INFLUENCE OF THYMUS FEEDING UPON DEVELOPMENT 139 
In the above mentioned group (O. 1916) which consisted of 
larvae of Amblystoma opacum fed with equal quantities, the 
development of further advanced characteristics of the legs, 
of the shape of the head, of the gills and of the color of the skin 
proceeded much more rapidly in the thymus-fed animals than 
in those fed on worms. 
With respect to the development of the gills, the following 
should be remarked: In larvae of Amblystoma opacum fed with 
worms and kept at an average temperature of 22.6°C. as late as 
one or more days before metamorphosis the gills attain a stage 
not only of considerable size, but one in which they are char- 
acterized by considerable redness and above all by the fact that 
they are bent upwards in a crescent shape. The long well 
developed branches are widely extended and the points of the 
stem inclined forwards so as to bend over. In worm-fed 
animals kept at a high temperature (22.6°C.) this condition of 
the gills was only attained in the 23rd week, but in thymus-fed 
animals kept at the same temperature as early as the 11th week; 
in worm-fed animals kept at a low temperature (in average 14.8°C.) 
only in the 29th week; in thymus-fed larvae kept at the same 
temperature as early as the 11th week (although in the latter 
case the gills were less developed than in the case of the high 
temperature thymus-fed animals). 
A similar relationship is observed with respect to the color of 
the skin. In the case of the warm worm-fed animals the melano- 
phore spots only began to develop in the 13th week, at which 
time they had already attained very considerable development 
in the case of the warm thymus-fed animals; in the warm worm- 
fed individuals the blue-grey pigment did not appear until the 
24th week, but in that of the warm thymus-fed animals as early 
as the 12th week. In the cold worm-fed animals the fusion of 
the melanophore spots into a uniform black-brown coat only 
began in the 380th week, and occurred as early as the 13th week 
in the case of the cold thymus animals; but in the cold worm-fed 
animals no trace of a silver-grey pigment can be detected after 
32 weeks, although this appeared in the cold thymus-fed in- 
dividuals as early as the 13th week. These differences in the 
