EXPERIMENTS ON THE FORE LIMB OF AMBLYSTOMA 421 
seen to be bowed toward the body. Further changes in the 
form of the limb are concerned largely with the lengthening of 
the various segments, notably the digits, and the more distinct 
demarcation of the arm, fore arm, and manus. 
Rotation takes place at the shoulder, the arm pointing more 
laterally and ventrally, so that the tip of the first digit rests on 
the bottom. Further rotation at this joint, coupled with flexion 
at the elbow, brings the manus much further forward beneath 
the gills, and the animal now rests upon two digits of each limb. 
The balancers, which serve to support the larva on its belly, 
are not lost till this stage is reached. ‘The first muscular move- 
ments take place at the shoulder before this period, and later, 
movement at the elbow and wrist joints begins; the limb is then 
used in crawling, the positions just described being those of 
normal rest. These changes are completed just about the time 
the yolk is entirely gone and the larva begins to feed. 
SIMPLE EXTIRPATION OF THE LIMB BUD 
: Mode of operation 
The first experiment which will be considered consisted in the 
simple extirpation of the body wall of the limb region. ‘To per- 
form this operation the scissors are inserted through the outer 
layers of the embryo at the anterior (cephalic) border of the re- 
gion to be removed and then the embryo is turned while a cir- 
cular incision of the desired size is cut. The part of the body 
wall thus isolated may be readily lifted from the underlying 
tissue and removed in entirety. Care has to be taken at the 
upper border of the wound to disengage the limb rudiment from 
the pronephros, if the latter is to be saved intact. When the 
limb dise is lifted, both somatopleure and splanchnopleure come 
away with the overlying ectoderm, since the mesoderm has not 
split at that time. The operations were mostly done in the 
stage shown in figure 2, though the age of the embryos used 
varied from the stage shown in figure 1 to that in figure 3. No 
difference in the results, which may be ascribed to the difference 
in age within these limits, has as yet been noted. In the younger 
