Work of Wolff.--Friedrich Kaspar Wolff, as a young man of twenty-six years, set himself against this grotesque doctrine of pre-formation and encasement in his _Theoria Generationis_, published in 1759. This consists of three parts: one devoted to the development of plants, one to the development of animals, and one to theoretical considerations. He contended that the organs of animals make their appearance gradually, and that he could actually follow their successive stages of formation. The figures in it illustrating the development of the chick, some of which are shown in Fig. 63, are not, on the whole, so good as Malpighi's. Wolff gives, in all, seventeen figures, while Malpighi published eighty-six, and his twenty figures on the development of the heart are more detailed than any of Wolff's. When the figures represent similar stages of development, a comparison of the two men's work is favorable to Malpighi. The latter shows much better, in corresponding stages, the series of cerebral vesicles and their relation to the optic vesicles. Moreover, in the wider range of his work, he shows many things--such as the formation of the neural groove, etc.--not included in Wolff's observations. Wolff, on the other hand, figures for the first time the primitive kidneys, or "Wolffian bodies," of which he was the discoverer. Although Wolff was able to show that development consists of a gradual formation of parts, his theory of development was entirely mystical and unsatisfactory. The fruitful idea of germinal continuity had not yet emerged, and the thought that the egg has inherited an organization from the past was yet to be expressed. Wolff was, therefore, in the same quandary as his predecessors when he undertook to explain development. Since he assumed a total lack of organization in the beginning, he was obliged to make development "miraculous" through the action on the egg of a hyperphysical agent. From a total lack of organization, he conceived of its being lifted to the highly organized product through the action of a "_vis essentialis corporis_." He returned to the problem of development later, and, in 1768-1769, published his best work in this field on the development of the intestine.[5] This is a very original and strong piece of observational work. While his investigations for the _Theoria Generationis_ did not reach the level of Malpighi's, those of the paper of 1768 surpassed them and held the position of the best piece of embryological work up to that of Pander and Von Baer. This work was so highly appreciated by Von Baer that he said: "It is the greatest masterpiece of scientific observation which we possess." In it he clearly demonstrated that the development of the intestine and its appendages is a true process of becoming. Still later, in 1789, he published further theoretical considerations.