EXPERIMENTS ON THE FORE LIMB OF AMBLYSTOMA 447 
It is clear from these experiments, even though small in 
number, that a single perfectly normal limb will regularly develop 
out of two limb buds fused together. 
EFFECT OF REMOVING THE ECTODERM ALONE 
In the early days of the study of the limbs the question whether 
the first sign of their appearance was in ectoderm or mesoderm 
was much discussed as one of phylogenetic significance. What- 
ever our attitude toward such questions may now be, it is im- 
portant from the standpoint of developmental mechanics to in- 
quire whether the factor which determines the development of 
the limb is located in the outer or the middle germ layer.'2 
Comparison of the experiments with cleaned and with un- 
cleaned wounds, in which the ectoderm is treated the same way 
in both series while the mesoderm is differently handled, already 
suggests what the answer will be. The experiments described 
in this and the following sections are intended to add further and 
more direct evidence that the power to produce the limb rests 
primarily in the mesoderm. 
It is not always possible to avoid injuring the mesoderm to a 
slight extent in the operation of removing the ectoderm. There- 
fore the experiments in which it was attempted to remove the 
ectoderm alone differ only in degree from the cases of simple 
extirpation with non-cleaned wounds. Five operations of this 
kind were performed, and in all five’ cases limbs developed 
promptly, though there was some delay as compared with the 
normal. The wound was not covered with grafted skin in any 
of the cases. | 
® Braus (’09) has touched upon this question in connection with his study of 
the origin of the skleroblasts of the limb. On p. 165 he says, ‘‘In einer spiiteren 
Arbeit, werde ich nachweisen kénnen, dass in ilteren Stadien ebenfalls das Ecto- 
derm nicht fiir das Zustandekommen eines typischen Skelets notwendig ist; 
denn dasselbe bildet sich eben so gut unter einem ortsiremden Eectoderm, welches 
experimentell gegen die typische Nachbarepidermis ausgetauscht wird, und in 
grossen Entfernung von jeglicher Haut wie in der ungestorter Entwicklung.” 
The paper: dealing especially with this work (Braus, ’08) is unfortunately not 
accessible to me at present. 
