DEDIFFERENTIATION IN PEROPHORA 679 
a retardation of growth in weight, then an acceleration, 
and finally lived about 12 per cent. longer than normal 
controls. Other experiments had led him to conclude that 
tethelin (cr pituitary extract) caused increased growth in 
cellular tissues, a conclusion strengthened by the recent 
crafting experiments of Allen (1920) on tadpoles. His explana- 
tion of the facts is as follows. ‘Tethelin causes at first an 
absolute increase in the growth-rate of the cellular tissues of 
the body ; this involves, as we have seen, a relative decrease 
in the weight of the supporting tissues. Since these latter 
are the heavy tissues, this involves an absolute decrease in 
total weight. Eventually, however, the characteristic relation 
between the amounts cf cellular and supporting tissues is 
established, but later than normal. Relative increase of the 
supporting tissues characterizes old age; and the onset of 
senility is delayed by that period by which the establishment 
of the cellular-supporting balance was postponed. ‘The 
reason for the more rapid growth of the cellular tissues at the 
beginning is that the tethelin stimulates them to greater 
activity, and that consequently they obtain first call on the 
available foodstuffs. 
This view-point, it will be seen, is very similar to that of 
Child. 
A beautiful example of differential inhibition depending only 
on the two quantitative factors of size and distance is given in 
the interesting paper of Detwiler (1920; see especially pp. 149- 
51). Detwiler transplanted the limb-rudiments of Ambly- 
stoma autoplastically, cutting the rudiments out and trans- 
planting them a varying number of segments posteriorly 
from their normal position. The experiments were undertaken 
at a stage when the rudiments were represented only by 
circular thickenings of somatopleuric mesoderm in segments 
3-5. He found, as had previous workers such as Harrison, 
that in many cases the rudiment was not completely excised, 
a few of its cells being left in the normal position. When this 
was so, these cells usually begin to regenerate on their own 
account. It is of interest to note that this regeneration is 
