No. 4-] 



THE DEVELOPMENT OF ANIMALS. 



183 



' or 4 C. The weather usually 



water is not more than 3 

 remains cool for at least a month after the laying of the eggs, 

 so that during the early development the water never becomes 

 very warm. The eggs of Rana virescens are not laid until the 

 water is much warmer ; thus they are never exposed to such 

 low temperatures as Amblystoma. 



a. Normal Temperature Range. - - The extent of the early 

 development measured in both forms was from the first or sec- 

 ond cleavage to the last stage of disappearance of the yolk plug ; 

 these periods were selected as being most sharply marked 

 ones ; and the time occupied even at the optimum is suffi- 

 ciently long to reduce the amount of error to a very small 

 proportion of the total time involved. 



TABLE III. Table of the time of development of the ova of Amblystoina tigrinuni from the 

 first cleavage to the last stage of disappearance of the yolk-plug. One observation does not 

 mean necessarily a single egg, but often all the eggs of a bunch. 



TABLE IV. Table of the time of development of the ova of Rana virescens from the first, 

 second, or third cleavage to the last stage of disappearance of the yolk-plug. Each obser- 

 vation means at least 30 eggs. The period between the first and third cleavages is relatively 

 so short that it may be ignored in comparison with the whole time involved. 



