Driesch formulated an hypothesis of development in his Analytische Theorie, but has modified and changed it in several later contributions. It is difficult to give in a few words the subtile analysis which Driesch has made of the phenomena of development. His analytical theory rests on the dictum that the prospective value of each blastomere is a function of its position in the whole. By “function” is meant “a relation of dependence of a general unknown kind.” This idea is connected with the following one, viz. that any blastomere could be interchanged with any other one without altering the end-result. A few elementary processes are supposed to be “given” in the structure, or in the composition of the egg. Each elementary process is the outcome of a cause, and each elementary process must release the succeeding causes,—i.e. if the organization of the phase A is present, one of the causes of the next phase B is also then present. The first elementary process is the cleavage, that is initiated (“ausgelöst”) by the fertilization. After a fixed number of divisions has taken place, the cleavage process comes to an end. It has led to the production of a number of cells having similar nuclei but having a different plasma structure, and the result is the blastula stage. Organs whose formation starts from the blastula stage are called primary organs; the archenteron, the mesenchyme, the ciliated band, and the mouth of the sea-urchin embryo belong to this class. Secondary organs are those that arise from the primary ones, as the cœlom sacs, for instance, in the sea-urchin embryo. The primary organs are started by the setting free (“Auslösung”) of a new elementary process in the blastula, and later the secondary organs are started by new elementary processes that arise in the gastrula, which cannot appear until the gastrula stage itself is present as a starting-point. In other words, the elementary processes that are “given” in the egg can only come into action, or be set free after a certain stage has come into existence. This means that we must think of each organ that responds to a stimulus as having the possibility of receiving that stimulus, and also of answering to it. Even in inorganic nature every reaction must depend on a specific receptiveness and a specific answer. Driesch supposes that the receptivity is in the protoplasm, and the power to respond is in the nucleus of each cell. In this way he attempts to meet the difficulty that the nucleus is, in every cell, the bearer of the totality of all the “Anlagen”; but inasmuch as it is surrounded by a specific plasma, it is in a position to receive only certain stimuli, and can therefore only respond to certain causes.
In the specific nature of the cytoplasm of the cell lies the prospective potence of every organ, and the possibilities of each cell are limited by its plasma; the cell becomes more and more limited as development proceeds. It may be said, therefore, that in the course of development the cells become actually limited in their possibilities, although they may still retain within themselves, in the nucleus, the potentialities of the entire organism.
In the course of development each causal reaction brings about not only chemically specific differences, and thereby makes possible the introduction of new elementary processes, but the reaction also brings about by this very means a lessening of the possibilities of the cell, because each cell will now only respond to a more limited set of causes. We may say that the elementary process is not only the cause of the next change, but by virtue of its specific nature it is the beginning stage of the future reactions. Development proceeds from a few prearranged conditions, that are given in the structure of the egg, and these conditions, by reacting on each other, produce new conditions, and these may in turn react on the first ones, etc. With every effect there is at the same time a new cause, and the possibility of a new specific action, i.e. the development of a specific receiving station for stimuli. In this way there develops from the simple conditions existing in the egg the complicated form of the embryo.
In this brief summary of some of the essential features of Driesch’s hypothesis, I have omitted some parts that seem to me to go beyond the legitimate field of a scientific hypothesis,—such, for instance, as the causal harmony of the reactions; and other parts have been omitted because they are improbable in the light of more recent work. It would not be difficult to show that many difficulties beset each stage of the argument, or to show how slender a basis of fact there is to support some of the hypotheses. In fact, Driesch himself has modified very greatly some of the views of his Analytische Theorie in his later writings. The merits of the analysis should not be overlooked, however, since it is one of the first philosophical attempts to show how, in the light of recent discoveries, the process of epigenetic development may receive a causal interpretation. Even if the argument should break down, the hypothesis will remain an interesting contribution, opening the way to newer points of view in regard to the process of development. In later papers, especially in those dealing with the localization of morphogenetic processes, Driesch attempts to show that certain experimental results demonstrate that there is a vitalistic principle at work in the development of the organism from the egg, as well as in the process of regeneration. He bases his argument on the results of the experiment in which the gastrula of the sea-urchin egg is cut in two, as described already on page 234. The archenteron has not, at the time of the experiment, subdivided itself into its three characteristic parts. The posterior piece, that contains the posterior part of the archenteron (the anterior part having been removed with the anterior piece), produces a new whole embryo of smaller size, in which the archenteron is subdivided into three parts, that are in the same proportion to each other and to the whole embryo as are the same divisions of the normal archenteron. This proportionate formation of the parts of the archenteron on a smaller scale cannot, Driesch claims, be accounted for on any known chemical or physical principle. There must be, therefore, a different sort of principle involved, and this Driesch calls the vitalistic principle.
It may be pointed out that this illustration that Driesch has selected is only an example of all proportionate development, which many observers have described as taking place in pieces of embryos. It is only a striking case of what has been also known in many cases of regeneration, of small pieces producing whole structures, and there is nothing new or startling in this demonstration of a vitalistic principle. The fact may be stated in another way, viz. that the proportionate development of an organ is, within certain limits, self-determining, or is self-determined by its size. The vitalistic principle that Driesch sees demonstrated in these results is the now familiar process of a smaller piece producing the typical structure on a smaller scale; a phenomenon that a number of other writers had already called attention to as one of the most remarkable phenomena connected with the regeneration of pieces of an adult organism, or of an egg.
It is something of this same sort that the older zoologists must have had in mind when they spoke of “formative forces” as peculiar to living things. The use of the word “force” in this connection has often been objected to, and not without justification; since it seems to imply that the action is of the sort for which the physicist uses the word “force.” The fundamental question turns upon whether the development of a specific form is the outcome of one or more “forces,” or whether it is a phenomenon belonging to an entirely different category from anything known to the chemist and the physicist. If we state that it is the property of each kind of living substance to assume under certain conditions a more or less constant specific form, we only restate the result without referring the process to any better-known group of phenomena. If we attempt to go beyond this, and speculate as to the principles involved, we have very little to guide us. We can, however, state with some assurance that at present we cannot see how any known principles of chemistry or of physics can explain the development of a definite form by the organism or by a piece of the organism. Indeed, we may even go farther and claim that it appears to be a phenomenon entirely beyond the scope of legitimate explanation, just as are many physical and chemical phenomena themselves, even those of the simplest sort. To call this a vitalistic principle is, I think, misleading. We can do nothing more than claim to have discovered something that is present in living things which we cannot explain and perhaps cannot even hope to explain by known physical laws.
Wilson (’94) has also rejected Roux’s hypothesis of qualitative nuclear division, and adopts the view of the totipotence of the early blastomeres. He has also advanced the view that there is during development a progressive differentiation of the cells. In a later contribution (’96), he accepts “the view of Hertwig and of Driesch that the various degrees of partial development beginning with the echinoderm egg and culminating in the gasteropod may be due to varying conditions of the egg cytoplasm in the different forms.” Wilson points out that the series of forms represented at one end by amphioxus and at the other end by the ctenophore and the gasteropod may be brought under a common point of view, “for it is certain that development must be fundamentally of the same nature throughout the series, and the differences must be of secondary moment.”
If we reject, as several students of experimental embryology and of regeneration have done, the Roux-Weismann idea of the existence of pre-formed germs in the nucleus, and also the idea of Hertwig of the equivalency of the first-formed blastomeres, and Driesch’s vitalistic principle, what position can we take in regard to the problem of development? We may at least attempt to formulate our present position.
There must be assumed to exist in the egg an organization of such a kind that it can be divided and subdivided during the cleavage without thereby losing its primary character. The refusion of the cells after each division by means of protoplasmic connections indicates how this may be possible. The organization must be thought to be of such a kind that the factors determining the cleavage may be different from those that determine the median plane of the body. This is demonstrated by Pflüger’s experiment in which the position of the cleavage planes is changed, but the embryo appears in relation to the primary meridians. The first-formed blastomeres, that result from the division of the egg, do not seem to be strictly equivalent, but they appear to be in most cases, at least, totipotent. The characteristics of each part of the protoplasm may be a factor in determining what sort of structure may come from that part of the egg, but back of this lies the fundamental character of the protoplasm itself, that determines what each part, in its relation to the whole, can do. The division of the nucleus appears to be in all cases an exact quantitative division, and there is some evidence to show that the early nuclei are all equivalent,—or at least totipotent. The division of the protoplasm is often into unlike parts, and the kind of cytoplasm contained in a part may or may not limit the potencies of each part.
One of the most important facts in connection with the organization is that a part, if separated from the rest, may become a new whole, and this appears to be a fundamental peculiarity of living things. Analogies can be found, perhaps, in inorganic phenomena, as for instance a storm dividing into two or more parts and each developing a new storm centre of its own, or when a suspended drop is divided and each half becomes a new sphere; but these comparisons lack some of the essential features of the organic phenomenon.