INFLUENCE OF THYMUS FEEDING UPON DEVELOPMENT 143 
whereas in the case of the Anura larvae the thymus animals 
must be larger than the worm animals before metamorphosis 
can occur. 
However, in addition to the facts mentioned above, still an- 
other phenomenon must be described which seems to aid greatly 
our understanding of the relation between development and 
metamorphosis. If we refer metamorphosis neither to the time 
which has passed since hatching nor to the size of the animals but 
to the stage of development of certain structures, metamorphosis 
does not appear to be accelerated in the thymus animals but 
rather retarded. 
For example, when comparing the warm worm animals with 
the warm thymus animals of the Opacum group (O 1916) we 
see that as early as the 11th week the warm thymus animals 
attained the same stage of development at which the warm worm 
animals commenced to metamorphose. At this stage, however, 
a remarkable phenomenon is noted; the warm thymus animals 
fail to metamorphose while some of their organs continue to 
develop; the structures of their skin, which are responsible for 
the development of the color of the skin attain, while the animal 
is still larval a phase of development reached by the worm animals 
only some time after metamorphosis has been accomplished. 
After the warm thymus animals have entered upon the stage 
characterized by the crescent-shaped gills and the fusion of the 
melanophore spots, they should, if compared with controls, 
undergo metamorphosis, but instead they develop the silver- 
grey pigment and undergo reduction of the size of the fin. Simul- 
taneously (a point to be specially emphasized) they stop growing 
and become reduced in length, a condition which also occurs in 
the case of worm animals before metamorphosis. They assume 
an aspect which on the whole resembles that of a worm-fed 
animal which had undergone metamorphosis about two weeks 
previously. As can already be seen, these relationships can 
be noted much more distinctly in the cold Opacum series; but 
as the animals of these series have not yet all undergone meta- 
morphosis and the worm animals have not yet begun to meta- 
morphose, we will not describe the phenomena already noted. 
