CHAPTER 3 



The Cellular Basis of Growth 



One of the great biological generalizations of the nineteenth century is 

 the cell theory, commonly attributed to the botanist Schleiden and the 

 zoologist Schwann and formally stated in 1839. This was a recognition of 

 the fact that all organisms are composed of living units, the cells. The 

 theory provided a common foundation for a study not only of structure 

 but of growth (cell multiplication) and development (cell differentia- 

 tion ) . It has served as a unifying concept for all biology, somewhat com- 

 parable to the atomic theory in the physical sciences. 



The implications of the cell theory for morphogenesis are important. 

 In the minds of those who promulgated it, it meant that the cell is the 

 true biological individual and that an organism is the result of the ac- 

 tivities of its constituent cells. That the cell is thus the primary agent of 

 organization is the opinion of some biologists today. In such a view the 

 organism is looked on as a sort of cellular state, built by the cooperative 

 efforts of its citizens among whom, as in a human society, there is a high 

 degree of division of labor. In support of this idea are cited cases such 

 as that of certain of the slime molds, where some thousands of individual 

 cells (myxamoebae), entirely independent in the early stages of the life 

 cycle, become aggregated into a cellular mass and then by their mutual 

 interactions build up a fruiting body of a specific size and form (p. 223). 



Other biologists, however, believe that the true individual is the 

 organism, essentially a mass of protoplasm divided into cellular units. 

 Such division has the advantage that it makes possible the differentiation 

 of parts and the segregation of various physiological activities within 

 particular cells. The organism may thus be said to make the cells rather 

 than the cells to make the organism. In this conception the multicellular 

 plant body is to be thought of as having arisen not through the aggrega- 

 tion of individual cells, originally separate, but by cellular multiplica- 

 tion. 



That this organismal theory gives a better picture of the growth and 

 activities of plants and animals is suggested by the high degree of co- 

 ordination and self-regulation that exists in a living thing. The produc- 

 tion of individuals essentially alike by a variety of developmental routes 



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