EXPERIMENTS ON THE FORE LIMB OF AMBLYSTOMA 415 
which develops into a normal limb when transplanted to new 
and strange surroundings. This important discovery, which was 
confirmed independently by Banchi (’04, ’05) and later by the 
present writer (’07) shows that once the development of the 
limb has started, its position in the organism as a whole does 
not affect its specific character. 
The experiments taken up in the present paper, which deal 
with the fore limb of the urodele embryo, are designed to apply 
the tests of self-differentiation to a much earlier period of develop- 
ment. Considering for the present only those stages with 
closed medullary folds, the experiments show that the immedi- — 
ate organic environment of the limb rudiment, even before vis- 
ible differentiation sets in, has no specific influence in deter- 
mining that a limb shall develop, though it may affect the pos- 
ture and laterality of the limb that does arise (Harrison, ’15, ’17). 
The essential process of differentiation, whereby the potency to 
form a fore limb becomes localized in certain cells of the body 
wall, must therefore be relegated to very early embryonic life.‘ 
It is further shown that in the stages under consideration this 
potency is localized in the mesoderm of the region involved, the 
ectoderm remaining indifferent. , 
The object of the experiments, aside from their purpose in 
further analyzing the factors taking part in the formation of the 
vertebrate limb, was. to study certain relations which have a 
direct practical bearing upon the experiments on the laterality 
of limbs to be described later. An effort has been made to de- 
termine precisely the extent of the limb rudiment in the stages 
used for operation. In addition, the question of the equipoten- 
tiality of the elements composing the limb rudiment has been 
investigated both by defect experiments and by transplantation. 
In interpreting defect experiments there is always a serious 
source of error in the occurrence of processes of regeneration or 
regulation, whereby elements which do not normally give rise 
to the part which has been removed may do so vicariously 
under the changed conditions. Such processes are known to 
occur in connection with experiments upon the limbs and must 
4 Before the formation of the medullary folds according to Detwiler (718). 
