CH. XVI] EFFECTS OF TEMPERATURE AND LIGHT 169 



nevertheless, as subsequent development showed, the egg had 

 been injured. Many of these eggs developed in the same way 

 as did those kept at a temperature of 25 degrees C., i.e. the 

 segmentation of the yolk-hemisphere was retarded. 



Schultze ('95) has also made some experiments on the 

 eggs of Rana fusca in which the eggs were subjected to a 

 temperature of zero C. Embryos in the following stages of 

 development were used : stage A, when the dorsal lip of the 

 blastopore had just appeared; stage B, at the end of the "gas- 

 trula" period; stage C, embryos with closed medullary folds. 

 Three days after these had been placed in a chamber at zero C. 

 they were examined and found in the same stage as when put 

 into the cold. Some of the eggs were then removed, and con- 

 tinued to develop normally at a higher temperature. After 

 fourteen days in the cold the remaining eggs were examined. 

 The eggs were still in the same stage as when put into the 

 cold chamber, but those of stage C had died. The others 

 developed normally when brought into a higher temperature. 



Thus while Hertwig found that the eggs of Rana fusca were 

 injured by only twenty-four hours at a temperature of zero C., 

 Schultze saw that certain stages, at least, were not affected by 

 fourteen days at the same temperature. It is to be noted that 

 Hertwig put the eggs into cold water soon after fertilization, 

 while Schultze used later stages of development. 



Not only is the rate of development of the frog-embryo 

 affected by the temperature, but also by the kind of light in 

 which it develops. Schnetzler in 1874 compared the develop- 

 ment of Rana temporaria in white and in green light. The con- 

 ditions of the two sets of embryos were nearly the same except 

 as regards the kind of light. The embryos developed much 

 faster in the white light, and the tadpoles underwent sooner 

 their metamorphoses. Yung ('78) made a much more careful 

 and elaborate series of experiments in which the eggs and 

 embryos were subjected to a series of different lights. Instead 

 of colored glass, which is seldom monochromatic, Yung used 

 solutions of different sorts. The eggs were placed in a vessel 

 containing about 5 litres of water ; this vessel was then placed 

 in a larger vessel of the same form. A space of 5 to 10 mm. 

 was left between the two vessels. This space was filled with 



