

“ From the view-point of cell-differentiation, which seems to me the most tenable position, there does not seem any evidence for, or any probability of, the occurrence of any roundabout mode of development of the germinal cells which could enable them to pick up acquired characters en route.
(d) We come now to the contention that the organism, being one and continuous, if any member suffers, the germ suffers with it. The organs of the body are not isolated or insulated; the blood is a common medium; the nerves ramify everywhere ”

“ In sexless reproduction one parent is enough; in sexual reproduction equivalent masses of germ-plasm from each parent combine to form the new individual. The germ-plasm resides in the nucleus of cells, and Weismann identifies it with the nuclear material named chromatin. Like ordinary protoplasm, of which the bulk of cell bodies is composed, germ-plasm is a living material, capable of growing in bulk without alteration of structure when it is supplied with appropriate food. ”