456 ROSS G. HARRISON 
No attempt was made to orient the engrafted tissue, if indeed: 
this were possible, but the results which have been obtained 
may be interpreted in accordance with the results from trans- 
planting the whole limb bud, where the orientation of the graft 
is known (Harrison, 717). In three of the cases the limb tissue 
was grafted on the same side of the embryonic body; one of these 
yielded a limb of the same side, while in the other two the later- 
ality was reversed. In four cases the tissue was implanted on 
the opposite side of the embryo; in one of these the limb pre- 
served the original laterality of the tissue, while in the re- 
maining three reversal occurred. 
CONCLUSION 
The purpose and results of each of the experiments having 
already been pointed out, it remains only to state briefly their 
significance as a whole. 
The tissue which is destined normally to form the fore limb 
has been delimited, and within the period of development dealt 
with in the experiments, it has been shown to be a self-differen- 
tiating system. It is a group of mesoderm cells formed as a 
proliferation of the somatopleure, and no specific stimulus from 
any particular portion of the ectoderm is necessary for its de- 
velopment. The exact boundary of the embryonic tissue which 
normally enters into the limb cannot be determined by the pres- 
ent methods; for, surounding the group of cells which constitutes 
the limb bud, there is a zone of mesodermal tissue, which, in 
case of removal of the original limb rudiment, may move in 
and assume the character of the excised material, giving rise 
after a time to a normal limb, as was first shown by Miss Byrnes 
(98 b) in the case of Rana embryos. The limb rudiment may be 
thus regarded not as a definitely circumscribed area, like a stone 
in a mosaic, but as a center of differentiation in which the in- 
tensity of the process gradually diminishes as the distance from 
the center mecreases until it passes away into an indifferent 
region. Many other systems, such as the nose, ear, hypophysis, 
gills, seem to have the same indefinite boundaries which may 
even overlap one another. 
