AD? ROSS G. HARRISON 
specimens the layers of the embryo are more readily separated, 
though in the older ones the tissues are of a firmer consistency 
and can be handled more satisfactorily. 
After the disc is excised the wound at first gapes, but a few 
minutes later it contracts. Then in the course of the next 
twenty-four or forty-eight hours the ectoderm stretches itself 
over the wound bed and covers it entirely. There is much 
variation in the time required to complete this process, and in 
some cases the wound has been found still partly open four 
days after the operation. Sections show that the wound is first 
covered by ectoderm and that the mesoderm creeps in soon after 
between that layer and the yolk. 
The first problem is to determine whether by an operation of 
this kind the development of the limb can be prevented, and, if 
so, how the size of the wound affects the outcome of the experi- 
ment. Two kinds of operations were done. In the first the limb 
disc was simply lifted and the wound left without further treat- 
ment. In the second the wound bed was afterwards carefully 
cleaned of all mesoderm cells. In some cases of each kind the 
pronephros was left intact and in others it was removed. Ex- 
tirpation of this organ facilitates the cleaning of the wound, but 
since many cases of non-regeneration with intact pronephros 
occurred as well as some cases of regeneration without its pres- 
ence, its influence, if any, upon the development of the limb 
must be unessential. 
Relation of regeneration to size of wound and completeness * of 
removal of mesoderm 
The smallest wounds were three somites in diameter, encom- 
passing in all but two cases the region ventral to myotomes 3, 
4. and 5. In the two exceptional cases the wound, which was 
not cleaned, included the area below somites 4, 5, and 6; one of 
these regenerated and one died. The largest were 4% somites 
in diameter, extending from the boundary between the second 
and third to the vertical line dividing the seventh myotome in 
half. 
