Biology and Its Makers / With Portraits and Other Illustrations

by William A. Locy

As a preliminary to discussing Wolff's position, we should bring under consideration the doctrine of pre-formation and encasement.

Rise of the Theory of Pre-delineation.—The idea of pre-formation in its first form is easily set forth. Just as when we examine a seed we find within an embryo plantlet, so it was supposed that the various forms of animal life existed in miniature within the egg. The process of development was supposed to consist of the expansion or unfolding of this pre-formed embryo. The process was commonly illustrated by reference to flower-buds. "Just as already in a small bud all the parts of the flower, such as stamens and colored petals, are enveloped by the green and still undeveloped sepals; just as the parts grow in concealment and then suddenly expand into a blossom, so also in the development of animals, it was thought that the already present, small but transparent parts grow, gradually expand, and become discernible." (Hertwig.) From the feature of unfolding this was called in the eighteenth century the theory of evolution, giving to that term quite a different meaning from that attached to it at the present time.

This theory, strange as it may seem to us now, was founded on a basis of actual observation—not entirely on speculation. Although it was a product of the seventeenth century, from several printed accounts one is likely to gather the impression that it arose in the eighteenth century, and that Bonnet, Haller, and Leibnitz were among its founders. This implication is in part fostered by the circumstance that Swammerdam's Biblia Naturæ, which contains the germ of the theory, was not published until 1737—more than half a century after his death—although the observations for it were completed before Malpighi's first paper on embryology was published in 1672. While it is well to bear in mind that date of publication, rather than date of observation, is accepted as establishing the period of emergence of ideas, there were other men, as Malpighi and Leeuwenhoek, contemporaries of Swammerdam, who published in the seventeenth century the basis for this theory.

Malpighi supposed (1672) the rudiment of the embryo to pre-exist within the hen's egg, because he observed evidences of organization in the unincubated egg. This was in the heat of the Italian summer (in July and August, as he himself records), and Dareste suggests that the developmental changes had gone forward to a considerable degree before Malpighi opened the eggs. Be this as it may, the imperfection of his instruments and technique would have made it very difficult to see anything definitely in stages under twenty-four hours.

In reference to his observations, he says that in the unincubated egg he saw a small embryo enclosed in a sac which he subjected to the rays of the sun. "Frequently I opened the sac with the point of a needle, so that the animals contained within might be brought to the light, nevertheless to no purpose; for the individuals were so jelly-like and so very small that they were lacerated by a light stroke. Therefore, it is right to confess that the beginnings of the chick pre-exist in the egg, and have reached a higher development in no other way than in the eggs of plants." ("Quare pulli stamina in ovo præexistere, altiorémque originem nacta esse fateri convenit, haud dispari ritu, ac in Plantarum ovis.")

Swammerdam (1637-1680) supplied a somewhat better basis. He observed that the parts of the butterfly, and other insects as well, are discernible in the chrysalis stage. Also, on observing caterpillars just before going into the pupa condition, he saw in outline the organs of the future stage, and very naturally concluded that development consists of an expansion of already formed parts.

A new feature was introduced through the discovery, by Leeuwenhoek, about 1677,[4] of the fertilizing filaments of eggs. Soon after, controversies began to arise as to whether the embryo pre-existed in the sperm or in the egg. By Leeuwenhoek, Hartsoeker, and others the egg was looked upon as simply a nidus within which the sperm developed, and they asserted that the future animal existed in miniature in the sperm. These controversies gave rise to the schools of the animalculists, who believed the sperm to be the animal germ, and of the ovulists, who contended for the ovum in that rôle.

It is interesting to follow the metaphysical speculations which led to another aspect of the doctrine of pre-formation. There were those, notably Swammerdam, Leibnitz, and Bonnet, who did not hesitate to follow the idea to the logical consequence that, if the animal germ exists pre-formed, one generation after another must be encased within it. This gave rise to the fanciful idea of encasement or emboîtement, which was so greatly elaborated by Bonnet and, by Leibnitz, applied to the development of the soul. Even Swammerdam (who, by the way, though a masterly observer, was always a poor generalizer) conceived of the germs of all forthcoming generations as having been located in the common mother Eve, all closely encased one within the other, like the boxes of a Japanese juggler. The end of the human race was conceived of by him as a necessity, when the last germ of this wonderful series had been unfolded.