160 CELLS, TISSUES, AND ORGANISMS 



Theoretically this is not the only possible meaning of reproduction. 

 We may imagine, for instance, that a cell will replace in a precise way 

 all of its molecules, so that one "old" cell gives rise to one "new" one. 

 Indeed, such a thought is implicit in the somewhat unclear concept of 

 the "dynamic state" of cell constituents. If a cell were perfectly dy- 

 namic in this sense, we might expect that it could survive indefinitely 

 as an individual, without dividing. In practice, we know that "turnover" 

 is quite variable. Some kinds of molecules, such as the DNA of the 

 primary genetic material, are thought to turn over very little {e.g., 

 Brachet, 1957, p. 228), although reasons for suspecting a degree of 

 turnover of genetic substances have been developed recently (e.g., 

 Ryan, 1959). In other cases, such as the proteins, the intensity of turn- 

 over seems to vary greatly with functional circumstances. 



Schemes of reproduction 



We can imagine at least two general schemes for the reproduction 

 of a system composed of many elements. In one of these, which I shall 

 call a fission model, every element has the power of self -replication and 

 is directly involved in the making of copies of itself; the copies have 

 the same powers of self-reproduction as the original; and changes are 

 propagated in future generations of copies. Such a model may be 

 represented simply by 



/ ABCD Z 



(ABCD Z) -^ I m 4 I 



\ ABCD Z 



(ABCD Z) + (ABCD Z). 



The capital letters symbolize all of the elements of the system possess- 

 ing specificity. Each produces a copy of itself, and when all the copies 

 are made, the two sets cleave to produce two independent systems. 



At the other extreme is what we may call a generative scheme of 

 reproduction, in which only one or a few elements of the system are 

 capable of self -reproduction, and these direct the assembly of a second 

 system : 



(Abed z) 



