I hardly need to say, that the histology, form, and size of elementary organs are equally an expression of their present or future physiological function. At least they prepare for this function by a specific sort of metabolism which sets in very early.
The whole sequence of individual morphogenesis has been divided by some embryologists into two different periods; there is a first period, during which the foundations of the organisation of the “type” are laid down, and a second period, during which the histo-physiological specifications are modelled out (von Baer, Götte, Roux). Such a discrimination is certainly justified, if not taken too strictly; but its practical application would encounter certain difficulties in many larval forms, and also, of course, in all plants.
Our mention of plants leads us to the last of our analytical results. If an animal germ proceeds in its development from a stage d to the stage g, passing through e and f, we may say that the whole of d has become the whole of f, but we cannot say that there is a certain part of f which is d, we cannot say that f is d + a. But in plants we can: the stage f is indeed equal to a + b + c + d + e + a in vegetable organisms; all earlier stages are actually visible as parts of the last one. The great embryologist, Carl Ernst von Baer, most clearly appreciated these analytical differences between animal and vegetable morphogenesis. They become a little less marked if we remember that plants, in a certain respect, are not simple individuals but colonies, and that among the corals, hydroids, bryozoa, and ascidia, we find analogies to plants in the animal kingdom; but nevertheless the differences we have stated are not extinguished by such reasoning. It seems almost wholly due to the occurrence of so many foldings and bendings and migrations of cells and complexes of cells in animal morphogenesis, that an earlier stage of their development seems lost in the later one; those processes are almost entirely wanting in plants, even if we study their very first ontogenetic stages. If we say that almost all production of surfaces goes on outside in plants, inside in animals, we shall have adequately described the difference. And this feature again leads to the further diversity between animals and plants which is best expressed by calling the former “closed,” the latter “open” forms: animals reach a point where they are finished, plants never are finished, at least in most cases.
I hope you will allow that I have tried to draw from descriptive and comparative embryology as many general analytical results as are possibly to be obtained. It is not my fault if there are not any more, nor is it my fault if the results reached are not of the most satisfactory character. You may say that these results perhaps enable you to see a little more clearly and markedly than before a few of the characters of development, but that you have not really learnt anything new. Your disappointment—my own disappointment—in our analysis is due to the use of pure description and comparison as scientific methods.
The Limits of Pure Description in Science
We have analysed our descriptions as far as we could, and now we must confess that what we have found cannot be the last thing knowable about individual morphogenesis. There must be something deeper to be discovered: we only have been on the surface of the phenomena, we now want to get to the very bottom of them. Why then occurs all that folding, and bending, and histogenesis, and all the other processes we have described? There must be something that drives them out, so to say.
There is a very famous dictum in the Treatise on Mechanics by the late Gustav Kirchhoff, that it is the task of mechanics to describe completely and in the most simple manner all the motions that occur in nature. These words, which may appear problematic even in mechanics, have had a really pernicious influence on biology. People were extremely pleased with them. “‘Describing’—that is just what we always have done,” they said; “now we see that we have done just what was right; a famous physicist has told us so.” They did not see that Kirchhoff had added the words “completely and in the most simple manner”; and moreover, they did not consider that Kirchhoff never regarded it as the ultimate aim of physics to describe thunderstorms or volcanic eruptions or denudations; yet it only is with such “descriptions” that biological descriptions of given bodies and processes are to be compared!
Physicists always have used both experiment and hypothetical construction—Kirchhoff himself did so in the most gifted manner. With these aids they have gone through the whole of the phenomena, and what they found to be ultimate and truly elemental, that alone may they be said to have “described”; but they have “explained” by the aid of elementalities what proved to be not elemental in itself.[8]
It is the method of the physicists—not their results—that morphogenesis has to apply in order to make progress; and this method we shall begin to apply in our next lectures. Physiology proper has never been so short-sighted and self-satisfied as not to learn from other sciences, from which indeed there was very much to be learned; but morphology has: the bare describing and comparing of descriptions has been its only aim for about forty years or more, and lines of descent of a very problematic character were its only general results. It was not seen that science had to begin, not with problematic events of the past, but with what actually happens before our eyes.