THE RISE OF EMBRYOLOGY 21 1 



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 " i'is essential is 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 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 Yon 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 currencv. This retarded 



o - 



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. Sci. Petrop., 

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



