418 Marion L. Durbin 
muscle appears later than ectoderm, notochord and connective 
tissue. A cross-section of the tail at an early stage of regenera- 
tion has the space, that is later occupied by muscles, filled with 
embryonic connective tissue. When the rate has reached its 
maximum the cells which form the muscle fibers appear, at the 
level of injury, in the part of the muscle region nearest to the 
ectoderm. Since muscle tissue does not begin to produce new 
cells until after the measurable regeneration is well begun, it is 
evident that it does not contribute to the formation of the first 
layer of cells which covers the wound; furthermore regions of the 
tail in which much of the cross-section is composed of muscle will 
be slow to complete this first wound-covering layer, and hence 
slow to begin rapid regenerations. ‘The upper levels of the tad- 
pole tail contain proportionally more muscle in cross-section than 
is found near the tip, so it is not difficult to explain the relatio 
between the amount removed and the length of the initial inter 
val of low rate upon the theory that the rate in the initial interva 
is low because time is spent immediately after the operation 1 
building a layer of embryonic tissue over the wound. 
A combination view of the slowness of regeneration in the first 
period is probably more sane. The low rate is perhaps inevi- 
table because undifferentiated tissue must be present at the level 
of injury before rapid regeneration can proceed; but the supply 
of nourishment to the new cells and the general body tone of the 
tadpoles may be influenced by the depressed condition they are 
in after the operation. 
Whether the formation of an embryonic tissue has anything 
to do with the low rate in the first period or not, it is true that by 
the end of a day and a half or two days in most cases, the wound 
made by the operation is covered with a thick layer of undiffer- 
entiated cells many of which are dividing. This condition is 
quite enough to account for the second region of the curve show- 
ing the rate of regeneration rapidly rising toa maximum. 
The third region of the curve, that representing the rapid 
decrease in rate, is more difficult to explain; it is closely associat 
with the fourth region, the one of gradual decline. An exa 
nation of Chart I (Experminent IV) will reveal a corresponde 
