FACTS AND FACTORS OF DEVELOPMENT 87 



lished in 1759 and entitled "Theoria Genera- 

 tionis/ J marked the beginning of a great epoch 

 in the study of development. Wolff demon- 

 strated that adult parts are not present in the 

 germ, either in animals or in plants, but that 

 these parts gradually appear in the process of 

 development. He held, erroneously, that the 

 germ is absolutely simple, homogeneous and 

 undifferentiated, and that differentiation and 

 organization gradually appear in this undiffer- 

 entiated substance. How to get differentia- 

 tions out of non-differentiated material, 

 heterogeneity out of homogeneity, was the 

 great problem which confronted Wolff and his 

 followers, and they were compelled to assume 

 some extrinsic or environmental force, some 

 vis formativa or spiritus rector, which could 

 set in motion and direct the process of 

 development. 



The doctrine of preformation, by locating 

 in the germ all the parts which would ever 

 arise from it, practically denied development 

 altogether; epigenesis recognized the fact of 

 development, but attributed it to mysterious 

 and purely hypothetical external forces; the 



