INFLUENCE OF THYMUS FEEDING UPON DEVELOPMENT 137 
muscle tissue, and before the latter had developed front limbs. 
This last series of experiments suggests the assumption that the 
influence exerted by the thymus on development must be de- 
pendent on factors not specific for the thymus. The experi- 
ments on Salamander larvae now to be described led to similar 
conclusions. 
Both Gudernatsch and Romeis took as index of the rate of 
development, the growth of the hind and front limbs, the 
absorption of the tail and the abandonment of the water. The 
latter phenomenon however, which we will refer to as meta- 
morphosis seems in Salamanders, to be dependent on a mechan- 
ism different in many respects from that which controls develop- 
ment, such as the growth of limbs, ete.; for in the first place 
even under conditions of normal feeding, different individuals 
show a different stage of development when they leave the water, 
and secondly the effect of thymus upon development and upon 
metamorphosis does not seem to be the same in the’ Salaman- 
der larvae examined. Therefore we shall distinguish between 
development and metamorphosis; growth and differentiation 
of the limbs, certain changes of the fin and gills not being in 
direct relation to the abandonment of the water, and the changes 
of the color pattern of the skin which finally lead to the definite 
coloration of the skin, will be referred to as development; while 
the abandonment of the water together with the sudden reduc- 
tion of the gills to mere stumps and the complete absorption of 
the fin will be called metamorphosis. 
In a group of eight series, O 1916, in which larvae of Ambly- 
stoma opacum were used, four series were fed with small frag- 
ments of earthworms and four series with equal sized pieces of 
thymus. As will be explained later on, these experiments were 
conducted with the intention of feeding the respective animals 
with equal quantities of worms and thymus. So far as develop- 
ment and metamorphosis is concerned, it would seem at least 
possible, that besides the quality of food, the amount of food may 
also have some influence upon these two phenomena; but at any 
rate only if we make the quantities of food alike in the experi- 
mental series and the controls, can we be sure that the differences 
