372 The American Naturalist. [May 



cance of gravity as well as light, heat, magnetism in directing- 

 the cleavage processes from rotation experiment upon frogs' 

 eggs. Using a vertical wheel, revolving rapidly enough to 

 produce centrifugal effect, about double that of gravitation, he 

 found the eggs develop normally, though constantly present- 

 ing their white poles away from the centre of the wheel and 

 being thus acted upon alternately above and below, by 

 gravitation. Having eliminated the constant action of grav- 

 ity as a directive force, he concluded it was unnecessary for 

 the appearance of cleavage planes and assigned this to causes 

 within the egg. 



By numerous other contributions and complete devotion to 

 a definite line of embryological research, this author has be- 

 come as it were, the apostle of a new branch of embryology, 

 Entwicklungsmechanik. Judging from the heterogenous char- 

 acter of the 262, 384, and 277 works for. 1 887, '88, '89, ranged 

 under the above heading, as forming a separate department 

 in Hermann and Schwalbe's Jahresberichte we conclude that 

 this term is by no means synonymous with physiological or 

 with experimental embryology, but has a much wider applica- 

 tion, including the last as one of its subdivisions. 



The first definite use of the term together with outlines of 

 the problems to be attempted in this pre-determined field of 

 work was made by Roux in 1885 7 . We there find Entwick- 

 lungsmechanik to be the science of the character and action of 

 the combinations of energy which produce development. Also, 

 development being the origin of observable multiplicity, there 

 may be either read production of or merely transformation of 

 non-observed into observable manifoldness. Epigenesis is 

 then the actual creation of complexity : Evolution only the 

 sensualization of latent diversities. 



The key to the causal knowlege of development lies in the 

 determination of the relative value of two possibilities: self- 

 differentiation : interaction with the environment. Self-differ- 

 entiation of a system of part is the result of the energy of the 

 system itself. Correlative differentiation is the change of a 



