Editor’s note: The late Italian geneticist Giuseppe Sermonti (1925-2018) is the author of the 2005 book Why Is a Fly Not a Horse? This year we observe the 100th anniversary of his birth. To mark the occasion, we are offering a FREE digital copy of his book if you sponsor Science and Culture Today at any level. Please do so now! The following is excerpted from his book’s Chapter 2.
The greatest problem in biology for centuries — for millennia even — has been: How does an egg turn into an embryo? Take the human egg, the size of a speck of dust and known to science only since the early 1800s. As this egg develops, the fundamental plan of the embryo becomes established in a short time — from one to four weeks following fertilization. During this phase a homogeneous lump of cells turns into an animal in miniature; the tiny animal then becomes recognizably human. But the real mystery of morphogenesis, and of this life of ours, is compressed into those three weeks when the spatial organization (or “regional specification”) of the organism emerges.
Most eggs are globe-shaped. The frog’s egg, the one most studied in this regard, is no larger than a pinhead. Under the microscope many eggs exhibit a polar region, with the nucleus and various planes of substance stratified in relation to the pole; and there these lie, one would think, with no intention of forming an animal. As the egg matures, the chromosomes become discernible in the nucleus. After a pair of cell divisions, these are reduced by half (the rest being ejected from the egg), and the remaining ones are ready to welcome the other half from the sperm that is about to arrive. Following fertilization, the chromosomes will show themselves at every division, in pairs.
When chromosomes were discovered in 1875, biologists soon realized that they were the key to solving many problems — except the problem of development. Chromosomes split in regular and symmetrical fashion at each cell division and eventually distribute themselves uniformly in all cells, where they maintain the same number and the same forms. Each of the cells of an organism — whether in the various embryo layers, in the liver, hand, or heart — has exactly the same chromosome set. This uniformity is quite incapable of explaining the distribution of functions and forms in the embryo. True, it has been shown that in different cells different chromosome regions are in operation. In some cells chromosome activity manifests itself in localized puffs. Nevertheless, these manifestations are not due to spontaneous initiatives by the chromosomes themselves, but are responses to stimuli originating in the region of the embryo where the nuclei (with their chromosomes) have ended up.
Regional Differentiation in the Embryo
In an attempt to understand regional differentiation in the embryo, research has concentrated on two phenomena: the spatial distribution of the egg cytoplasm; and “induction,” which is the effect of local stimuli (external or internal) on the developing embryo. The geography of the cytoplasm has been studied chiefly in the eggs of invertebrates such as the sea urchin, while induction has been studied in the embryos of vertebrates such as the frog. In this latter case, experimenters have been able to mimic certain stimuli with a needle, micropipette, thread, or tip of a lancet, and they have ended up with instructive monstrosities in the process.
The problem of morphological development is first of all a problem of asymmetry. Physicists have a law (P. Curie’s principle) which states that, if a phenomenon exhibits a certain asymmetry, the same asymmetry will be found in at least one of the conditions determining the phenomenon. If a body has a front and a back, the cause, too, must have a front and a back. But chromosomes have no such asymmetry; they have no back and no belly, no right and no left.
A Pre-Existing Bilateral Asymmetry
If you want a baby to understand right and left, there is no point in shouting at him. You can explain to him, and have him adopt the distinction, only because right and left are already present in his instinct. And then you will tell him that his right hand is the one he uses to hold his spoon, or, when he is a little older, the hand with which he makes the sign of the Cross. The “right”/”left” message does not teach anything unless it fits in with some pre-existing bilateral asymmetry in which two opposing and different half-worlds can originate and develop.
Embryologists of the late 19th century thought at first that they had discerned elements of asymmetry in the egg cytoplasm, this being readily observable in many invertebrate species. The ascidian (Styela) egg is as transparent as glass and reveals within its interior pigmented granules arranged in strips, concentric strata, and crescents. The pigmented granules run about and shift aside when the egg begins to develop, and they take up their positions in specific regions of the embryo. The organism thus seemed at first to be predestinated in the egg’s geography.
Yet research along these lines came to nothing. In ascidian eggs, just as in the eggs of other species, a brisk centrifugation could bring about shifts in the pigmented areas. A yellow half-moon band was displaced and ended up in different cells following segmentation; yet the embryo developed quite normally. Some thought that the intrinsic differentiation of the cytoplasm was something stable and built-in, and that the pigments were merely secondary manifestations of it. But by then the pigment game was lost.
The spatial frame of the future organism was then entrusted by some embryologists to the egg cell wall, or cortex, which does not become deformed under centrifugation. To distinguish this from the endoplasm, or the cell’s central cytoplasm, the part adhering to the cortex was given the name of ectoplasm — a sort of ghost leaning against the wall.
Form as a Matter of Distribution of Cell Material
An experiment conducted by Wilhelm Roux in the late 19th century seemed to confirm spatial distribution within the egg of the future areas of the organism — a mosaic of local destinies. Roux worked on frog’s eggs at the stage of the first two cells (blastomeres). With a needle he killed one of the two cells, and the other cell developed into a half-organism with one side only. This was interpreted as proof that form was a matter of distribution of cell material.
Hans Driesch (1892) introduced a variant into Roux’s experiment by separating completely the first two blastomeres of a sea urchin egg. From each of them — i.e., from each half-egg — a larva (pluteus) developed that was perfectly normal, only smaller. The same result was obtained if the egg was subdivided after several cells had formed. From each cell a “twin” developed that was identical to the others and (except for size) to the larva that would have formed from an egg that had been left alone. So there was no preformed mosaic.
A Lost Thread Re-Emerges
It sometimes happens that from the latest failure or disappointment a lost thread re-emerges — the elusive figure we had sought in vain. The fact that an egg can be divided into two half-eggs that develop into the same form after separation can be expressed by a magnetic field analogy. In the egg there is a “field” with this strange property: Cutting the egg in half produces two fields that are identical with the field of the original egg and with each other. Cutting the pieces again reproduces the whole field two-, four-, or even eightfold. Something similar happens with a magnetized iron bar, with its magnetic field lines revealed by iron filings. Break the bar in two and place the halves at a distance from each other, and the iron filings will show that each piece forms around itself a field similar to the original one. Just as the original bar had a south pole and a north pole, so also the broken pieces each have a south pole and a north pole.
Driesch called the egg’s field the “morphogenetic field” to indicate that it has the capacity to generate a form; but it does not have, nor is it, a form. It is a bodiless structure, an immaterial energy flux.
Pavel Florenskij, a Russian physicist and theologian (1882-1937), imagined a similar field on the surfaces of icons that portray sacred images. “The essence of the surface is dormant until it is evoked; but once paints are applied to it these awaken it … just as a magnet’s invisible lines of force become visible thanks to the iron filings.”
Driesch’s “field” has not only the property of forming the organism but also that of restoring it to its proper form following any perturbation (self-regulation). Every single part possesses morphogenetic properties higher than that which it will express (equipotentiality). We have seen how, for a short while, embryo cells remain totipotential. As development proceeds, totipotentiality tends to decline. The field is a morphological project that gradually differentiates and becomes regionalized. Such a procedure does little to explain the precision of the end result, of the perfected form, the marvel of nature. Driesch rounded off his field’s properties by assigning a causal role — autonomy, almost — to morphological outcomes. An identical final structure can be arrived at via different avenues of development, as though it beckoned morphogenesis to a stereotypical end-product (equifinality).