426 



EFFECT OF LIGHT 



[Ch. XVII 



two lots were very similar. After 30 days, and again after 

 60, 3 typical tadpoles of each lot were measured. Their 

 averages, together with those from a second series, are given 

 below : — 



TABLE XLI 



Showing for Two Lots of Tadpoles the Relative Sizes attained in the 

 Light and in the Dakk 



This table clearly shows that tadpoles grow faster in the light 

 than in the dark, and that the difference in the rate of growth 

 is more marked during the first than during the second month 

 of development.* 



Other animals have been experimented upon by Yung. He 

 placed recently fecundated eggs of the sea trout, Salmo trutta, 

 in 4-liter vessels, each of which contained 200 eggs. Those 

 lots which were reared in the light hatched a day earlier than 

 those reared in the dark. Also, the pond snails, Lymnsea stag- 

 nalis, reared in the light hatched in 27 days, whereas those 

 reared in the dark required 33 days: Perhaps less weight is to 

 be given to the observation of Hammond ('73), who found 

 that 20-day-old cats, reared under similar conditions except as 

 concerns daylight, grew faster in the light than in the dark. 



* With these experiments agree certain experiments of Lessona ('77) and 

 Cajierano ('93) upon the size of tadpoles talien from ponds so thickly covered 

 with vegetation as to shut out the light, as compared with tadpoles from fully 

 exposed ponds. Camerano finds that, at the same stage of development, the 

 sizes are as 9 to 14, or the tadpoles reared in the light are 156 % of the size of 

 those reared in the darker ponds. Such observations in which the other con- 

 ditions are not controlled are not, however, altogether satisfactory. 



