4 
EXPERIMENTS ON THE FORE LIMB OF AMBLYSTOMA 433 
TABLE 4 
Showing results of extirpation of circular areas of body wall of given diameter; 
wound covered with grafted skin 
WOUND NOT CLEANED - WOUND CLEANED 
' 1 A ' 
Be o D d PI ra a 4 
DIAMETER OF = Ae | Bins || Brey | Es 
= o eo ° oO + O o 
EXTIRPATED AREA Sn Ones |e 2s | bo Sell aes ee 
2 a SE g& Bel RE oo Total Oo & 
$5 | os | se! S -e | Be | 88 | ove nar 
ge} am) 55 | 6 | 5m | 2a | Su] 8s 5 
O75 |e A H a 1@) 1é) A Ay 
Not recorded........ 2 5 3 10 
SESOIMIGES eer eee 7 14] 3 |24 (21) 33.3 
SeESOMMNGeSie as as ae 1 Oi Oy a ad} 
SS TSOMMGeSs 4. 2 see 0 15; 4 {19 (15)} 00.0 
AG SOMUGCS HS 4 ete eee 0 3) Il BN Ss5 G3) 
AO taE scene 2 5 3 10 8 382} 9 |49 (40)) 20.0 
* These three cases were classed as negative in the tabulation previously pub- 
lished (Harrison, ’15). They are excluded here because they were not kept 
under observation for a sufficient length of time. 
in diameter, regeneration is altogether blocked, eighteen cases 
all having given a negative result. To these might be added 
the five cases given in Table 8, which differ from the present 
experiments only as to the region from which the covering 
ectoderm is taken (p. 448). 
When the, skin is grafted to the wound it soon sticks to the 
underlying tissue just as a transplanted limb bud does, and 
often in the course of several hours the wound becomes com- 
pletely healed. The wandering of the ectoderm over the de- 
nuded area, which takes place in the uncovered wounds, is 
blocked, and probably also the movement of the mesoderm 
cells. Prevention of the surrounding cells from reaching the 
proper position by the substitution of other tissues which do 
not have the potency to produce a limb thus effeetually prevents 
the regeneration of the appendage. The cells immediately 
surrounding the limb-producing area are evidently unable to 
form a limb unless they can reach the proper position. On 
the other hand, indifferent cells in this position cannot produce 
alimb. It is possible that when no mesoderm is grafted with the 
skin, the cells of this layer wandering in from the host may in 
