164 WITHROW MORSE. 



Pointing to the correctness of this interpretation is the fact 

 that during the development of the adult organs (legs, etc.) 

 no food is taken by the larva. Storage of fat, etc., has taken 

 place during the earlier period of larval existence, but the impor- 

 tant thing is to remember that protein is never stored as such in 

 the animal organism. The nitrogenous material for the legs, 

 etc., developing during metamorphosis must be supplied from some 

 source other than storage. The simplest explanation is to assume 

 that it is derived from the larval organs. 



VON BAER'S LAW AND LARVAL METAMORPHOSIS. 



The frog has been taken as a classic example of the operations 

 of the so-called Law of Von Baer, termed sometimes the Law of 

 Recapitulation and at others that of Repetition. The question 

 is, can the metamorphosis be explained wholly upon the assump- 

 tion that physico-chemical events other than those concerned 

 with heredity initiate the process? 



Those who have experimented with the agents accelerating 

 metamorphosis have found that these compounds are impotent 

 except in the case of larvae which have reached a certain period 

 of their existence. In other words, we know of no agent which is 

 operative regardless of the stage of development of the larvae. 

 We may suppress metamorphosis; we can not, or have not in- 

 duced it in stages far removed from those in which it would nor- 

 mally occur. This indicates that a certain cycle of events, 

 probably determined by heredity, are necessary before any stimu- 

 lating agent is effective. The growth of the pygostyle according 

 to Barfurth, initiates the process of atrophy in the tail, but what 

 events stimulate the growth of the pygostyle are unknown. In 

 some species, which do not metamorphose during the first sum- 

 mer, the larvae are subjected to relatively the same stimuli from 

 the environment during the second summer as during the first. 

 It is improbable that external conditions determine the time of 

 metamorphosis. Again, to offer the explanation that hormones 

 or enzymes initiate metamorphosis leaves open the more potent 

 question as to what determines the development of these com- 

 pounds. 



