THE RISE OF EMBRYOLOGY 211 
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 devel- 
opment ‘“‘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 devel- 
opment of the intestine.* 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 posi- 
tion 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. 
Opposition to Wolff’s Views.—But all Wolff's work was 
launched into an uncongenial atmosphere. The great physi- 
ologist 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 
* De Formatione Intestinorum, Nova Commentar, Ac. Sct. Petrop., 
St. Petersburg, XII., 1768; XIII., 1769. 
