AND EMBRYOLOGY OF FROGS. 391 



VII.— Effect of Temperature.- It is impossible to give any 

 exact time to the different stages of development, as the time is 

 direcdy proportional to the temperature of the water within certain 

 limits. The highest temperature is not always the optimum, for 

 several bunches placed in an incubator for hens' eggs were entirely 

 destroyed. The freezing of the water in which the eggs are kept, 

 does not seem to injure the eggs in the least, but simply to retard 

 their development. I have had eggs completely surrounded by 

 ice, and afterward development quite normally. However, when 

 the eggs themselves are actually frozen, they seem to be destroyed, 

 perhaps by the formation of ice spicules within them. The wood- 

 frogs, which lay their eggs so early, generally lose, in this locality, 

 great numbers of them, on account of getting caught in the ice. 

 Those which are not so caught, develop later, when the ice melts, 

 and do not seem, in any way, to be injured by water at the freezing 

 point. I think there is here a most interesting field for experimen- 

 tation by the physiological embryologist, and I regret I have not 

 kept exact records of the effects of heat and cold. 



VIII.— Time of Hatching, etc.— The different species of frogs 

 leave the jelly membrane at different ages. Some have the tail 

 well developed, and are quite active ; others have the tail just 

 appearing, and are only able to twist their bodies slowly from side 

 to side, as they cling to the jelly-mass by means of the suckers 

 below the mouth. 



The young tadpoles of the wood-frog leave the water as small 

 frogs in the late spring of the same year in which they were laid, 

 that is, become frogs in four to six months. Eggs collected about 

 March 17th, began to change to frogs about June ist to 14th. 

 These were kept in quite cool water, in a basement room, away 

 from the sun-light. 



\t the time of transformation into tadpoles, a sudden decrease 

 in the length of the intestine is brought about. The tadpoles cease 

 to eat, and the intestine is entirely freed from extraneous matter 

 during this time. The change takes place at the same time that 

 the tail is absorbed within the body (not dropped off, as popularly 

 supposed), and, at the same time, the pair of fore feet, which were 

 enclosed within the branchial fold, break through the exterior. 

 The intestines were removed and measured from the pyloric end 



