STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 105 



of phenomena of growth and division are maximal with such materials. 

 Indeed, one might suppose that the obvious separation of function within 

 cell parts is precisely what is meant by differentiation, since the development 

 of a division of labor in a multicellular animal requires the exaggeration of 

 one or another of the potentialities and functions of cells possessing a common 

 genome, and hence results in the exaggeration of some structures and acti- 

 vities among the cell parts. In unicellular organisms, such as bacteria, the 

 whole cell participates in the survival of the species and differentiation 

 cannot confer the selective advantage gaitied by organisms which possess 

 various kinds of tissues and which obtain improved habitats at the expense 

 of gamete number. 



In nervous tissue, the exaggeration of cytoplasmic function is effected to 

 the pomt of essentially ehminating mitosis and cell division in the adult 

 nerve cell. One could imagine that the problem of information transfer in 

 nerve and brain would be quite difficult to manage if new synapses were 

 continually being created as a result of cell division. On the other hand, 

 although mitosis in adult hver slows to 1 to 2 % per day, concomitant with 

 a massive cytoplasmic development, the enlarged differentiated cell has not 

 lost its capacity for division since m regenerating liver a division may occur 

 at the rate of about once per day. Therefore, the separation of growth and 

 division observed in differentiated cells may be further subdivided according 

 to the irreversibihty of the loss of mitotic activity. In such differentiated 

 systems the upper Hmit of the low rate of mitosis can be described by the low 

 rate of synthesis of DNA and of chromosome sets. 



In addition, many differentiated cells, characterized by active cytoplasmic 

 function and particularly that related to protein synthesis, display a very 

 active UNA synthesis. In the differentiated cells of multicellular forms, 

 therefore, it is frequently possible to dissociate ENA and DNA synthesis 

 completely, even as this is possible in Amoeba, Acetabularia, and their various 

 derived fragments. 



In some plant tissues, nuclear divisions may be separated readily from cell 

 divisions. In some tissues, as in the elongation and differentiation of cells 

 behind root tips, DNA may increase without cell division to produce poly- 

 ploid cells (Deeley et at., 1957). Tobacco pith tissue cultured on a nutrient 

 agar medium wiU fail to grow in the absence of indole acetic acid (lAA); 

 however, in the presence of this hormone, a 10-fold increase in cell size is 

 realized after nuclear divisions, which begin at 40 hours and reach a maximum 

 by 70 hours. Throughout this process, cell divisions do not occur (Jablonski 

 and Skoog, 1954; Naylor et al., 1954). When another substance, kinetin or 

 6-furfuryl aminopurine, which is formed in the chemical decomposition of 

 DNA (Mnier et al., 1956), is added together mth indole acetic acid, mitoses 

 are accompanied by cell division (Das et al., 1956). Cell enlargement induced 



