Chapter VIII: Part II: Thoughts Towards a Theory of the Development of Organisms.17 (2)

Differentiation in multicellular organisms takes a similar course. Every cell, by doubling division of the egg, receives all the rudiments of its kind; of these rudiments, some in one set of cells, others in another, come to develop, according to the part of the whole in which the cells come to lie during the progress of the development, and according to the relations to the whole they come to assume. Thus, here they assume the characters of the external skin; there, they become gland-cells of the intestine; here, muscle-fibres; there, sense-cells or nerve-cells; in one place they serve the whole organism, in the form of blood-corpuscles, as agents for nutrition and respiration; there, becoming connective tissue or bone, they form skeletal elements of the body.

Thus, during the course of development, they are forces external to the cells that bid them assume the individual characters appropriate to their individual relations to the whole; the determining forces are not within the cells, as the doctrine of determinants supposes. The cells develop those characters that are suggested by their relation to the external world and their places in the whole organism.

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; how each change in the general state exercises an influence on the individual's disposition freedom of will, and method of life (dwelling, food, institutions, health); then reflect how much greater in the animal and the plant is the domination of the whole, and the subordination of the units, as in them cell is directly joined to cell--indeed, in most cases united materially by threads of protoplasm. In such cases the self-sufficiency of the cell as an elementary, living organism is so far prevented, that it becomes a subordinate part, with its function in dependence on the whole.

One other point our comparison will make clearer: I refer to the relation of the specific nature of the rudiment to the specific nature of the product of the rudiment.

The different organisations and qualities of the communities formed by different animals may be explained by the special characters of the animals forming them. Those of the bee colonies depend on the nature of bees; of ant colonies on the nature of ants; of the societies of men on the nature of men; indeed, in the latter case we see how they differ as they are formed by Italians, Germans, Slavs, Turks, Chinese, or Negroes. Similarly, the specific organisation of the cell determines the kind of animal which may be built up by it.

In my theory two assumptions of totally contrasting nature are made: I assume a germplasm of high and specific organisation, and I assume that this is transformed into the adult product by epigenetic agencies. To a certain extent, therefore, I reconcile the opposition between evolution and epigenesis, these opponents so prominent last century.

But my theory does not pretend to explain all the many problems involved in the course of organic development. In this respect it differs from Weismann's doctrine of determinants, as that is a closed system, finding within itself a formal explanation of all development. So far it seems to me an abandonment of explanation rather than an explanation; for it explains by signs and tokens that elude verification and experiment, and that cannot encounter concrete investigation. His explanation is no more than a description, in other words, of the visible events of development. To be more than this, it would be necessary to explain how in each case the biophores and determinants and ancestral plasms are constituted, and how they are arranged in the architecture of the germplasm so as to produce the development of the egg-cell in this or that fashion. It must, at the least, offer such possibilities as the structural formulae of chemists offer. But in the present stage of our knowledge Weismann's method is unpromising; it merely transfers to an invisible region the solution of a problem that we are trying to solve, at least partially, by investigation of visible characters; and in the invisible region it is impossible to apply the methods of science. So, by its very nature, it is barren to investigation, as there is no means by which investigation may put it to the proof. In this respect it is like its predecessor, the theory of preformation of the eighteenth century.

FOOTNOTES:

[17] The second section contains references to the following treatises:

C. V. NAEGELI: _Mechanisch-physiologische Theorie der Abstammungslehre_ (1884).

HERTWIG, OSCAR: _Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere_; 4th edit.

SACHS: _Lectures on Plant Physiology_; English edition, Clarendon Press.

VOECHTING: _Ueber die Theilbarkeit im Pflanzenreich und die Wirkung innerer und aeusserer Kraefte auf Organbildung an Pflanzentheilen._ _Pflueger's Archiv._, vol. xv., 1877.

Ibid.: _Ueber Organbildung im Pflanzenreich_, 1, 2; Bonn, 1878, 1884.

GOEBEL: _Beitraege zur Morphologie und Physiologie des Blattes._ _Bot. Zeit._, 1880.

PFLUeGER: _Die teleologische Mechanik der lebendigen Natur_; Bonn, 1877.

MAUPAS: _Sur le determinisme de la sexualite chez l'Hydatina senta._ _Comptes rendus des seances de l'Academie des Sciences_; Paris, 1891.

WEISMANN: _Die Allmacht der Naturzuechtung. Eine Erwiderung an Herbert Spencer_; Jena, 1893.

HERBERT SPENCER: _A Rejoinder to Professor Weismann._ _Contemporary Review_, 1893.

Ibid.: _Die Unzulaenglichkeit der 'Natuerlichen Zuchtwahl.'_ _Biol. Centralblatt_, vol. xiv., No. 6.

EMERY: _Die Entstehung und Ausbildung des Arbeiterstandes bei den Ameisen._ _Biol. Centralb._, vol. xiv., No. 2, 1894.

HAACKE; _Gestaltung and Vererbung_ (1894).

[18] The assumption of doubling division does not involve the assumption that the germinal mass is unalterable. Although I do not regard the process of division as a mechanism for breaking up the idioplasm into dissimilar groups of determinants, I regard the idioplasm--and here I agree with Naegeli--as only relatively stable. In course of time external and internal forces may slowly alter it. On the one hand, the idioplasm of the reproductive cells in the course of generations may slowly alter, while, on the other hand, the idioplasm of cell groups in an organism may acquire a local character in correspondence with their different topographical and functional positions in the whole creature, and in relation to their place in the organic division of labour, just as in human communities individuals become altered by the lifelong exercise of some calling.

Nor does the doctrine of doubling divisions conflict with those conclusions of pathology according to which, in the process of regeneration, cells and tissues give rise only to cells and tissues of their own order. For further details see my treatise, _Ei und Samen-Bildung bei Nematoden_, pp. 97-99. These slight suggestions are only to prevent misconceptions.

INDEX AND GLOSSARY

A

ACINETA, a group of protozoa, development of, 41.

Acquired characters, question of their inheritance, x.

Amphioxus, a marine animal, representative of the primitive vertebrate stock, experiments on eggs of, 61.

Anabolism, the formation of more complex chemical bodies by the agency of protoplasm, 86.

Animal cells, characteristic mode of growth, 111.

Antennularia, Loeb's experiment, 117.

Ants, polymorphism in, 125.

Ascidians, tunicate animals, 46.

Atavism, the occurrence in an organism of a character abnormal in it, but normal in an ancestor, 24.

B

Bees, polymorphism in, 125.

Beetroot, grafting experiments, 121.

Begonia, reproduction from leaves, 46.

BEET, experiments on rats, 73.

BERESOWSKY, skin-grafting, 75.

BEYERINCK, upon galls, 51.

Biophores. Each determinant, according to Weismann, is composed of a number of ultimate living pieces, the biophores, which are the active agents that direct the functions of a mature cell, ix, 22.

Blastosphere, an early stage in embryonic development; the embryo consists of a hollow sphere, the walls of which consist of a single layer of cells, and the cavity of which is called the segmentation cavity, xvii; explanation of formation, 97, 98.

Blood, transfusion of, 75.

BLUMENBACH, _nisus formativus_, 5;
upon galls, 50.

Bone-grafting, 73, 74.

Bonellia, sexual dimorphism in, 122.

Bryozoa, a group of minute animals which form encrustations on seaweeds and stones, 46.

Buds, origin of, 28;
reproduction and regeneration by, 46.

C

Cell, description of, 31;
characters possible in, 88;
differentiation of, in development, 112;
as units in morphology and physiology, 113;
Sachs on, 114;
Voechting on, 114, 116.

Cell theory, relation of, to heredity, 31.

Centrosome, an organ of cells most obvious during nuclear division, 93.

Cerianthus, experimental heteromorphoses, 51.

CHABRY, destruction of segmentation sphere, 62.

Chromatin, a material found in the nucleus of cells, so called because it absorbs stains with avidity: germplasm and, viii, xiv; relation of, to specific character of cells, 36, 37.

Chromosomes, definite, visible bodies, as which the chromatin of a dividing nucleus appears, xiv, 93.

Crystal, growth of, compared with organic growth, 108.

Cione, experimental heteromorphoses, 52.

Clavellina, reproduction from buds, 46.

Cleavage-planes,
the planes separating the daughter-nuclei, or daughter-cells, in the
early division of a fertilised egg-cell, xvii;
relation between appearance of, and structure of eggs, 95.

Coelenterata, a major division of multicellular animals, including such creatures as sea-anemones, corals, and jelly-fish, 46.

Continuity of the germplasm, 26.

Continuity of life, the doctrine opposed to spontaneous generation, 2.

Correlations, 118, 121.

D

DARWIN, pangenesis, 21.

Determinants. Each _id_ of germplasm is supposed by Weismann to be composed of minor pieces, arranged in a complicated fashion that is the result of the past history of the species. For every part of the body, large or small, that may be different in different individuals or species, there is, at least, one determinant in the _id_. The determinants are so grouped in the _id_ that they are liberated and become active when the time comes for the development of that part of the body they control, viii, 22; arguments against, 82; relation to cells, 87.

Determinates, the smallest parts of an organism which vary independently, and which are supposed by Weismann to be represented in the germplasm by special pieces, 23, 25.

Differentiating division, such a division of the nucleus as would result in daughter-nuclei unlike each other, and unlike the parent nucleus. The qualities of the parent nucleus are supposed to have been distributed between the daughter-nuclei, xi; absence of visible evidence for, xv, 25; objections to occurrence of, 34, 78.

Dimorphism, the appearance of the same species in two different forms, sexual dimorphism, 122, 124.

Disharmonic union in grafting, 70.

Double monsters, as examples of heteromorphosis, 63.

Doubling division. When an amoeba reproduces by simple division, the daughter-amoebae are identical, and each is identical with the parent except in size; from one amoeba two have been formed. A doubling division of the nucleus is such as would result in the formation of two nuclei alike in every respect, ix; visible evidence for, xv, 24; in unicellular organisms, 40; occurrence of, with differentiating division, 78.

DRIESCH, experiments on eggs, 54;
separation of segmentation spheres, 60.

E

Echinoderms, a group of marine animals, of which the star-fish is the most familiar type, eggs of, 54.

Echinoidea, a group of echinoderms, 61.

Ectoderm, the tissue in an adult derived from the epiblast (which see), 19.

Egg, relation between structure and division of, 94;
specific character of, 135.

EMERY, on polymorphism in ants, 128.

Endoderm, the tissue in an adult, derived from the hypoblast (which see), 19.

Enfoldment. See Evolution.

Epiblast. In the development of all multicellular animals, the young embryo soon becomes divided into two sets of cells, the epiblast and hypoblast; where a gastrula is formed, the outer layer of cells is the epiblast, the inner layer the hypoblast, xviii.

Epigenesis, the doctrine that the formation of a new individual is not the
mere out-growing of particles hidden in the egg-cell, but the result of
moulding external forces, xiii;
Roux's definition of, 7;
Weismann's denial of, 9;
epigenetic explanation of stages in development, 98;
summary of Hertwig's acceptance of, 136.

Evolution. Originally the term was applied, not to the origin of existing forms of life from common ancestors, but to the doctrine that every living creature contained within it the whole series of its future descendants, and that the growth of a living creature was evolving of one of these enfolded miniatures, xiii, 1, 2, 3; Roux's contrast of, with epigenesis, 6; the new evolution, 10; Hertwig's partial agreement with, 135, 136.

Experiment, Weismann's caution against, 10.

F

Fertilisation, the union of the nuclear matter of a male cell with the nuclear matter of a female cell, xii, xiv.

Foraminifera, a group of protozoa provided with shells, 44.

FOREL, on eyes of ants, 126.

Frogs' eggs, Hertwig's experiments upon; development of, under compression, 57-60.

Funaria, reproduction from chopped pieces, 46.

G

Galls, 50.

Gastrula, an early embryonic stage, most simply formed from the blastosphere by the invagination of one side of the wall, and consisting of a hollow sac, the walls of which are formed by two layers of cells, xviii, 60; formation of, 99.

Gemmules. See Pangenesis.

Germ, the youngest embryonic stage of an individual or organ, 10.

Germplasm, the substance supposed to be the material bearer of inherited
qualities: Weismann's conception of, viii, 20;
identification of, with nuclear matter, 21;
account of Weissmann's theory, 21-28.

Germ-tracks, the hypothetical paths along which germplasm passes in an unaltered condition during development, 27; objections to, 81.

GOEBEL, on plasticity of plants, 120.

Grafting, 68, 70;
of Hydra, 72;
bone-grafting, 73, 74;
skin-grafting, 74, 120, 121.

GRASSI, polymorphism due to food, 129.

Gregarines, a group of parasitic protozoa, development of, 41.

H

HAACKE, declaration that Hertwig is evolutionary, 135.

Haemoglobin, the red colouring matter of blood, 75.

Harmonic union in grafting, 70.

Heteromorphosis, explanation of, 49;
cases of, 51, 52;
embryonic cases, 54.

His, presence of foci in the germ, 13.

Histogenous, producing microscopical characters, 20.

Histology, study of the microscopical characters of cells and tissues, differentiation, 115.

Hydatina, determination of sex, 5;
temperature, 123.

Hydra, regeneration in, 47;
grafting of, 72.

Hydromedusae, a group of invertebrate animals, the typical members of which are branched colonies of polyps: Weismann's investigations on, viii, xii.

Hypoblast. See Epiblast, xvi.

Hypotrichous infusoria, a group of protozoa, 41.

I

_Ids_, hypothetical individual pieces, a number of which are supposed by Weismann to be present in the germplasm of every sexual cell, and each of which is supposed to contain the inherited material necessary for a complete new organism. It has been suggested that tiny beads seen within the chromosomes of a sexual cell are the _ids_, viii, 23, 33.

Idioblasts, Hertwig's name for hypothetical ultimate units of living matter, 22, 82; the ultimate units of living matter, according to De Vries, 22.

Idioplasm, as opposed to germplasm, which is the nuclear material of germ-cells; idioplasm is the nuclear material of tissue-cells, xi, 38.

Immortality, definition of, 82;
of germ-cells, ix;
of unicellular organisms, 17;
of germ-cells, 80.

Individuality of cells, 115.

Invagination, the infolding of a layer of cells, as, for instance, in the transformation of a blastosphere into a gastrula, xvii.

Isotropism, explained in footnote, 33.

K

Karyokinesis, a complicated process of nuclear division, xiv.

Katabolism, the formation of less complex chemical bodies by the agency of protoplasm, 86.

L

Labile, unstable, constantly changing, 38.

LANDOIS, experiments on transfusion of blood, 75.

LEIBNITZ, on immortality, 82.

LOEB, on heteromorphoses, 49;
on plasticity of animals, 117.

M

MAUPAS, experiments on sex of rotifers, 123.

Melons, determination of sex by temperature, 124.

Mesoblast, in the development of the coelomata, or three-layered multicellular animals; a third set of cells, the mesoblast, arises between the epiblast and hypoblast, xviii.

Monsters, relation of, to division of egg-cell, 63.

Mosaic theory of Roux, 56.

Morphoplasm, the general protoplasm of a cell, 35.

Multicellular organisms, those in which the body is composed of many cells, specialized in different directions; cell-division in, 43.

Mus, experiments on grafting among mice and rats, 74.

Myxomycetes, sometimes called 'slime fungi,' a group of low organisms, consisting of creeping masses of protoplasm with many nuclei, 33.

N

NAEGELI, biological units, 30;
cross-fertilization and grafting compared, 69;
heredity, 92;
environment in development, 104;
on plasticity of plants, 119;
on specific characters of eggs, 134.

Nais, regeneration in, 47.

Notochord, formation of, from unusual cells, 117.

Nucleus, a specialized portion of the protoplasm of cells, different in chemical and physical properties (see Chromatin, Chromosomes), as the bearer of heredity, 19.

NUSSBAUM, views on origin of germ-cells, 17.

Nutrition, influence of, on development, 2.

O

OLLIER, bone-grafting, 73.

Ontogeny, the development of an individual from the egg upwards, 9.

Osteoblasts, cells which are the active agents in bone-formation, 73.

Ovogenesis, the formation of egg-cells in the ovary, 13.

P

Pangenesis, Darwin's provisional hypothesis, that the sexual cells were composed of minute particles (gemmules), given off by all the cells of the body, 21.

Periosteum, a cellular sheath of bones, 73.

Physiological units, Herbert Spencer's name for hypothetical ultimate units of living matter, 22.

Pistachio, influence of temperature on, 121.

Plant-cells, mode of growth, 110.

Plasomes, Hertwig's name for theoretical units of protoplasm, 32.

Plasticity of plant tissues, 117, 119, 120.

Pluteus, a free-swimming larval stage in the development of echinoderms, 54.

Podophrya, reproduction of, 41.

Polymorphism, the appearance of the same species in several different forms in ants and social insects, 125.

PONFICK, on transfusion of blood, 75.

Preformation, identical with the original meaning of evolution, which see.

Prothallus, the leaf-shaped green organism that grows from the spore of a fern and produces sexual organs, 49.

Pseudopodia, extensions of protoplasm beyond the general contour of the cell, 41.

R

Radiolaria, a group of protozoa, 44.

Regeneration in plants and animals, 45, 47.

Rhipsalis grafted on Opuntia, 71.

ROUX, contrast between epigenesis and evolution, 6;
mosaic theory of, 56.

Rudiment, used here as a translation for the word _anlage_, which means the first plotting-out or beginning of a living structure. Darwin showed that rudimentary organs in adult creatures were for the most part vestiges of organs that had lost their use. In this treatise 'rudiment' is applied to an organ or structure in its incipient condition, whether that incipient state be visible in a young embryo, or a hypothetical structure in the germplasm, 6; latent rudiments, 37.

S

SACHS, on cells, 114;
on reaction and protoplasm, 133.

Salix purpurea, reproduction from galls, 51.

SCHMITT, bone-grafting, 74.

Segmentation, the early division of a developing egg, xvii.

Segmentation spheres, the cells resulting from the early divisions of a developing egg, separation of, by Wilson and Driesch, 60.

Segmentation cavity. See Blastosphere.

Sex, determination of, by temperature, 123, 124.

Sexual cells (spermatozoa in male, ova or egg-cells in female), the nucleated pieces of protoplasm which are the starting-point of the new generation in sexual reproduction, origin of, 18.

Soma, the body of a plant or animal as contrasted with the reproductive cells contained within it, 45.

Somatic cells, the cells of the soma; mortality of, 17.

SPENCER, HERBERT, controversy with Weismann on polymorphism in insects, 125.

Spermatogenesis, the formation of spermatozoa in the testis, 13.

Spontaneous generation, 2.

Stolon, a strand of tissue connecting the individuals of colonial animals, 46.

STRASBURGER, the value of the nucleus in heredity, 13, 18.

T

Termites, polymorphism in, 125.

Transfusion of blood, 75.

Transplantation of bone, 73, 74.

TREMBLEY, grafting of Hydra, 72.

Triton, an amphibian, experiments on the egg by constriction, 64.

Tubularia, experimental heteromorphoses, 51.

Tunicata, a group of marine animals clad with a leathery tunic, 14.

U

Unicellular organisms, animals (protozoa) and plants (protophyta) with the simplest structure, each being a single cell: immortality of, 17; division doubling in, 40.

Unit, definition of a biological, 30.

V

Vegetative affinity, 66 _et seq._

Vertebrates, regeneration of lost parts, 47.

VOECHTING, experiments on grafting, 70;
harmonic and disharmonic union, 70;
on cells, 114, 116;
on plasticity of plants, 117, 119;
on grafting, 120.

W

WEISMANN and preformation, 8-10;
caution against experiment, 12;
sources of his theory, 20, 21;
Hertwig's description of his theory, 22;
absence of proof for differentiating division, 34;
symmetry of egg and adult, 55;
immortality of germ-cells, 17, 80, 82;
germ-tracks, 83;
doubling division, 102;
controversy with Spencer, 125.

Willow, reproduction from slips, 46.

Wilson, separation of segmentation spheres of amphioxus egg, 60.

WOLFF, _Theoria Generationis_, 4.

Wounds, healing of, in relation to idioplasm, xii.

Y

Yolk, nutritive material stored in an egg-cell, xvi.

THE END.

BILLING AND SONS, PRINTERS, GUILDFORD.

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