THE MECHAjSTISM OF LIFE 529 



of change of form, i.e., the phenomena of growth, reproduction, 

 development, and hereditary transmission, may also be derived as 

 mechanical consequences from these metabolic relations. 



a. Groivth as the Fundamental Fhenomenon of Change of Form 



Groivth constitutes the fundamental phenomenon of changes of 

 form in organic nature, for not only is growth of the cell the 

 simplest case of change of form in general, but at the same time, 

 as the following considerations will at once show, it contains the 

 internal causes of the more complex phenomena of cell-reproduc- 

 tion and development. We have referred elsewhere ^ to the mode 

 of growth of living substance. We know that there are molecules in 

 living substance, that possess an extraordinary tendency toward 

 polymerisation, i.e., under given conditions, by continually taking 

 on similar groups of atoms, they endeavour to enlarge and to form 

 chains of many similar links. We have become acquainted with 

 such polymeric molecules in the native proteids. It is, a prioi-i, 

 probable that the so-called living proteids, or biogens, likewise 

 possess this property, and the more probable because there is 

 reason to assume with Pfltiger, that the radical cyanogen, which 

 tends strongly towards polymerisation, is contained in the biogen 

 molecule. Moreover, the fact of growth requires the assumption of 

 polymerisation in the biogen molecule. Growth can be conceived 

 only as a process in which a biogen molecule attaches to itself little 

 by little similar groups of atoms from the materials of the environ- 

 ment (food-stuffs) ; these groups then proceed in the same manner 

 to attract to themselves certain atoms from the environment and 

 place them in similar positions, and so on. 



This process, which is here pictured in a uniform substratum, 

 goes on in a much more complex manner in the cell whose living 

 substance and whose metabolism are very widely differentiated. 

 In the cell the constituents of both the nucleus and the protoplasm 

 with all their special differentiations share more or less closely in 

 the formation and the growth of the biogen molecule. But with 

 this close relationship and dependence of the individual constituents 

 of the cell upon one another, it is easily understood that the 

 growth of certain biogens of the protoplasm by polymerisation 

 is only possible when at the same time other constituents of the 

 protoplasm or of the nucleus increase in a definite measure; in 

 other words, whenever a single substance of the protoplasm or of 

 the nucleus grows, other substances also will grow. 



It is important to consider somewhat fully the relations that, 



with this close correlation of the individual parts of the cell, are 



developed by growth. For example, let us imagine a, free-living, 



spherical cell which has at its disposal in the surrounding medium 



' Gf. pp. 30.5 and 486. 



JI M 



