170 DEVELOPMENT OF THE FROG'S EGG [Ch. XVI 



a fluid that allows only certain parts of the spectrum to 

 pass through. The top of the dish containing the eggs was 

 covered by an opaque lid. An alcoholic solution of " fuchsine 

 cerise " was used to produce a monochromatic red light ; a 

 solution of potassium chromate for a yellow light (although 

 this allows a little red and green to pass through) ; a solution 

 of nitrate of nickel (which is perfectly monochromatic) for a 

 green light ; an alcoholic solution of aniline "bleu de Lyon" 

 for a blue light ; and an alcoholic solution of aniline violet for 

 a violet light. Parallel experiments took place in the daylight 

 ("white light") and in the dark. The other conditions were 

 the same for all the aquaria ; they had the same amount of 

 water, the same extent of surface for aeration, the same tempera- 

 ture, and were placed in the same position before a window. 

 Eggs of R. esculenta and of R. temporaria were used. 



At the end of seven days it could be seen that the embryos 

 in the violet and in the blue light were more vigorous and in a 

 later stage of development than the others. At the same time 

 the development in the red and in the green was retarded. At 

 the end of a month the tadpoles were in good condition, and the 

 following table shows their mean length in each aquarium. 



Larv.e of Rana Esculenta at the End of One Month. 



The breadth of the embryos shows the same differences. It 

 is interesting to see that in the red and in the green light the 

 tadpoles were even less developed than those in the white light 

 or even in the dark. The result of this series of experiments 

 on R. esculenta agrees with other experiments made by Yung 

 at different times, upon other species of frogs and upon other 

 animals. 



