CHAPTER XVI 



EFFECTS OF TEMPERATURE AND OF LIGHT ON 

 DEVELOPMENT 



IT has been long known that the rate of development within 

 certain limits is dependent on temperature. The development 

 of the frog's egg is very much retarded, or even stopped, in 

 water at the freezing-point. In North America, Rana tempo- 

 raria often lays its eggs so early in the spring that the water 

 is afterward frozen. The eggs that are caught in the ice are 

 generally killed, but those that lie in the water below often 

 remain alive and will subsequently develop normally. 



Hertwig ('94) has shown that the maximum temperature 

 for normal development of the eggs of Rana fusca is about 

 25 degrees C. Eggs develop very rapidly at this tempera- 

 ture, and in twenty-four hours have reached a stage of ad- 

 vancement corresponding to that at the end of the second day 

 for the average temperature of 16 degrees. A temperature 

 of 25 to 30 degrees C. long continued, or a temperature of 30 

 to 35 degrees for a short time, injures the eggs ; their develop- 

 ment is arrested and many die. Eggs that have been partially 

 injured by heat (after two or three hours at 30 degrees C. or 

 after three to eight hours at 26 to 28 degrees C. and then 

 brought into a normal temperature) continue to develop at a 

 slower rate than eggs under normal conditions. The yolk- 

 hemisphere of the egg is first affected, so that the cleavage- 

 furrows do not appear in it. The injured or dead half of the 

 egg lies below, and the segmented portion above. 



Hertwig obtained similar results by cooling the eggs. Soon 

 after fertilization the eggs were placed in water at zero C. and 

 kept there for twenty-four hours. During that time they did 

 not segment, but when brought back to a higher (normal) 

 temperature, the egg divided into two, four, etc., blastomeres ; 



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