424 ROSS G. HARRISON 
eases of this kind available. In these cases the healing is 
usually very slow. The yolk, being long exposed, begins to 
disintegrate after a time, and the embryo rarely recovers if 
this process sets in. It is possible that when a few mesoderm 
cells are left in, they cover the yolk and facilitate the overgrowth 
of the ectoderm. 
Although the regenerative capacity is dependent to some 
extent on the size of the wound, there are experiments with 
wounds of each of the several sizes in which no regeneration 
took place. Within the limits of these operations it is there- 
fore impossible to say that wounds beyond a certain size pre- 
clude regeneration altogether. This can only be said of those 
eases in which the wound is covered with indifferent skin, as 
described in another section (p. 432). 
It is reasonably certain from a study of the normal develop- 
ment that the cells which ordinarily give rise to the limb bud 
take origin in the region below the fourth and the neighboring 
parts of the third and fifth somites. In this region, as the limb 
bud becomes more prominent, numerous mitoses are found, 
while the rest of the somatopleure is almost devoid of dividing 
cells. In those cases in which the region corresponding to the 
whole of these three somites, or even more, is removed the cells 
which give rise to the regenerated appendage are, therefore, 
probably such as in normal development do not participate in 
its development. Miss Byrnes (’98) has emphasized this in 
her paper on the hind limbs of Rana. 
Relation of regeneration to age of embryo at time of operation 
There seems to be no definite correlation between the age of the 
embryo at the time of operation and the occurrence of regenera- 
tion. The tabulation here given (Table 2 a), which includes both 
covered and uncovered wounds shows that when cases are re- 
corded in significant numbers, both positive and negative results 
are obtained after operation in both older and younger stages. 
The number of cases operated in the extreme stages is small 
and no statistical value can be placed upon the figures there. 
