B. EXPERIMENTAL AND THEORETICAL 

 MORPHOGENESIS 



1. THE FOUNDATIONS OF THE PHYSIOLOGY OF DEVELOP- 

 MENT. " EVOLUTIO " AND " EPIGENESIS " 



THE THEORY OF WEISMANN 



OF all the purely hypothetic theories on morphogenesis 

 that of August Weismann l can claim to have had the 

 greatest influence, and to be at the same time the most 

 logical and the most elaborated. The "germ-plasma" 

 theory of the German author is generally considered as 

 being a theory of heredity, and that is true inasmuch as 

 problems of inheritance proper have been the starting-point 

 of all his hypothetic speculations, and also form in some 

 respect the most valuable part of them. But, rightly under- 

 stood, Weismann's theory consists of two independent parts, 

 which relate to morphogenesis and to heredity separately, 

 and it is only the first which we shall have to take into 

 consideration at present ; what is generally known as the 

 doctrine of the " continuity of the germ -plasm ' will be 

 discussed in a later chapter. 



Weismann assumes that a very complicated organised 

 structure, below the limits of visibility even with the 



1 Das Keimplasma, Jena, 1892. 

 52 



