His 'Formatione Intestinorum' embodies his greatest contribution to embryology rather than his 'Theoria Generationis'; not only is it a more fitting model of observation, but in it he foreshadows the idea of germ-layers in the embryo, which, under Pander and Von Baer, became the fundamental conception in structural embryology. Throughout his work, both early and late, he likens the embryonic rudiments, which precede the formation of organs, to leaflets. In his work of 1768, he describes in detail how the leaf-like layers give rise to the systems of organs: Showing that the nervous system arises first from a leaf-like layer, and is followed, successively, by a flesh-layer, the vascular system and, lastly, by the intestinal canal—all arising from original leaf-like layers.
In these important generalizations, although they are verbally incorrect, he reached the truth as nearly as it was possible at the time, and laid the foundation of the germ-layer theory.
Wolff was a man of great power as an observer, and although his influence was for a long time retarded, he should be recognized as the foremost investigator in embryology before Von Baer.
The little known of his life is gained through his correspondence and a letter by his amanuensis. Through personal neglect, and hostility to his work, he could not secure a foothold in the universities of Germany, and, in 1764, on the invitation of Catharine of Russia, he went to the Academy of Sciences at St. Petersburg, where he spent the last thirty years of his life.
His sincere and generous spirit is shown in his correspondence with Haller, his great opponent. "And as to the matter of contention between us, I think thus: For me, no more than for you glorious man, is truth of the very greatest concern. Whether it chance that organic bodies emerge from an invisible into a visible condition, or form themselves out of the air, there is no reason why I should wish the one were truer that the other, or wish the one and not the other. And this is your view also, glorious man. We are investigating for truth only: we seek that which is true. Why then should I contend with you?"
I have not been able to locate a portrait of Wolff, although I have sought one in various ways for several years. The Secretary of the Academy of Sciences at St. Petersburg writes that no portrait of Wolff exists there, and that they will gratefully receive information regarding any existing portrait of the great academician.
The Period of Von Baer.
What Verworn says of Johannes Müller's position in physiology, may with equal appropriateness be applied to Von Baer in the science of embryology. He was: "One of those monumental figures that the history of every science brings forth but once. They change the whole
