DEVELOPMENTAL MECHANICS. 157 



a multicellular organism ; while at the same time, new effects 

 are produced which are peculiar to the higher unit in question 

 and which would naturally depend on the reciprocal operations 

 of the lower constituents. After ascertaining the formative 

 functions of each such unit, the modi operandi on which these 

 functions depend must be established by themselves; this 

 holds good in the case of the lowest independent parts of the 

 cell (7) : the isoplassons, autokineons, automerizons , idioplas- 

 sons, and the parts which they constitute, the nucleus, centro- 

 some, and protoplasm. It also holds good in the case of the entire 

 cells themselves, of the tissues, organs, and the organism which 

 is composed of the latter. 



Inasmuch as each of these vital units of different orders is 

 distinguished by its individual functions, whenever such a unit 

 cooperates with an "external" factor, we are often interested 

 only in the behavior of the unit and we call this its reaction. 

 In a complete estimate of the phenomena we should, of course, 

 have to take cognizance of the way in which the external, or 

 more correctly speaking, "other" factor is affected, especially 

 when this happens to be also a living part. 



Thus we speak of the formative reactions of cells, tissues, 

 organs, or of the whole organism which these go to make up, 

 e.g., of the influence of increased functional stimuli on bones 

 through the activity of the muscles, etc. 



Besides the modi operandi or energies of development, the 

 modi operandi or energies of the maintenance and of the involu- 

 tion of organic forms and their bearers must be investigated 

 by themselves, although it is probable that maintenance often 

 represents merely the equilibration of diverse components which 

 are also active and formative during development ; and that 

 during subsequent involution this equilibrium is upset by alter- 

 ing, destructive components. Besides searching for such con- 

 ditions we must, on the other hand, seek to determine whether 

 each of these phases has not formative modi operandi peculiar to 

 itself. 



Furthermore, in accordance with the double course of devel- 

 opment, viz., the phylctic and ontogenetic, developmental 

 mechanics must look for the causes, or modi operandi, of each 



