ABSTRACT
T H E N A T U R E OF L I F E of the lens or the retina of an eye unless we realize the relations between these parts and the eye as a whole functioning unit.It is perhaps clear on first principles that the development of entities which are organized in this sense must demand something more than a purely atomistic theory. We have then in the study of development rather the opposite situation to that which confronts us in the study of heredity. Whereas the latter has seemed, since Mendel's day, to cry aloud for an atomistic theory, the former seems to demand organismic or non-atomistic theories. But, as we saw in the last chapter, it is necessary to supplement the atomistic theories of heredity by some considerations of the continuum type, and one of the points which we shall have to explore in this chapter is whether, or how far, atomistic notions are valuable as correctives to purely continuum theories of biological organization.The first major fact to be noted about biological organization is that it is essentially a dynamic affair, involving the lapse of time. The organization of an animal arises gradually as the egg develops into the adult, and to understand the organization we must follow the changes and processes by which it comes into being. The first experimental studies of such questions led to theories which were very far indeed from being atomistic. In fact they were most often completely at the other end of the spectrum of possible types of theory.For instance, towards the end of the last century, Driesch performed some of the earliest experiments on developing eggs. He took the newly fertilized eggs of sea urchins and cut them in fragments. When these were allowed to develop he found that each fragment seemed capable of producing a complete and normal embryo. Driesch concluded that no mechanistic, let alone atomistic, system could possibly explain such a result, and that it was necessary to suppose that development is controlled by some non-material agency, which he called an entelechy. His theory was in fact a vitalistic one.We need not, however, pursue it much further, since the actual facts of the situation are not quite as Driesch thought. In practice, when he cut his eggs into fragments, he sliced them always from top to bottom. If he had turned them on the side,
D E V E L O P M E N T and cut them through so as to separate the top half from the bottom half, he would not have found that these fragments gave complete embryos. Since his day, such experiments have been very extensively performed, particularly by a school of Swedish biologists led by Runnstrom and Horstadius. They have shown that the upper half of the sea urchin egg has a character quite different from that of the lower half. Normal embryos arise only when the upper, or as it is called ‘animal’, character is in correct balance with the lower, or ‘vegetative’, character. This balance can be achieved even in certain abnormal combinations of fragments. For instance, if a small portion of the upper end of the egg (less than half of the whole egg) is combined with a suitably sized small portion from the lower end, the egg has a correct balance between the properties of its different regions, and these interact with one another to control normal development. This brings the system into the range which can be accounted for in terms of organismic theories, and removes the need to postulate any non-material vitalistic principle.The great importance in development of interaction between different parts of the system was particularly brought home to biologists by some classical experiments by the German embryologist Spemann. Although Spemann’s work was perhaps not so revolutionary in its consequence as that of Mendel, his experiments can not unjustly be regarded as the starting point for the growth of modern ideas on development, just as Mendel’s were the origin of modern genetics. They were carried out before the Swedish work on sea urchin eggs referred to in the last paragraph, and in fact provided a stimulus for them.The basic experiment was very simple. It was performed on the eggs of newts. These eggs, like all others, begin their development by becoming divided up into a large number of cells, each of which is considerably smaller than the original egg at the time of fertilization. This packet of moderately small cells becomes arranged as a hollow sphere. One can put the point of the experiment in its simplest possible terms with the aid of a diagram like that in Fig. 6. This shows the egg, as seen from the side as it floats in water. What Spemann showed was that the two quadrants labelled 1 and 2 are at first com-
FIG. 6 The newt's egg, seen from the side, and marked off into four quadrants.
