Opposition to Wolff's Views.--But all Wolff's work was launched into an uncongenial atmosphere. The great physiologist Haller could not accept the idea of epigenesis, but opposed it energetically, and so great was his authority that the views of Wolff gained no currency. This retarded progress in the science of animal development for more than a half-century. Bonnet was also a prolific writer in opposition to the ideas of Wolff, and we should perhaps have a portrait of him (Fig. 64) as one of the philosophical naturalists of the time. His prominent connection with the theory of pre-delineation in its less grotesque form, his discovery of the development of the eggs of plant-lice without previous fertilization, his researches on regeneration of parts in polyps and worms, and other observations place him among the conspicuous naturalists of the period. His system of philosophy, which has been carefully analyzed by Whitman, is designated by that writer as a system of negations. [Illustration: Fig. 64.--Charles Bonnet, 1720-1793.] In 1821, J. Fr. Meckel, recognizing the great value of Wolff's researches on the development of the intestines, rescued the work from neglect and obscurity by publishing a German translation of the same, and bringing it to the attention of scholars. From that time onward Wolff's labor was fruitful. His _De Formatione Intestinorum_ rather than his _Theoria Generationis_ embodies his greatest contribution to embryology. 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 researches both early and late, he likens the embryonic rudiments, which precede the formation of organs, to leaflets. In his work of 1768 he described 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. Few Biographical Facts.--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 Catherine of Russia, he went to the Academy of Sciences at St. Petersburg, where he spent the last thirty years of his life.