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]
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).
Fig. 5.—Illustration of Experiments on Echinus.
| a1 and b1. | Normal gastrula and normal pluteus. |
| a2 and b2. | “Half”-gastrula and “half”-pluteus, that ought to result from one of the first two blastomeres, when isolated, according to the theory of “evolutio.” |
| a3 and b3. | The small but whole gastrula and pluteus that actually do result. |
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.