Oscar Hertwig

“ As a matter of fact, the idea that parts of the tissues of different animals may serve to replace one another has been employed repeatedly in science, especially in the science of medicine. But I believe that our ideas are not yet clear upon the matter. The erroneous impression to which I have alluded has arisen because we do not bear in mind that each tissue, each part of an organ, each cell, possesses, in addition to its obvious characters, very many characters that are invisible to us. ”

“ What is development? Does it imply preformation or epigenesis? This perplexing question of biology has reappeared recently as a problem of the day. Of late years there have been set forth contradictory doctrines, each seeking to explain the process by which the fertilised egg-cell, an apparently simple beginning, gives rise to the adult organism, which often is exceedingly complicated, and which has the capacity of producing new beginnings like that from which it itself arose. ”

“ Between somatic cells and reproductive cells Weismann places a gulf that cannot be bridged. Only the reproductive cells contain real germplasm, and only these contain the conditions for maintaining the species, as they alone serve for the starting of new generations of development. The somatic cells, on the other hand, are endowed only with fragments of germplasm, and hence they are incapable of preserving the species, and are doomed to death. The reproductive cells, like unicellular organisms, are regarded as immortal, the somatic cells as mortal. ”

“ Inquiry into the problems of heredity is beset with many difficulties, of which not the least is the temptation to argue about the possible, or the probable, rather than to keep in the lines of observation. Setting out from a laborious and beautiful series of investigations into the anatomy of the Hydromedusæ, Weismann came to think that the organic material from which the sexual cells of these animals arose was not the common protoplasm of their tissues, but a peculiar plasm, distinct in its nature and possibilities. ”

“ We must regard the functions of a cell that is part of an organism, disregarding external influences, as determined by the whole organism, and only by the cell itself, in so far as that forms a greater or less part of the whole organism. When not part of an organism, the cell is independent, and entirely determines its own function. ”

“ Supposing for the moment that we accept the assumption that different character in cells implies different character in their nuclear matter, we have at once a new and important decision to make. Does the nuclear matter in the different cells, that has arisen by division from the nuclear matter of the egg-cell, become unlike by the process of division itself? ”

“ Unfortunately, Weismann's hypothesis tells us nothing at all about these internal causes, that depend upon the physical nature of the idioplasm; that is to say, nothing at all about the causes which, working in a fashion so contradictory and astonishing, really produce the whole development.
In such a state of affairs it is better to turn to Nature herself; and to see whether or no the occurrence of differentiating division of the nucleus in the organic world is at all supported by the actual observations and investigations of those who study cells. ”

“ And so they regarded development as a simple process of growth resulting from nutrition. Their mental picture of the germ or beginning of an organism was an exceedingly reduced image of the organism, an image requiring for its development nothing but nutrition and growth. ”

“ The products of the sexual cells show us clearly enough that out of each kind of egg only its own species of organism can be developed. Certainly it is not so plain that, besides their visible microscopical characters, the tissues and organic parts are in possession of more general characters, identical in all the differently-specialised tissues of a single organism; but we may infer the existence of such latent characters, at least partly, from the results obtained, in the case of plants, by grafting, in the case of animals, by transplantation and transfusion. ”

“ The cells, indeed, become different from each other as the organism grows, some becoming muscle-cells, others nerve-cells, others digestive-cells, and so forth. Weismann thinks that the differences occur because, in the disintegration of the germplasm-microcosms, according to a prearranged plan, only the determinants for nerve-cells are marshalled into nerve-cells, only those for muscle-cells into muscle-cells, and so forth. The development is an evolution, an unfolding or unwrapping of little rudiments that lie in the germplasm. ”

“ A unicellular animal takes organic particles bodily into its protoplasm, and forming around them temporary cavities known as food vacuoles, treats them chemically. The multicellular animal has become shaped so as to enclose a space within its body into which solid organic food-particles are carried and digested, thereafter, in a state of solution, to be shared by the single cells lining the cavity. In this way the animal body does not require so close a relation with the medium surrounding it; its food, the first requirement of an organism, is distributed to it from inside outwards. ”

“ 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. ”

“ Nearly all kinds of animals appear as male or as females. These differ from each other not only in that they produce eggs or spermatozoa, but frequently in a number of more or less striking characters affecting different parts of the body, and known as secondary sexual characters. In fact, the difference between the sexes may be so great that a systematic naturalist, unacquainted with the mode of development of the creatures, might place them in different species, genera, or even families, on account of the striking differences in external characters. ”

“ In truth, therefore, all organic bodies would be miracles. Would not this change for us the presence of Nature? Would it not spoil her of her beauty? Hitherto we had a living Nature, displaying endless changes by her own forces. ”

“ Although from the point of view of morphology it has become more and more imperative to regard the cell as the unit of the higher organism, still, from the physiological point of view the higher organisms must be regarded as masses of material acting as wholes, and composed of several grades of structural parts, subordinate in function to the whole, and displaying only a limited division of capacities. ”

“ But I must insist here that the subordination of the cells to the whole organism, in both multicellular animals and in plants, is much more complicated than that of the units to the human state. In the latter case, the individuals are separate from one another; they are independent organisms and are bound together only in social relations. None the less, consider how in a civilized state the apparently sovereign individual is conditioned in all his circumstances ”

“ The invagination of the blastosphere comes about by the co-operation of all the cells of its wall, by local differences in the rates of growth in that wall, from dissimilarities in its curvature, from many causes which have not yet been sufficiently sought out and investigated. As cell division itself depends not upon special particles, but upon changes in the entire nuclear contents, it follows that the growth of the blastosphere-wall, which is merely the sum of the growth of all the cells in it, cannot be determined by special groups of determinants. ”

“ As an instance, take Bonellia, a gephyrean, the strange case of which has been remarked upon by Hensen and by Weismann. The male is about a hundred times smaller than the female, in the respiratory chamber of which it lives as a kind of parasite, and appears, so far as outward shape goes, more like a turbellarian than a gephyrean. None the less, male and female are alike not only while they are in the egg, but as larvæ, and it is only towards the period of sexual maturity that the great difference between them begins to appear. ”

“ My conception receives strong support from the way in which Vöchting set forth the relations of the cell to the whole:
'Is the circumstance that a cell, separated from the organism, is able to survive and build up the whole again a proof of the independent life of the cells while in the organism? I believe it to be only a proof that the life of the organism is always dependent upon the cell, that the life is inherent in the cell, and that the life of a compound organism is merely the resultant of the vital phenomena of its single cells ”

“ In the first place, the mass of germplasm which is the starting-point of a new individual consists of several, sometimes of many, pieces termed ids, each of which contains all the possibilities—generic, specific, individual—of a new organism. Each id is a veritable microcosm, possessed of a historic architecture that has been slowly elaborated during the multitudinous series of generations that stretch backwards in time from every living individual. This microcosm, again, consists of a number of minor vital units called determinants, which cohere according to an orderly plan. ”

“ For the supposition that the nucleus is the hereditary mass transmitting the characters of the species necessitates the conclusion, in the case of unicellular forms, that the hereditary mass remains in possession of all the rudiments of the cell while it passes through the various phases of its cycle of development. ”

“ With right and reason one may adduce, against his own determinants, what Weismann has said about pangenes, for exactly the same reasons: 'There cannot be zebra-determinants or serration-determinants, because zebra-striping, like the serrated edge of a leaf, is no cell character.'
The error in Weismann's doctrine of determinants may be made clearer by an analogy.
The human state may be conceived as a high and compound organism that, by the union of many individuals, and by their division into classes with different functions, has developed into a form always becoming more complicated. ”

“ The cell, which consists of protoplasm and a nucleus, is an elementary organism, that, by itself, or in combination with other cells, forms the basis of all animal and vegetable organisation. In minute structure it is so extraordinarily complicated that its essential constitution (its micellar or molecular structure) eludes our observation. It is a medley, composed of numerous, chemically distinct particles that may be divided into two groups, organised and unorganised. ”

“ It is true that Weismann regards his almost countless germinal particles as being held together in an architectural structure of almost inconceivable complexity. For him the germ is an exceedingly complicated living being, a microcosm in the truest sense, in which every independently variable part that ever appears throughout the whole life is represented by a living particle, and in which each of the living particles is endowed with a definite, inherited position, a constitution, and the power of rapid multiplication. ”

“ On the other hand, we regard the development of the rudiments as depending upon motive forces or causes that are external to the germplasm of the ovum, but that none the less arise in orderly sequence throughout the course of the development. The causes we recognise are first, the continual changes in mutual relations that the cells undergo as they increase in number by division, and second, the influence of surrounding things upon the organism. ”

“ But in another way the multiplication of cells leads to differentiation among the cells arising from the egg. Although each of these resembles the parent egg, from which they arose by doubling division, yet they differ from it in one point: they are no longer a whole, but have become the subordinate parts of a higher unity, that is, of a higher organism. A cell that is no longer a whole, but the part of a whole, has entered upon reciprocal relations with other cells, and in [Pg 113] the functions of its life is limited by these others and by the whole. ”