EXPERIMENTS ON THE FORE LIMB OF AMBLYSTOMA 457 
Self differentiating as is the system as a whole, the parts 
within the system do not constitute a developmental mosaic, 
with the exception of certain portions of the shoulder girdle. 
The system itself is equipotential, as shown by the two tests to 
which it can be subjected; a whole will develop out of a part, 
and a single normal whole will develop out of two separate rudi- 
ments when fused together. The experiments of extirpating half 
buds and of superimposing buds, respectively, make this clear. 
The limb rudiment, therefore, is an entity, which, except for 
its dependence for nourishment, is independent of its surround- 
ings in the attainment of its specific form. In one important 
respect it has been found to be influenced by its position in the 
organism as a whole, and that is as regards its relations of sym- 
metry and its power to form reduplications. The study of these 
phenomena will be the subject of a continuation of the present 
work. 
SUMMARY 
The fore limb of Amblystoma punctatum develops normally 
as a thickening of the somatopleure centering in the area ventral 
to the fourth myotome and extending over into the regions 
ventral to the third and fifth. 
Numerous mitoses indicate that the growth of the limb bud 
is due to rapid proliferation of the cells in situ rather than to 
inwandering from surrounding territory. 
Extirpation of the tissues of this region results in disturbances 
of development in the limb, the intensity of which depends upon 
the size of the wound, the care with which it is cleaned of meso- 
derm cells, and whether it is covered with grafted ectoderm or 
not. 
If the wound is small or not entirely cleaned of mesoderm, a 
normal limb usually develops (regenerates) with some delay. 
After more radical operations the limb regenerates much less 
frequently. 
When nothing further is done beyond excising the circular disc 
of tissue constituting the limb rudiment, the limb has been 
found to develop subsequently in 81.4 per cent of the cases 
