NATURALIST. [Vol. XXXVII. 



Main- larva? have metamorphosed readily after introduction into 

 the almost total darkness of a closely covered, deep cistern. 



As to temperature similar facts may be cited ; indeed, many 

 of those already c ited are obviously applicable. The instance of 

 prolonged larval growth in the east window is especially interest- 

 ing, for on several of the hottest days of the season other larvae, 

 in like jars in the same window, died of the heat. On the other 

 hand scores of small larva? have undergone metamorphosis in 

 shaded aquaria supplied with currents of cool tap water. The 

 instance, already cited, of the phenomenally early metamorphosis 

 of numerous larvae in the cool water of the fountain basin may 

 be recalled. Even six small larvae which I introduced into a 

 tank supplied with a stream of the coldest spring water, heavily 

 shaded with trees and covered with two-inch plank, all underwent 

 the change in the course of a few weeks. In short, these 

 Amblystoma larvae have, with the writer, proved singularly 

 indifferent to wide variations of temperature and luminosity. 



What then does control the metamorphosis ? But one cardinal 

 factor in the animal's economy is left, and observation and 

 experiment show it to be the dominant factor in question : nutri- 



accurately, it mav be said that metamorphosis is a matter of 

 metabolism, of anabolism passing into sudden katabolism, as the 



far the most effective and the most frequently operative. 



Liability to metamorphosis at any given time is great, in 

 direct proportion to the prevalence of anabolic change at that 

 time ; the certainty of metamorphosis at any time is great, in 



