EXPERIMENTS ON THE FORE LIMB OF AMBLYSTOMA 427 
days. This is an average of nineteen cases. Daily observa- 
tion would undoubtedly have revealed a greater difference be- 
tween the two classes of cases in time of appearanec of the re- 
generating limb bud. In a total of 177 cases there are but 
twenty-eight in which regeneration was first noted as late as 
ten days after the operation, and but six when the period was 
fifteen days or over. In only four of the twenty-eight cases 
was the last previous observation made as late as ten days after 
the operation, and only once was a negative observation, ‘with 
subsequent regeneration, recorded as late as fifteen days. It 
is safe to conclude, then, that when regeneration has not be- 
come visible from the outside two weeks after the operation 
it will not occur at all. Nearly all of the cases have been kept 
alive at least three weeks, many for four, and some as long as 
twelve (fig. 14), and no instances of regeneration in the later 
periods of development have been observed. 
The experiments described .in this section show that it is 
not possible to prevent the regeneration of the fore limb with 
certainty when the wound is left uncovered, even if the circular 
area extirpated has a diameter of 4 somites. It may be said 
in general, however, that the larger the area removed and the 
more carefully the wound is cleaned of mesoderm the less likely 
will regeneration occur. This can only mean that the meso- 
deri cells of the region surrounding the limb bud have, in a 
gradually diminishing degree as the distance from the limb 
increases, the potency to form a limb. Their praspective po- 
tency is, therefore, greater than their prospective significance. 
Regeneration, when it takes place, is usually perfect, though 
subject to delay in a varying degree. 
The process of regeneration 
The actual process of regeneration has not been followed in 
detail, though some observations may be recorded here. 
The earliest case (R. E. 127—) which need be considered was 
preserved six days after extirpation of the limb rudiment. 
The regenerating limb appears on the surface (fig. 10) as a 
