334 ZOOLOGY 



rate at which the development of frogs' eggs proceeds de- 

 pends closely upon the temperature of the water. They 

 develop most rapidly at about 30-32 C. If the tempera- 

 ture is elevated above this point, the rate of development 

 is retarded, and finally ceases at about 40 C. So, likewise, 

 as the temperature is lowered, the development is retarded, 

 until at the temperature of freezing water it ceases 

 (Fig. 306). If the temperature is too high, development 

 may be abnormal, so that a monstrosity is produced. 



Light has a less striking effect on the development of 

 the frog. If light is excluded from the developing eggs, 

 they will develop more slowly. The acceleration of 

 development by rather high temperature and by daylight 

 is probably due to a chemical effect of these agents. It 

 indicates that development is a complex chemical process. 



Healing and Regeneration. If the egg of a frog be 

 pricked slightly, there will be a loss of substance and the 

 resulting embryo will be at first abnormal. Later, how- 

 ever, this abnormality will become smoothed over by 

 appropriate development. So, also, if the tail of the 

 developing larva is mutilated, the wound will heal and the 

 missing parts will be re-formed. This capacity of the living 

 organism to restore the normal form after mutilation is 

 seen also in man. For if some of the skin be cut away, or 

 even if parts of internal organs are removed, the wounds 

 will not merely heal, but the lost part will regenerate. 

 The remarkable thing is that in regeneration almost 

 exactly that is produced which was lost. Both regenera- 

 tion and healing in the adult are a survival of the same 

 capacity for development which we see in the egg. 



Postembryonic Development of the Frog. After reach- 

 ing a certain stage of development the embryo frog, called 



