516 S. R. DETWILER 
The position of the stained disc is not always an absolute indi- 
cator of the region of the excised mesoderm, since it may be 
changed somewhat by ectodermal shifting. There is, however, 
very little shifting in the immediate limb region. The more 
pronounced ectodermal shifting, which does not usually occur 
until a somewhat later stage, takes place dorsal to the limb 
region and proceeds in a dorso-posterior direction; the general 
character and direction of the movement being similar to that 
observed in frog embryos by Harrison (’03) in his experiments on 
the development of the lateral line. 
The migrating ectoderm, however, occasionally involves the 
dorsal portion of the stained area which then loses its circular 
shape and lengthens out dorso-posteriorally. Even though the 
stained disc is not involved in the ectodermal migration, it 
gradually becomes larger through continued cell division and 
the stain, now being distributed over a larger area, gradually 
loses its intensity. 
It is impossible to excise equal areas in all cases and a certain 
degree of variation is to be expected. Measurements of the 
antero-posterior and dorso-ventral diameters of the inserted disc, 
after healing, were taken in many cases. These, of course, rep- 
resent approximately the diameters of the excised piece. The 
size of this area was compared with that of a typical limb bud, 
which, in an embryo of the tail-bud stage has a diameter of three 
and one-half somites, an actual measurement of 0.93 mm. The 
diameters of the discs extirpated from the embryos in the neural 
fold stage varied between 0.66 and 0.93 mm. 
Harrison (’15 and 718) showed in a series of extirpation experi- 
ments on embryos in the tail-bud stage that no regeneration oc- 
curred from wounds the width of three and one-half somites in 
diameter, provided they had been cleaned and covered, but when 
they were only three somites in diameter 33 per cent regeneration 
occurred. In most of the experiments on the neural-fold stage 
the average diameter of the excised areas was equal to the width 
of from three to three and one-half somites, or 0.73 to 0.93 mm. 
The results of the simple extirpations are given in table 7. 
Although slight variations in the position from which these 
