808 



MORPHOGENESIS 



expressed — can be induced to assume the dominant generative functions. 

 The regenerative behavior following simple, compound, or successive 

 operations is another illustration of what Child (1929) referred to as 

 physiological correlation: the relations of dominance, or control and 

 subordination between parts. The single apical cell of Zoothamnium 

 colonies exercises the controlling influence over growth and differentia- 

 tion in subadjacent cells. 



Figure 186. A, a seventy-two-hour regenerate produced from a lateral cell of the first 

 branch generation (a cell which ordinarily represents the presumptive ciliospore) ; B, 

 schematic diagram of the apical portion of the colony at the time of cutting (see arrow) ; 

 C, a similar diagram of the regenerating colony seventy-two hours later, or as shown in A. 

 (From Summers, 1938b.) 



Regional coordination, according to Child, depends primarily upon 

 quantitative rather than specific differences in the protoplasmic condi- 

 tion of the dominant region. Evidence that this is not necessarily the 



