94 EXTERNAL FACTORS III. 4 



no relation between the amplitude of the deviation and the 

 length of the exposure. 



Nor are the processes of growth and differentiation necessarily 

 affected in any way at all by the presence or absence of light, 

 or by the kind of light to which the eggs are subjected. 



Thus Driesch, who has experimented with the eggs of 

 Echinus, Planorbis, and Rana, maintains that neither red, yellow, 

 green, blue, nor violet light has the slightest effect upon the eggs 

 during the early stages of segmentation and gastrulation, in 

 what he calls the organ-forming period of development; and 

 Loeb has asserted that the development of the embryos of the 

 fish Fundulus is as rapid in darkness as in the light, except that 

 on the yolk-sac (not in the embryo) far fewer pigment-forming 

 cells are produced. 



Yung, on the other hand, has brought forward evidence to 

 show that in later stages, at any rate, the embryos of the Frog 

 react differently to lights of various wave-lengths, some of which 

 are harmful, others, apparently, beneficial. 



Yung obtained his colours from solutions of fuchsin (red), 

 potassium bichromate (yellow), nickel nitrate (green), bleu de 

 Lyon (blue), and viole de Parme (violet). The colours, it may 

 be noticed, are not absolutely monochromatic. 



Freshly laid eggs of Rana temporaries were placed under the 

 influence of these lights. After one month, samples of the tad- 

 poles were measured, with the following result in millimetres : 



TABLE IX 



Red. Yellow. Green. Blue. Violet. White. 



Length 21-58 25-91 18-83 26-83 29-66 25-75 



Breadth 4-83 5-58 4-16 5-75 6-83 5-25 



The mortality in the green light was great. 



After two months the dimensions were as follows : 



TABLE X 



Red. Yellow. Green. Blue. Violet. White. 



Length 26-25 31-83 All 33-50 41-30 31-00 



Breadth 6-00 7-50 dead. 8-00 10-16 7-33 



All the tadpoles in the red light eventually died. 

 White and yellow light gave the greatest number of perfect 

 frogs, but, as will be seen, those in the violet were larger. They 



