526 METAMORPHOSIS IN AMPHIBIANS 



the tailless amphibians. Allen''' has shown that extirpation of the thyroid in 

 the larvae of Rana pipiens, though it prevents metamorphosis, does not retard 

 development of the sex organs, and Swingle's*' experiments prove that feeding 

 thyroid to tadpoles, though it enforces metamorphosis, does not accelerate 

 development of the germ cells. 



The fact that the germ cells in amphibians are independent of 

 metamorphosis as regards their development has recently given rise 

 on the part of Professor Allen'^ to renewed consideration of Weismann's 

 hypothesis postulating the existence of a fundamental difference be- 

 tween the germ plasma and the somatic plasma, since the observations 

 made in amphibia may be interpreted to mean that the germ plasma is 

 independent of the somatic plasma. This interpretation, however, 

 would be correct only if it could be shown that the independent be- 

 havior during development is peculiar only to the germ cells, and that 

 consequently the soma as a unit is opposed to the germ cells as an 

 independent unit. But we will show, in the present paper, that upon 

 further analysis of amphibian development, it is found that the so 

 called soma, in the Weismannian sense, does not exist; for it is pos- 

 sible to demonstrate that in salamanders this soma can be made to 

 disintegrate into a number of organs which, like the sex organs, are 

 independent of metamorphosis and, moreover, are independent of 

 each other as regards their development. Furthermore, the nature 

 of metamorphosis will have to be defined more accurately to mean 

 not the development of the animal as a whole, but the development 

 of certain organs of the amphibian organism. The remarkable 

 feature in the development of amphibians is not the independence of 

 the germ plasma from the somatic plasma, but the independence of 

 various groups of organs from one another, due to the fact that the 

 development of each of these groups is controlled by substances dif- 

 ferent from those controlhng the other groups, and that each of these 

 substances separately may be supplied to or withheld from the organ- 

 ism by the experimenter at will, either directly or by means of 

 changing the environmental conditions. 



5 Allen, B. M., /. Exp. ZooL, 1917-18, xxiv, 499. 

 « Swingle, W. W., /. Exp. Zool, 1917-18, xxiv, 521. 



