SECT. 2] AND WEIGHT 529 



and though the form of its curve is sigmoid the point of inflection 

 depends on the ratio of the two velocity constants, so that it may be 

 very asymmetrical. 



Before embarking on the exposition of the applications of the 

 Arrhenius formula to growth in general and embryonic growth in 

 particular, it was said that there was much difference of opinion as 

 to whether the time/temperature relation could be expressed best 

 by a hyperbola or an exponential curve. It was often pointed out 

 by Loeb and others that even a straight line might be produced by 

 the action of various factors on a true exponential curve causing it 

 to flatten out. Snyder suggested that an important factor which 

 might be expected to have such an effect was the protoplasmic 

 viscosity. This idea has since had a considerable popularity, although 

 Crozier has vigorously combated it. "The attempt to introduce con- 

 siderations of protoplasmic fluidity, presumably as influencing dif- 

 fusion, requires a theory of the general control of organic activities 

 by the whole body of the cell rather than at surfaces. This is un- 

 necessary", says Crozier, "and at present inadmissible." But it is by 

 no means easy to see why, and the possibilities contained in cor- 

 rections for viscosity should certainly be explored. The difficulty is 

 that viscosity varies in different ways with temperature according to 

 the animal or plant used. Some authors, moreover, seem to wish to 

 obtain a constant Q_io, although even in inorganic in vitro reactions 

 Q^io is never constant, but increases as the temperature is lowered. 

 Pantin, for instance, corrected various temperature coefficients, on 

 the basis of the viscosity changes in Nereis eggs, and found a much 

 greater approach to constancy, though even after correction, the Q^^q 

 rose a good deal at the lower temperatures. Krafka's work, already 

 referred to, is rather an obstacle to this point of view. 



The same line of thought was carried further still by Belehradek, 

 who criticised the use of the Arrhenius equation on the ground that, 

 like the van't Hoff" formula, it also did not give a constant for all 

 temperatures at which normal development will proceed, but shows 

 up critical points which may or may not be real. This criticism was 

 also made by Heilbrunn. Belehradek used a modification of the 

 Esson-Harcourt equation, which he found would represent the time/ 

 temperature relation fairly accurately: 



y = ^ or logy = log a — b log x, 



NEI 34 



