SECT, 2] 



AND WEIGHT 



523 



adalb 

 body- 

 temp. 



100K 



70 



65 



60 



j= 55 



50 



45 



40 



35 



i. 30 



25 



20 



15 



with rise of temperature, and in the last stage, i.e. from about the 

 14th day onwards, the effect is nil, for the line is horizontal; or, in 

 other words, the heat-regulating mechanism in the embryo is suffi- 

 ciently developed at this stage to enable it to keep its body-tempera- 

 ture constant within the limits of the experimental temperatures so 

 that they do not get a chance of affecting its growth-processes. It 

 could indeed be argued that the 

 three curves show a progressively 

 increasing command of body- 

 temperature, so that higher 

 temperature coefficients and 

 steeper growth-rate/age curves 

 are inevitably found the younger 

 the embryo is. This question of 

 temperature-regulation will be 

 taken up again later in the book 

 (see Section 4'2i). 



From these data the /x con- 

 stants of Arrhenius' equation 

 was calculated. For phase i it 

 was 25,700 below 37° and 7100 

 above it, while for phase 2 it was 

 39,050 below 37° and 6500 above 

 it. These values agree well with 

 those obtained for other in- 

 stances of embryonic growth, a 

 fact which is especially important 

 in view of the restriction of the 

 other data to amphibia, fishes 

 and arthropods. For the third 

 phase there was obviously no 



35^ 



37= 



39° 



Temperature (c) 

 Fig. 83 b. 



41° 41-2'= 



temperature characteristic. Thus, before the age of about 13 days and 

 between the temperatures of 35 and 37° increase of temperature by 

 ten degrees more than doubles but does not quite triple the rate of 

 growth, while after the 14th day the growth of the embryo is practically 

 unaffected by change of temperature owing to its acquired homoio- 

 thermicity. 



Taking the values which have been obtained for /x for growth and 

 development, it is clear that they all cluster rather closely round 



