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 

 1 768-1 769, published his best w^ork 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 1 768 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 v.hich 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 Commentary Ac. Sci. Petrop., 

 St. Petersburg, XII., 1768; XIII., 1769. 



