SECT. 2] 



AND WEIGHT 



447 



development see also the maximal relative size of brain and lens, 

 but these fall very rapidly away from their pre-eminence. Towards 

 the end of the developmental period, the fore limb gains much in 

 importance, and about that time also the metanephros reaches a 

 maximal point of growth. 



For further comparison further calculations are necessary. The 

 relative instantaneous percentage growth-rate could be obtained 



from the equation ^ , 1 



^ (^v _ log v-^ — log V 



Cw log Wi — log w ' 



1 I 3 V J 6 7 



9 10 Tl 12 13 n IS 16 1? 18 IS ZO 21 

 Days 



Fig. 58. £ = fore limb; G = brain; ^=heart; Z,=lens; 

 jV= metanephros; f/ = mesonephros. 



where Cv is the instantaneous percentage growth-rate for the organ 

 or part in question, v-^ and v the weights of the organ at the be- 

 ginning and end of the period in question, Cw the instantaneous 

 percentage growth-rate of the organism as a whole and w-^ and w 

 the weights of the organism as a whole at the beginning and end of 

 the period in question. But this would not take into account the 

 time of formation of the various anlages. More complicated expres- 

 sions have, therefore, to be found, but as they do not at present 

 seem to have any direct importance for the chemical embryologist, 

 a reference to the original paper must suffice. They involve the com- 

 putation of an "extensity factor" which is usually the same as the 



