SECT. 2] AND WEIGHT 527 



very considerable slowing at the low temperatures, so that although 

 the coefficient between 17 and 27° was 2-6, between 8 and 18° it 

 was 6-0. The temperature coefficient for the respiratory rate, how- 

 ever, did not show this, and, while it was 1-5 approximately round 

 30°, never rose higher than 2-45 even including the data for between 

 3 and 13°. There was thus no correspondence between the two sets 

 of coefficients below 15°. This circumstance, they thought, harmonised 

 with the fact that potassium cyanide in concentrations which stopped 

 development altogether still permitted 25 per cent, of the oxidations 

 to proceed, and further, with the fact that when no development was 

 going on normally, i.e. before fertilisation, there was still a certain 

 respiration. Of the two processes the morphological development 

 rate seemed to be the more easily abolished by external agents. 

 But above 32° when cleavage became impossible, respiration was 

 also impossible, and the oxygen uptake rapidly fell off. "These 

 facts", said Loeb & Wasteneys, "do not contradict the view that 

 oxidations are the independent variables in developmental processes. 

 But on the other hand they do not furnish any convincing evidence 

 in favour of such a view." The whole incident, as a matter of fact, 

 may now be regarded as an instance of the incapacity of the ordinary 

 van't Hoff equation to differentiate between two alternatives in a 

 complicated biological system, and the question which Loeb & 

 Wasteneys were trying to answer may be said to be solved by the 

 frequency polygon of temperature characteristics. 



The results of all this work may be summed up by saying that by 

 the use of the heat effect on embryonic growth and development a 

 kind of dissection has been made of the process, for at one tempera- 

 ture one controlling reaction or pacemaker is laid bare, as it were, 

 and, at another temperature, another controlling reaction. This 

 exposing method has led to the unveiling of four or five reactions 

 of quite clear-cut temperature characteristics, which according to 

 the temperature and the circumstances may be in control of em- 

 bryonic growth. What these reactions are we do not yet know, but 

 admittedly a good step forward has been taken by the identification 

 of the heats of activation of their active molecules. 



Crozier has discussed in some detail the relation between the 

 temperature characteristic of growth and the autocatakinetic growth 

 curve. Difficulties arose out of the experimental fact that temporary 

 alteration of the temperature during a period of development 



