SECT. 2] AND WEIGHT 533 



of the data are very interesting, and possess advantages as well 

 as disadvantages in comparison with the equations of definite 

 physical meaning, such as that of Arrhenius. For, although 

 they do not seem to give immediately so great an insight into 

 the fundamental processes of the living cell, they are in value 

 independent of advances in physics which may destroy the basis 

 on which the other kind of treatment rests. In other words, their 

 status as short descriptions of the phenomena is safer if not so exalted. 

 I have now completed what it was necessary to say about 

 the effects of temperature on embryonic growth and develop- 

 ment. This knowledge is very valuable, but the unveiling of the 

 limiting factor by temperature control, and the identification of 

 the "master reaction" by fitting growth-curves, are perfect ex- 

 amples of what may be called the "short-cut" method. To a 

 certain type of mind the attractions of the short-cut are too great 

 to be resisted, but, as we have seen, the results are liable to be 

 enigmatic. 



2-17. The Effect of Light on Embryonic Growth 



The influence of light upon the course of embryonic growth has 

 been little studied, though the results of such work cannot now be 

 looked on with the scepticism of twenty years ago. Schnetzler's 

 paper of 1874 was the first serious contribution to the study, although 

 Edwards fifty years before had stated that frog eggs would not develop 

 at all in the dark, a groundless assertion which had been disproved 

 by Higginbottom and McDonnell. Schnetzler; and Baudrimont & 

 M. de St Ange also found no difference in hatching time, though the 

 former thought that the growth of the tadpoles was more rapid in 

 the light than in the dark. This was substantiated quantitatively by 

 Auerbach and by Yung, who placed freshly laid frog's eggs in vessels 

 containing 60 eggs in 4 litres of water, some in diffuse light, others 

 in the dark. Yung's figures were as follows: 



Days 



Size of light tadpoles compared 

 with dark (100) 



development Length Breadth 



25 125 124 



30 117 118 



60 106 107 



It is interesting that the light effects diminish proportionally with 



