The Determination of Size 



629 



wing size was correlated with increase in 

 developmental time, and larvae whose pupa- 

 tion was much delayed developed into small 

 flies with exceptionally large wings. On the 

 other hand, Hersh and Ward ('32) had 

 found the size of the wing to be an expo- 

 nential function of temperature. 



In conclusion, the determination of size 

 of either organisms or their organs remains 

 a problem which is definable but little 

 understood. Fundamentally, size is due to 

 the operation of several factors. The initial 

 size of the growth unit (cf. Berrill, '41), at 

 the moment it ceases to grow at the maximal 

 rate, determines the final size, other things 

 being equal. This is a matter of measure- 

 ment, but it may be difficult to determine 

 when growth ceases to be maximal. The 

 energy of growth is but vaguely understood 

 but its intensity determines the magnitude 

 of the maximal growth rate. The decrement 

 of growth is widely recognized as basically 

 significant, but its origin remains a mystery. 

 Its value determines the duration of growth. 

 Finally, growth may be polarized and each 

 growth unit, whatever it may actually be, 

 gives rise to a part that continues growth 

 at a maximal rate and a part that exhibits 

 decrement, that is, the establishment of 

 growth and differentiation gradients. 



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