The Biological Problem of To-day: Preformation Or Epigenesis? / The Basis of a Theory of Organic Development

by Oscar Hertwig

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, assimilates 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 space 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 fashion.

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 assumed in the course of growth.

Shape in many respects appears to be a function of growth in an organic material.

A few examples will make clear this important relation. A limit is set to increase in the size of a blastosphere by the nature of the material of its walls. Its wall is a membrane, composed of one or more layers of cells; that this may preserve its curvature, a definite pressure from within must be maintained, proportioned to the cohesive force of the cells; at the same time the wall of the sphere must be able to withstand the strain and pressure put upon it by external forces. All these, and many other factors less easy to conceive, must be delicately adjusted to one another. If in any direction a definite limit be exceeded, then either the structure will be destroyed by disintegration of the component parts, or a new shape will be assumed. The latter is the event in the case of a living substance capable of reaction. The blastosphere, growing beyond its limits, folds into a cup-shaped organism. Did we know all the influences affecting the wall of the blastosphere, then we would understand the causes by which growth beyond a definite limit must result in invagination. From the occurrence of the gastrula in all the divisions of the animal kingdom, we may conclude that it is a temporary phase, inevitable in the growth of animals.

There may be noticed here a second connection between shape and organic growth, exceedingly simple in its nature, but of fundamental importance in its consequences. It may be stated in this saying: Growth always must be such as to produce the greatest possible extension of surface. The reason of this is simple, depending on the different natures of inorganic material and living organic material.

A crystal in its mother liquor grows by attracting new particles and depositing them upon its outer surface, according to the kind of crystallisation peculiar to the material of which it is composed. These particles, once crystallised, retain their position even when new layers are deposited on their outer surfaces, and remain unchanged, perhaps, like rock crystals, for thousands of years, until changed outer forces loosen the bonds that bind them.