444 ROSS G. HARRISON 
half, though the small figures given in the table cannot be deemed 
significant in this direction. Moreover, this defect, like that of 
syndactyly, has been found in cases which had not been oper- 
ated upon at all, and is probably to be regarded as due to slight 
general disturbances of growth. 
On the other hand, the removal of the anterior portion seems 
to have a definite tendency to bring about reduplication, which 
is probably due to a more or less complete separation of the re- 
maining portion of the limb rudiment from a regenerative center 
in front of the wound scar. 
Notwithstanding these anomalies, the experiments speak as a 
whole for the equipotentiality of the four quadrants of the limb 
bud, at least in a qualitative sense. Quantitatively, the lines 
which divide the limb-forming tissue equally are anterior, and 
dorsal to the vertical and horizontal diameters, respectively, of the 
limb bud as defined by the experiments (fig. 2). 
These statements are valid for the free extremity only, and 
must be held in abeyance with respect to the shoulder girdle, 
where it is known that localization has taken place in the em- 
bryo at the time of operation (p. 429). 
While the prospective potency of the limb-forming cells is the 
same as regards the topographic divisions of the limb, the experi- 
ments give no evidence regarding histogenetic potencies. Whether 
certain cells at the operating stage are already differentiated 
into cartilage, bone, connective tissue, or muscle-forming ele- 
ments cannot be determined either by direct observation or by 
any of the experiments yet devised. 
What the prospective significance of the cells of each of the 
four quadrants of the limb bud is, i.e., what part of the free ex- 
tremity is formed in normal development out of each, has not 
yet been determined, though the study of normal embryos points 
to the view that the distal part of the limb is developed more par- 
ticularly from cells lying in the posterior half, the ulnar half aris- 
ing from the dorsal and the radial from the ventral quadrant. It 
is difficult to devise experiments to test this hypothesis. Graft- 
ing of tissue colored by vital stains, such as neutral red and 
Nile blue sulphate, is not feasible because the stain is all de- 
% 
