44 HELEN DEAN KING. 



very great care was taken that all of the individuals being ex- 

 perimented upon were kept under similar external conditions, the 

 only factor that was intentionally varied being that of nutrition 

 whose action it was proposed to study. 



Two separate series of experiments were made which, for con- 

 venience, will be called Series I., and Series II. The eggs used 

 in Series I., were laid in the laboratory and normally fertilized on 

 the morning of April 13, 1906 ; while those used in Series II. 

 were laid under similar conditions on April 16, 1906. Both lots 

 of eggs were kept in large aquaria until the tadpoles hatched, and 

 the experiments began in each case five days after the eggs were 

 laid. 



Series I. was started with a total of 1,500 individuals ; but, as 

 will be explained later, only 1,100 of these can be taken into 

 account in considering the results. Eight hundred individuals were 

 used in the second series of experiments, making a total of 1,900 

 individuals upon which to base conclusions from the results ob- 

 tained. Glass dishes of uniform size were used throughout the ex- 

 periments, each dish containing, in the beginning, 100 tadpoles. 

 The dishes were kept together so that the tadpoles were all under 

 the same conditions of temperature. The water used was " tap " 

 water obtained from an artesian well and used for drinking pur- 

 poses, so presumably it was free from unicellular organisms. 

 Approximately the same quantity of water was kept in each dish. 



Out of the total of 1,900 individuals, only 364, or 19.15 per 

 cent, died before it was possible to ascertain the sex. This com- 

 paratively low rate of mortality during the early stages of develop- 

 ment I attribute in great part to the fact that the water in the 

 dishes was never allowed to become foul. When the tadpoles 

 were very small the water was changed on alternate days and the 

 dishes carefully cleaned. Later, as the tadpoles became larger, 

 it was necessary to change the water every day. During some 

 very warm weather in June when the tadpoles were beginning to 

 undergo metamorphosis, the water was renewed as often as four 

 or five times daily. 



In Bnfo the genital organs are apparently much better devel- 

 oped at the time of metamorphosis than they are in Rana, as in 

 the majority of individuals it is possible to distinguish the males 



