THE RATE OF GROWTH 89 



four degrees centigrade ; the other two at temperatures 

 between. They are all descendants from the same 

 batch of frogs' eggs, and you can see readily that the 

 first one is still essentially nothing but an egg. The 

 second one, which has had a little higher temperature, 

 already shows some traces of organisation, and those 

 familiar with the development of these animals can 

 see in the markings upon the surface the first indica- 

 tions of the differentiation of the nervous system. 

 The third has been kept at a considerably warmer 

 temperature, and is now obviously a young tadpole ; 

 here are the eyes, the rudimentary gills, the tail, etc. 

 While the fourth tadpole, which was maintained at 

 the best temperature for the growth of these animals, 

 has advanced enormously in its development. Ob- 

 viously, should we make experiments upon animals 

 of this class it would be necessary to keep them at a 

 uniform temperature, if we wished to study their rate 

 of development, and that is, for very practical rea- 

 sons, extremely difficult and unsatisfactory. Far bet- 

 ter it has seemed for our study of growth to turn 

 to those animals which regulate their own tempera- 

 ture. This, accordingly, I have done, and the animal 

 chosen for these studies was the guinea-pig, a crea- 

 ture which offers for such investigations certain de- 

 finite advantages. It is easily kept ; it is apt to 

 remain, with proper care, in good health. Its food 

 is obtainable at all seasons of the year, in great 

 abundance, and at small expense. The animals them- 

 selves being of moderate size do not, of course, re- 



