The Biological Problem of To-day: Preformation Or Epigenesis?. Oscar Hertwig

mass to the growing organism. Weismann and some others think that there is evidence for a difference in the nature of the process, which may in some cases correspond to his distinction between doubling and differentiating divisions, but it may be said at once that the record of observations is yet too conflicting for any such general interpretation.

      Along with the increase in bulk and distribution of the nuclear matter, there goes an increase in bulk and segregation of the ordinary protoplasm. The simplicity of the actual development of most back-boned animals is disguised by provision for the nutrition of the growing embryo. In a large number of cases, as, for instance, in birds and reptiles, the egg-cell, a microscopic structure at its first formation, is bloated out into the large eggs with which we are familiar, by the addition of quantities of food-yolk. These eggs, although morphologically single cells, do not divide as cells. A small disc of protoplasm, surrounding the nucleus, floats upon the surface of the yellow yolk, and, when the nucleus divides, furrows appear in this between the daughter-nuclei, but stretch very little way into the inert food-yolk. The subsequent marshalling of the cells is disguised by their association with a preponderating mass of inert material. In a far-distant period in the history of evolution, the eggs of mammals like man were large, and contained, as in the lowest existing mammals, a store of food-yolk. Now the food-yolk is not formed, as the developing embryo obtains its nourishment from the blood of the mother. But the course of development is distorted, partly as a legacy from the old large-yolked condition, and still more to suit the new method of nutrition. Some of the simpler animals even among existing vertebrates still exhibit a marshalling of cells common among invertebrates, and to be traced under the complications of higher forms. In these, now, as in the marine ancestors of all the vertebrates, the fertilised egg is a tiny cell provided with very little yolk, and set adrift in the sea-water. The first division of the nucleus, and each subsequent division of the daughter-nuclei, is at once followed by division or segmentation of the whole cell. The plane between the two cells thus formed is called the first cleavage-plane, and is regarded as vertical. The second cleavage-plane is at right angles to the first, and is also vertical, so that the little embryo consists of four cells, all on the same horizontal plane. The third cleavage-plane is horizontal, and divides the four cells into an upper and lower tier of four cells. In the course of a series of divisions the eight cells come to form a hollow sphere—the blastosphere—enclosing a cavity known as the cleavage or segmentation cavity.

      The first great modelling then occurs. At one side the single layer of cells, of which the wall of the blastosphere is composed, begins to bend inwards, just as a dimple forms in a hollow india-rubber ball if a pin-prick allow some of the contained air to escape. Further cell-divisions occur, and the invagination becomes deeper, until the invaginating wall nearly touches the wall which has retained its primitive position. The embryo has thus become a hollow cup, the walls of which are double. The cup elongates, and its mouth, originally wide open, becomes more and more narrow, until it forms a small pore opening into an elongated blind sack. The embryo in this stage is known as a gastrula. The central cavity becomes the cavity of the gut; the pore leading into it marks the hind end of the future animal, in the case of vertebrates, and is known as the blastopore. The layer of cells lining the cavity of the sack is known as the hypoblast, and gives rise chiefly to the cells lining the alimentary canal of the future animal. The outer layer of cells is known as the epiblast, and forms the outer layer of the skin, and, along the future dorsal line, gives rise to the nervous system. The muscles and skeleton and the reproductive cells arise from a set of cells known as the mesoblast, that are formed chiefly from the hypoblast, and that push their way in between the hypoblast and epiblast.

      This general course of development may be traced in all members of the vertebrate group, and, with slight modifications, may be applied to a large number of invertebrates. As the modelling of the general contour of the whole body and of the separate organs proceeds, the protoplasm of the cells gradually assumes the characters of the substance of muscle-cells, liver-cells, nerve-cells, blood-cells, and so forth. The problem of this book will become clearer if it be considered with special reference to what goes on in these early stages. Hertwig says that all the cells of the epiblast, hypoblast, mesoblast, and of the later derivatives of these primary layers, receive identical portions of germplasm by means of doubling nuclear divisions. The different positions, relations to each other and to the whole organism, and to the environment in the widest sense of the term, cause different sides of the capacities of the cells to be developed, but they retain in a latent form all the capacities of the species. Weismann says that the nuclear divisions are differentiating, and that the microcosms of the germplasm, in accordance with their inherited architecture, gradually liberate different kinds of determinants into the different cells, and that, therefore, the essential cause of the specialisation of the organism was contained from the beginning in the germplasm.

THE BIOLOGICAL PROBLEM OF TO-DAY

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INTRODUCTION.

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      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.

      The opposing views of to-day were in existence centuries ago, and they are known in the history of science as the theory of preformation or evolution, and the theory of epigenesis. That most of the great biologists of the seventeenth and eighteenth centuries were decided upholders of evolution was the natural result of the contemporary knowledge of facts. For they knew only the external signs of the process of development. All they saw was the embryo becoming adult, the bud growing out into a blossom, as the result of a process in which nutrition transformed smaller to greater parts. 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. That the material eye failed to recognise the miniature they attributed to the imperfection of our senses, and to the extreme minuteness and resulting opacity of the object.

      That it might satisfy our human craving for final causes, the theory of preformation had to be accompanied by a corresponding explanation of the origin of the miniatures. Biologists had already abandoned the error of such spontaneous generation as the origin of flies from decaying meat, and, in its place, had accepted the doctrine of the continuity of life, formulating it in the phrase, Omne vivum e vivo (Each life from a life), and in the similar phrase, Omne vivum ex ovo (Each life from an egg). One creature issued from another, within which it had lain as a germ, and the series was continuous. Thus, the theory of preformation gave rise to the conception that living things were a series of cases or wrappings, germ folded within germ. The origin of life was relegated to the beginning, at the creation of the world: it became the work of a supernatural Creator, who, when He formed the first creatures, formed with them, and placed within them, the germs of all subsequent creatures.

      To reckon at their proper value the theory of preformation, and, still more, the doctrine of enfolded germs, the standard of appreciation must not be the present range of our knowledge. They must be viewed historically, in the light of the knowledge of these days.

      Nowadays it is not so much pure reason as a wider empirical knowledge of nature, with its consequent transformation of ideas, that makes the doctrine of enfoldment difficult. Abstract thought sets no limit to smallness or greatness; for mathematics deals with the infinitely small and with the infinitely great. So long as actual observation had not determined the limits of minuteness in the cases in question, there were no logical difficulties in the doctrine of enfolded germs. The biology of earlier centuries had not our empirical standard. What appeared then to be a simple organic material