ZOOLOGY: R. G. HARRISON 
543 
ment of a normal limb. Since these differ only in degree from those 
experiments of the first group in which the wound was not carefully 
cleaned, they have been included in the tabulation under that class. 
6. To transplant the mesoderm alone, the ectoderm is first torn free 
from the underlying tissues after the usual circular incision is made. 
The mesoderm of the region is then cut out in a single piece as large 
as possible, and transferred with a blunt needle to a small pocket under 
the ectoderm of another embryo previously prepared. The mass of 
cells is then pushed into the pocket and left, a procedure that is not 
always easily carried out because of the extreme stickiness of these cells. 
Nineteen experiments were made, in four of which the embryo died 
before yielding results. In five cases the transplanted tissue was entirely 
resorbed; in three others it remained a small nodule; in one a long 
appendage without digits resulted; and in six there developed a limb 
of approximately full size which, 
however, in most cases showed 
some irregularities, usually in the 
form of reduplications. 
The most perfect of these 
showed reduplication of only a 
single digit; another gave rise to 
a perfect limb with complete re- 
duplication of fore-arm and 
manus. The one shown in figure 
3 has the beginning of a super- 
numerary digit on the radial side 
of the hand. The other cases 
were more irregular. Sections show that in addition to the free ap- 
pendage a small shoulder girdle is formed from the transplanted tissue. 
This is also the case when the entire limb rudiment is transplanted, as 
is shown both by Braus's^ experiments upon Bombinator and my own 
upon Ambly stoma.. 
Conclusions. The anterior limb of Ambly stoma, according to the fore- 
going experiments, is already determined, at the time of appearance of 
the tail bud, in the mesoderm cells of that region of the body wall which 
lies close to the pronephros and ventral to the third, fourth, and fifth 
myotomes. The prospective significance of this group of cells as a 
whole thus is defined some time before differentiation becomes visible. 
Forming a narrow zone around this region there are cells which may 
vicariously give rise to a limb in case the rudiment proper is removed. 
These cells, responding to the stimulus of the wound, gradually move 
up toward the center, and covering the bare yolk, afterward in many 
FIG. 3. AMBLYSTOMA LARVA WITH SUPER- 
NUMERARY FORE LIMB, DEVELOPED FROM 
CELLS INOCULATED FROM THE MESODERM OF 
THE LIMB REGION OF AN EMBRYO IN THE 
STAGE SHOWN IN FIGURE 1. EXPERIMENT TR. 
MES. 1. PRESERVED 16 DAYS AFTER OPERA- 
TION. X 10. 
