EXPERIMENTS ON THE FORE LIMB OF AMBLYSTOMA 417 
facts with those of Braus, though they leave uncertainty as to 
whether in Amblystoma there is complete qualitative restitution 
of the shoulder girdle in either place.’ Whichever view may be 
accepted for the shoulder girdle, all of the evidence accumulated 
here goes to show that the free extremity, while self-differenti- 
ating,-must be considered as an equipotential system in Driesch’s 
sense. ® 
MATERIAL, METHODS AND NORMAL DEVELOPMENT 
All the experiments have been made upon embryos of Ambly- 
stoma punctatum in stages ranging between those shown in 
figures 1 and 3. This species offers great advantages over the 
anurans, which have been largely used in previous experi- 
ments upon the limbs. The fore-limb rudiment develops very 
early and rapidly, and may be located with precision shortly 
after the medullary folds close, before the embryo can move, 
and before blood vessels or nerves have developed. Experi- 
ments can, therefore, be carried out without anaesthesia, and 
the different tissues or embryonic organs can be easily separated 
from one another. The absence of a closed operculum, which 
in the anurans covers up the fore limb, is also a great advantage 
in that it permits the study of the progress of the experiment 
on the living individual, which may be examined from day to 
day without difficulty. 
The technique of the surgery of the Amphibian embryo is 
now so well known that no general description is required here. 
The special operations used in the present investigation will be 
described as far as needed in the separate sections. 
In the earliest stages at which operations were done the limb 
rudiment cannot be distinguished by any definite characteristic 
save its location. In the stage with beginning tail bud (fig. 1, 
stage 25°) the pronephric swelling becomes visible and the muscle 
7 A more complete analysis of the case is given by Detwiler (’18). 
8 Driesch, ’99, ’05, p. 679. 
9 In the absence of a set of ‘normal plates’ of Amblystoma, a series of stages 
have been designated arbitrarily and type specimens preserved. In course of 
time, if the necessary drawings can be prepared, it is hoped that they may be 
published. 
