9 • DEVELOPMENT OF THE 

 FROG EGG 



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e have just carried an analysis of early embryonic development to 

 the point where most of the organs are determined. During the course of the 

 analysis we have drawn upon experiments and observations from many 

 sources, and the reader does not as yet have a clear picture of the orderly 

 progressive changes in any one form. Largely because of its accessibility the 

 frog egg has been extensively studied, and the stages from fertilization 

 through larval development have been well described. The fact that pituitary 

 injections cause ovulation makes possible the artificial insemination of the egg 

 and the continuous observation of the changes in form with time. 



The rate of these changes with respect to time is directly proportional to 

 absolute temperature. The frog egg has no means of regulating its tempera- 

 ture, and thus the speed of its development depends upon the temperature 

 of the environment. For example, the egg reaches any given stage almost 

 three times faster at 20° C. than at 10° C. The increase in development rate 

 is a case of the general phenomenon of the increase in the rate of chemical 

 reactions with increase in temperature. 



Although temperature may be used to control the rate of development, 

 there are limits to the temperature range which the frog egg will withstand. 

 The eggs of the species described here (Rana sylvatica) will develop from 



in 



