Part 6 (2/2)

VON GaRTNER: _Versuche und Beobachtungen ueber die b.a.s.t.a.r.derzeugung im Pflanzenreich_, 1849.

LeOPOLD OLLIER: _Recherches experimentales sur la production artificielle des os au moyen de la transplantation du perioste, etc._ _Journal de la physiologie de l'homme et des animaux_, tom. ii., 1859, pp. 1, 169, 468.

LeOPOLD OLLIER: _Recherches experimentales sur les greffes osseuses_. The same, tom. iii., p. 88, 1860.

PAUL BERT: _Recherches experimentales pour servir a l'histoire de la vitalite propre des tissus animaux_. _Annales des Sciences naturelles, Ser.

V., Zoologie_, tom. v., 1886.

VON RECKLINGHAUSEN: _Die Wiedererzeugung (Regeneration) und die Ueberpflanzung (Transplantation)_. _Handbuch d. Allgem. Pathologie des Kreislaufs aus Deutsche Chirurgie_, 1883.

TREMBLEY: _Memoires pour servir a l'histoire d'un genre de Polypes d'eau douce_, 1744.

LANDOIS: _Die Transfusion des Blutes_; Leipzig, 1875.

ADOLF SCHMITT: _Ueber Osteoplastik in klinischer und experimenteller Beziehung_. _Arbeiten aus der chirurgischenklinik der Konigl. Universitat, Berlin._

PONFICK: _Experimentelle Beatrage zur Lehre von der Transfusion_.

_Virchow's Archiv._, vol. lxii.

BERESOWSEY: _Ueber die histologischen Vorgange bei der Transplantation von Hautstucken auf Thiere einer anderen Species_. _Ziegler's Beitrage zur pathologischen Anatomie und zur allgemeinen Pathologie_; Jena, 1893.

PART II.

THOUGHTS TOWARDS A THEORY OF THE DEVELOPMENT OF ORGANISMS.[17]

Now that criticism of the germplasm theory has given us a bias in the right direction, it is necessary to map out more clearly the path along which solution of the problem may be sought. In general terms, our problem is the necessary origin from an egg, always of the same organism, with its manifold characters, and the explanation must avoid the attribution to the egg of characters foreign to its nature as a cell. This is the more necessary as Weismann objects to the supposition that cell-division is doubling, holding that the supposition allows neither an explanation, nor even the beginning of an explanation, of the differences that arise among cells while the differentiation of the body occurs. 'Any explanation must in the first place account for this differentiation,' says Weismann (_Germplasm_, p. 224); 'that is to say, the diversity which always exists amongst these cells and groups of cells arising from the ovum must be referred to some definite principle. In fact, no one could even look at it as giving a partial solution of the problem, if differentiation is supposed to be due to that part alone of the germplasm becoming active which is required for the production of the cell or organ under consideration. But the higher we ascend in the organic world, the more limited does the power of producing the whole from separate cells become, and the more do the numerous and varied differentiations of the soma claim our attention and require an explanation in the first instance. The presence of idioplasm in all parts containing the primary const.i.tuents does not help us in this respect.'

With this I cannot agree. Naturally, Naegeli, De Vries, Driesch and I a.s.sume that, of the many rudiments present in every cell, only some come to activity in each special case, and that the selection of those that become active is due to causes arising in the course of development. Our conception of the nature of these causes, and of their place of origin, is diametrically opposed to Weismann's.

Weismann would make the causes of this orderly development of the rudiments reside in the germplasm itself; for he considers that to be not only the material but the motive force of the course of development. According to him, every cell _must_ have become what it is, because it was provided only with the definite rudiments a.s.signed it beforehand, according to the plan of the development of the germplasm.

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.

One may group together as _centrifugal causes_ of the process of development the characters of the fertilised cells and the interrelations between the products of their divisions, and distinguish them from the _centripetal causes_, or motive forces that are provided by the action of surrounding things. None the less, it must be borne in mind that there is no sharp distinction between centrifugal and centripetal forces. On page 86 I showed how what is external in one stage of the process becomes internal in the succeeding stage. The external constantly is becoming internal, and the sum of the internal factors increases only at the expense of external factors.

From the physiological point of view I regard the divergent differentiation of cells as a reaction of the organic material to unlike impelling forces--that is, to factors shown by experimental physiology to be actually present and to rule the building up of the organism. 'It were superfluous to detail,' as Naegeli says, 'how continually other forces external to the idioplasm, but belonging to the individual, influence the idioplasm; every cell, indeed, as it grows and divides, takes up a definite place in the growing whole, and finds itself in a peculiar combination of conditions of organisation.' 'Not only influences within the individual affect the idioplasm, as that may be altered by external influences, and so may be forced to grow in a new direction.' 'The influence of surroundings in determining which of the rudiments contained in the idioplasm shall achieve development is shown in the following example: it depends on their nutrition whether certain trees shall bear foliage or flowers; while in an unpropitious climate many plants refuse to bear flowers at all, but content themselves with vegetative reproduction.'

This principle indicates the path along which explanation of the differentiation of cells is to be sought. Although in no single case is it yet possible to refer a known action to its appropriate cause--in other words, to show a definite stimulus producing a definite reaction upon the rudiment--this failure is not to be attributed to error in the principle.

It is the natural result of the enormous difficulties besetting an attempt to understand the highly involved events of development. We can only ask whether or no our general principle is harmonious with the facts displayed in nature.

In the following pages I shall try to develop this view, taking, as formerly, a few instances. I shall now proceed further with suggestions I made in my treatise on _Old and New Theories of Development_. I start from the conception that the ovum is an organism that multiplies by division into numerous organisms like itself. I shall explain the gradual, progressive organisation of the whole organism as due to the influences upon each other of these numerous elementary organisms in each stage of the development. I cannot regard the development of any creature as a mosaic work. I hold that all the parts develop in connection with each other, the development of each part always being dependent upon the development of the whole.

The power of the egg to multiply by division is a chief and most important factor in the production of complexity during the course of development. It is only because the nuclear material, by a series of intricate, chemical changes, a.s.similates reserve material from the egg and oxygen from the atmosphere that it can give rise to continually increasing complexity within itself. The increase in bulk results in a cleavage into two, four, eight, and sixteen pieces, and so forth. The cleavage produces a constantly changing distribution in s.p.a.ce of the nuclear material. The two, four, eight, and sixteen nuclei that arise by division diverge from each other and take up new positions inside the egg, in definite relations to each other. At first the particles of the egg were arranged around the fertilised nucleus, which was a single centre of force; they become grouped around as many centres of forces as there are nuclei, and so become segregated into as many cells. Clearly enough, the egg, in its single-celled condition, changes its quality in a marked degree when it becomes multicellular, even although the change has occurred by doubling division.

This, so clear in the early stages of development, continues to occur throughout the later stages of growth. The continued cell-multiplication causes not only changes of bulk, but also from time to time changes in quality; for each shape is bound up with definite conditions. When the conditions alter, the organic material, by its power of reaction, changes its shape in a corresponding fas.h.i.+on.

As the nature of architectural plans depends upon the properties of the wood, stone, or iron, as they must correspond with the material to be employed (_i.e._, the span of a roof, the construction of a bridge depend upon the material in shape and weight), so the nature of the organic material determines to a large extent the shapes a.s.sumed in the course of growth.

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