The Science and Philosophy of the Organism — Key Ideas to Explore

at was to follow. But it was Wilhelm Roux,[13] now professor of anatomy at Halle, who entered the field with a thoroughly elaborated programme, who knew not only how to state the problem analytically, but also how to attack it, fully convinced of the importance of what he did. “Entwickelungsmechanik,”--mechanics of development--he called the “new branch of anatomical science” of which he tried to lay the foundations.

[11] *Unsere Körperform*, Leipzig, 1875.

[12] *Die Entwickelungsgeschichte der Unke*, Leipzig, 1875.

[13] *Gesammelte Abhandlungen*, Leipzig, 1895. Most important theoretical papers:--*Zeitschr. Biolog.* 21, 1885; *Die Entwickelungsmechanik der Organismen*, Wien, 1890; *Vorträge und Aufsätze über Entwickelungsmechanik*, Heft i., Leipzig, 1905.

I cannot let this occasion pass without emphasising in the most decided manner how highly in my opinion Roux’s services to the systematic exploration of morphogenesis must be esteemed. I feel the more obliged to do so, because later on I shall have to contradict not only many of his positive statements but also most of his theoretical views. He himself has lately given up much of what he most strongly advocated only ten years ago. But Roux’s place in the history of biological science can never be altered, let science take what path it will.

It is not the place here to develop the logic of experiment; least of all is it necessary in the country of John Stuart Mill. All of you know that experiment, by its method of isolating the single constituents of complicated phenomena, is the principal aid in the discovery of so-called causal relations. Let us try then to see what causal relations Wilhelm Roux established with the aid of morphogenetic experiment.

THE WORK OF WILHELM ROUX

We know already that an hypothesis about the foundation of individual development was his starting-point. Like Weismann he supposed that there exists a very complicated structure in the germ, and that nuclear division leads to the disintegration of that structure. He next tried to bring forward what might be called a number of indicia supporting his view.

A close relation had been found to exist in many cases between the direction of the first cleavage furrows of the germ and the direction of the chief planes of symmetry in the adult: the first cleavage, for instance, very often corresponds to the median plane, or stands at right angles to it. And in other instances, such as have been worked out into the doctrine of so-called “cell-lineages,” typical cleavage cells were found to correspond to typical organs. Was not that a strong support for a theory which regarded cellular division as the principal means of differentiation? It is true, the close relations between cleavage and symmetry did not exist in every case, but then there had always happened some specific experimental disturbances, *e.g.* influences of an abnormal direction of gravity on account of a turning over of the egg, and it was easy to reconcile such cases with the generally accepted theory on the assumption of what was called “anachronism” of cleavage.

But Roux was not satisfied with mere indicia, he wanted a proof, and with this intention he carried out an experiment which has become very celebrated.[14] With a hot needle he killed one of the first two blastomeres of the frog’s egg after the full accomplishment of its first cleavage, and then watched the development of the surviving cell. A typical half-embryo was seen to emerge--an organism indeed, which was as much a half as if a fully formed embryo of a certain stage had been cut in two by a razor. It was especially in the anterior part of the embryo that its “halfness” could most clearly be demonstrated.

[14] *Virchow’s Archiv.* 114, 1888.

That seemed to be a proof of Weismann’s and Roux’s theory of development, a proof of the hypothesis that there is a very complicated structure which promotes ontogeny by its disintegration, carried out during the cell divisions of embryology by the aid of the process of nuclear division, the so-called “karyokinesis.”

To the dispassionate observer it will appear, I suppose, that the conclusions drawn by Roux from his experiment go a little beyond their legitimate length. Certainly some sort of “evolutio” is proved by rearing half the frog from half the egg. But is anything proved, is there anything discovered at all about the nucleus? It was only on account of the common opinion about the part it played in morphogenesis that the nucleus had been taken into consideration.

Things soon became still more ambiguous.

THE EXPERIMENTS ON THE EGG OF THE SEA-URCHIN

Roux’s results were published for the first time in 1888; three years later I tried to repeat his fundamental experiment on another subject and by a somewhat different method. It was known from the cytological researches of the brothers Hertwig and Boveri that the eggs of the common sea-urchin (*Echinus microtuberculatus*) are able to stand well all sorts of rough treatment, and that, in particular, when broken into pieces by shaking, their fragments will survive and continue to segment. I took advantage of these facts for my purposes. I shook the germs rather violently during their two-cell stage, and in several instances I succeeded in killing one of the blastomeres, while the other one was not damaged, or in separating the two blastomeres from one another.[15]

[15] *Zeitschr. wiss. Zool.* 53, 1891.

Let us now follow the development of the isolated surviving cell. It went through cleavage just as it would have done in contact with its sister-cell, and there occurred cleavage stages which were just half of the normal ones. The stage, for instance, which corresponded to the normal sixteen-cell stage, and which, of course, in my subjects was built up of eight elements only, showed two micromeres, two macromeres and four cells of medium size, exactly as if a normal sixteen-cell stage had been cut in two; and the form of the whole was that of a hemisphere. So far there was no divergence from Roux’s results.

The development of our Echinus proceeds rather rapidly, the cleavage being accomplished in about fifteen hours. I now noticed on the evening of the first day of the experiment, when the half-germ was composed of about two hundred elements, that the margin of the hemispherical germ bent together a little, as if it were about to form a whole sphere of smaller size, and, indeed, the next morning a *whole* diminutive blastula was swimming about. I was so much convinced that I should get Roux’s morphogenetical result in all its features that, even in spite of this whole blastula, I now expected that the next morning would reveal to me the half-organisation of my subject once more; the intestine, I supposed, might come out quite on one side of it, as a half-tube, and the mesenchyme ring might be a half one also.

But things turned out as they were bound to do and not as I had expected; there was a typically *whole* gastrula on my dish the next morning, differing only by its small size from a normal one; and this *small but whole* gastrula was followed by a whole and typical small pluteus-larva (Fig. 5).

That was just the opposite of Roux’s result: one of the first two blastomeres had undergone a half-cleavage as in his case, but then it had become a whole organism by a simple process of rearrangement of its material, without anything that resembled regeneration, in the sense of a completion by budding from a wound.

If one blastomere of the two-cell stage was thus capable of performing the morphogenetical process in its totality, it became, of course, *impossible* to allow that nuclear division had separated any sort of “germ-plasm” into two different halves, and not even the protoplasm of the egg could be said to have been divided by the first cleavage furrow into unequal parts, as the postulate of the strict theory of so-called “evolutio” had been. This was a very important result, sufficient alone to overthrow at once the theory of ontogenetical “evolutio,” the “Mosaiktheorie” as it had been called--not by Roux himself, but according to his views--in its exclusiveness.

After first widening the circle of my observations by showing that one of the first four blastomeres is capable of performing a whole organogenesis, and that three of the first four blastomeres together result in an absolutely perfect organism, I went on to follow up separately one of the two fundamental problems which had been suggested by my first experiment: was there anything more to find out about the importance or unimportance of the single *nuclear* divisions in morphogenesis?[16]

[16] *Zeitschr. wiss. Zool.* 55, 1892.

By raising the temperature of the medium or by diluting the sea-water to a certain degree it proved at first to be possible to alter in a rather fundamental way the type of the cleavage-stages without any damage to the resulting organism. There may be no micromeres at the sixteen-cell stage, or they may appear as early as in the stage of eight cells; no matter, the larva is bound to be typical. So it certainly is not necessary for all the cleavages to occur just in their normal order.

But of greater importance for our purposes was what followed. I succeeded in pressing the eggs of Echinus between two glass plates, rather tightly, but without killing them; the eggs became deformed to comparatively flat plates of a large diameter. Now in these eggs all nuclear division occurred at right angles to the direction of pressure, that is to say, in the direction of the plates, as long as the pressure lasted; but the divisions began to occur at right angles to their former direction, as soon as the pressure ceased. By letting the pressure be at work for different times I therefore, of course, had it quite in my power to obtain cleavage types just as I wanted to get them. If, for instance, I kept the eggs under pressure until the eight-cell stage was complete, I got a plate of eight cells one beside the other, instead of two rings, of four cells each, one above the other, as in the normal case; but the next cell division occurred at right angles to the former ones, and a sixteen-cell stage, of two plates of eight cells each, one above the other, was the result. If the pressure continued until the sixteen-cell stage was reached, sixteen cells lay together in one plate, and two plates of sixteen cells each, one above the other, were the result of the next cleavage.

We are not, however, studying these things for cytological, but for morphogenetical purposes, and for these the cleavage phenomenon itself is less important than the organogenetic result of it: all our subjects resulted in *absolutely normal* organisms. Now, it is clear, that the spatial relations of the different nuclear divisions to each other are anything but normal, in the eggs subjected to the pressure experiments; that, so to say, every nucleus has got quite different neighbours if compared with the “normal” case. If that makes no difference, then there *cannot* exist any close relation between the single nuclear divisions and organogenesis at all, and the conclusion we have drawn more provisionally from the whole development of isolated blastomeres has been extended and proved in the most perfect manner. There ought to result a morphogenetic chaos according to the theory of real “evolutio” carried out by nuclear division, if the positions of the single nuclei were fundamentally changed with regard to one another (Fig. 6). But now there resulted not chaos, but the normal organisation: therefore it was disproved in the strictest way that nuclear divisions have any bearing on the origin of organisation; at least as far as the divisions during cleavage come into account.

On the egg of the frog (O. Hertwig), and on the egg of annelids (E. B. Wilson), my pressure experiments have been carried out with the same result.[17]

[17] In the pressure experiments I had altered the relative position of the nuclei *in origine*. In later years I succeeded in disturbing the arrangement of the fully formed cells of the eight-cell stage, and in getting normal larvæ in spite of that in many cases. But as this series of experiments is not free from certain complications--which in part will be understood later on (see page 73)--it must suffice here to have mentioned them. (For further information see my paper in *Archiv. f. Entwickelungsmechanik*, xiv., 1902, page 500.)

ON THE INTIMATE STRUCTURE OF THE PROTOPLASM OF THE GERM

Nuclear division, as we have seen, cannot be the basis of organogenesis, and all we know about the whole development of isolated blastomeres seems to show that there exists nothing responsible for differentiation in the protoplasm either.

But would that be possible? It cannot appear possible on a more profound consideration of the nature of morphogenesis, it seems to me: as the untypical agents of the medium cannot be responsible in any way for the origin of a form combination which is most typical and specific, there must be somewhere in the egg itself a certain factor which is responsible at least for the general orientation and symmetry of it. Considerations of this kind led me, as early as 1893,[18] to urge the hypothesis that there existed, that there *must* exist, a sort of intimate structure in the egg, including polarity and bilaterality as the chief features of its symmetry, a structure which belongs to every smallest element of the egg, and which might be imagined by analogy under the form of elementary magnets.[19] This hypothetic structure could have its seat in the protoplasm only. In the egg of echinoderms it would be capable of such a quick rearrangement after being disturbed, that it could not be observed but only inferred logically; there might, however, be cases in which its real discovery would be possible. Indeed Roux’s frog-experiment seems to be a case where it is found to be at work: at least it seems very probable to assume that Roux obtained half of a frog’s embryo because the protoplasm of the isolated blastomere had preserved the “halfness” of its intimate structure, and had not been able to form a small whole out of it.

[18] *Mitteil. Neapel. 11, 1893.*

[19] But the elementary magnets would have to be bilateral!

Of course it was my principal object to verify this hypothesis, and such verification became possible in a set of experiments which my friend T. H. Morgan and myself carried out together,[20] in 1895, on the eggs of ctenophores, a sort of pelagic animals, somewhat resembling the jelly-fish, but of a rather different inner organisation. The zoologist Chun had found even before Roux’s analytical studies, that isolated blastomeres of the ctenophore egg behave like parts of the whole and result in a half-organisation like the frog’s germ does. Chun had not laid much stress on his discovery, which now, of course, from the new points of view, became a very important one. We first repeated Chun’s experiment and obtained his results, with the sole exception that there was a tendency of the endoderm of the half-larva of Beroë to become more than “half.” But that was not what we chiefly wanted to study. We succeeded in cutting away a certain mass of the protoplasm of the ctenophore egg just before it began to cleave, without damaging its nuclear material in any way: in all cases, where the cut was performed at the side, there resulted a certain type of larvae from our experiments which showed exactly the same sort of defects as were present in larvae developed from one of the first two blastomeres alone.

[20] *Arch. Entw. Mech.* 2, 1895.

The hypothesis of the morphogenetic importance of *protoplasm* had thus been proved. In our experiments there was all of the nuclear material, but there were defects on one side of the protoplasm of the egg; and the defects in the adult were found to correspond to these defects in the protoplasm.

And now O. Schultze and Morgan succeeded in performing some experiments which directly proved the hypothesis of the part played by protoplasm in the subject employed by Roux, *viz.*, the frog’s egg. The first of these investigators managed to rear two whole frog embryos of small size, if he slightly pressed the two-cell stage of that form between two plates of glass and turned it over; and Morgan,[21] after having killed one of the first two blastomeres, as was done in the original experiment of Roux, was able to bring the surviving one to a half or to a whole development according as it was undisturbed or turned. There cannot be any doubt that in both of these cases, it is the possibility of a rearrangement of protoplasm, offered by the turning over, which allows the isolated blastomere to develop as a whole. The regulation of the frog’s egg, with regard to its becoming whole, may be called facultative, whilst the same regulation of the egg of Echinus is obligatory. It is not without interest to note that the first two blastomeres of the common newt, *i.e.* of a form which belongs to the other class of Amphibia, after a separation of *any* kind, *always* develop as wholes, their faculty of regulation being obligatory, like that of Echinus.

[21] *Anat. Anz.* 10, 1895.

Whole or partial development may thus be dependent on the power of regulation contained in the intimate polar-bilateral structure of the protoplasm. Where this is so, the regulation and the differences in development are both connected with the chief relations of symmetry. The development becomes a half or a quarter of the normal because there is only one-half or one-quarter of a certain structure present, one-half or one-quarter with regard to the very wholeness of this structure; the development is whole, in spite of disturbances, if the intimate structure became whole first. We may describe the “wholeness,” “halfness,” or “quarterness” of our hypothetic structure in a mathematical way, by using three axes, at right angles to one another, as the base of orientation. To each of these, *x*, *y*, and *z*, a certain specific state with regard to the symmetrical relations corresponds; thence it follows that, if there are wanting all those parts of the intimate structure which are determined, say, by a negative value of *y*, by minus *y*, then there is wanting half of the intimate structure; and this halfness of the intimate structure is followed by the halfness of organogenesis, the dependence of the latter on the intimate structure being established. But if regulation has restored, on a smaller scale, the whole of the arrangement according to all values of *x*, *y* and *z*, development also can take place completely (Fig. 7).

I am quite aware that such a discussion is rather empty and purely formal, nevertheless it is by no means without value, for it shows most clearly the differences between what we have called the intimate structure of germs, responsible only for the general symmetry of themselves and of their isolated parts, and another sort of possible structure of the egg-protoplasm which we now shall have to consider, and which, at the first glance, seems to form a serious difficulty to our statements, as far at least as they claim to be of general importance. The study of this other sort of germinal structure at the same time will lead us a step farther in our historical sketch of the first years of “Entwickelungsmechanik” and will bring this sketch to its end.

ON SOME SPECIFICITIES OF ORGANISATION IN CERTAIN GERMS