INFLUENCE OF THYMUS FEEDING UPON DEVELOPMENT 155 
nal conditions, such as temperature and quantity of food andjin 
general by all factors which modify growth, rate of growth, size 
and velocity of development. No doubt such factors are of 
great importance in determining at what time, at what size and 
developmental stage of the animal, metamorphosis will occur. 
Since the relations between these different factors are very 
complicated and the number of experiments relative to them is 
rather small, discussion of these conditions must be postponed. 
Finally it should be mentioned that the thymus gland appar- 
ently contains all substances which are necessary to build up the 
substance of an Amphibian organism to maintain the animal 
growing and to sustain life permanently. This is demonstrated 
by a number of specimens of A. punctatum kept at low tem- 
perature which have been fed on thymus since about the 14th 
day of their life and are now about 14 months old; they are in- 
creasing in size. 
