THE ACCELERATION OF THE RATE OF CELL 



DIVISION. 1 



A. RICHARDS. 



Cell division is a property of all animal and all plant cells upon 

 which depends the ability of organisms to carry out progressive 

 differentiation. While individual cells may grow and become 

 specialized, a limit to the differentiation of the body cells is reached 

 if division is lacking, a fact which finds illustration in the work 

 on tissue cultures where the cells, though they may be maintained 

 alive for protracted periods, do not greatly change in character, 

 particularly if the mitotic ratio is low. 



Differentiation in an organism consists of processes of several 

 types. In an embryo the first differentiation processes must take 

 place among the cells as units. Subsequently other processes begin 

 within the cells themselves which result, for example, in the for- 

 mation of contractile fibers in muscle cells, or in other well-known 

 changes in cells of other kinds. These later processes do not 

 usually begin, however, until the rate of division becomes differ- 

 ential. Conklin has defined differential division as follows: ''Cell 

 division is typical and non-differential when it occurs at regular 

 intervals or at the same time in cells of the same generation 

 (rhythmical) when successive divisions are at right angles (alter- 

 nating), when daughter cells are of similar size (equal), and are 

 composed of similar materials (homogeneous). Divisions are 

 differential when they depart from these typical conditions in one 

 or more respects, becoming non-rhythmical, non-alternating, un- 

 equal, or heterogeneous." 



In an organism, then, the progressive differentiation of the cells 

 must be thought of as cumulative, since in each cell generation the 

 progress begins where it left off in the proceeding. The problem 

 of the organization of the early embryo is largely one of the 

 cumulative differentiation of the individual cells, of their increase 



i Studies from the Zoological Laboratory of the University of Oklahoma, 

 Second Series, No. 24. 



348 



