INFLUENCE OF THYMUS FEEDING UPON DEVELOPMENT 153 
develops characteristics of a metamorphosed animal, while the 
animal as a whole still is in a larval stage. At the time when 
metamorphosis should occur disturbances in the course of de- 
velopment begin to appear evidently due to the suppression of 
the development of some factor, without which further develop- 
ment is impossible. In most of the animals of a thymus-fed 
series this factor still develops much earlier than.in the controls; 
but even in these individuals metamorphosis becomes a grave 
danger to the animals’ life. In high temperature some animals 
die during metamorphosis and those which survive metamorpho- 
sis die a relatively short time after metamorphosis. In some in- 
dividuals the development of the factor necessary for meta- 
morphosis is still more disturbed and becomes delayed in com- 
parison with the controls; at high temperature all individuals in 
which this is the case die on the day when the gills and the rest 
of the fin undergo the sudden reduction in size, characteristic 
of the entrance into metamorphosis. In low temperature they 
may survive metamorphosis. In low temperature a very small 
percentage of the thymus-fed animals may remain at a low stage 
of development and not metamorphose for more than a year; 
but whether this is due to the action of the thymus diet is not 
yet certain, as a similar phenomenon was observed in one worm- 
fed animal of the stock. 
It seems that we cannot understand the results reported in 
thymus feeding experiments if we assume that they are the pure 
expression of the influence of the thymus substance. The rather 
great fluctuations reported in individuals of the same species 
as well as the surprising differences between larvae of Anura and 
Urodela when fed on thymus, indicate that quite a number of 
factors are involved in metamorphosis, some of which were not 
controlled in the experiments. It is of course clear, that differ- 
entiation of the organism is one of these factors; that a certain 
degree of differentiation is indispensable for metamorphosis, or 
at least to facilitate it, was shown by Gudernatsch in some recent 
experiments on the influence of thyroid. That some of the 
individuals among a thymus-fed series of Salamander larvae 
metamorphose earlier than the controls may be due in some degree 
