£22 S. R. DETWILER 
limb bud, only a portion of the girdle rudiment is included. 
From his experiments on Bombinator, Braus (’09) claimed that 
from this fraction of the rudiment a complete girdle of reduced 
size 1s developed, and he concluded that the shoulder-girdle rudi- 
ment, like that of the limb, constitutes an equipotential restitu- 
tion system. Experiments hitherto reported (Detwiler, op. cit.) 
have shown that this rudiment in Amblystoma punctatum con- 
stitutes a mosaic and is incapable of qualitative restitution. 
When transplanted, only such components of the girdle develop 
as are represented in the corresponding portions of the implanted 
rudiment. Such girdles, however, although qualitatively in- 
complete, may become topographically complete by a process of 
hyperplasia which compensates for qualitative deficiencies. This 
is well illustrated in cases where in the absence of the supra- 
scapular rudiment the dorsal zone of the girdle becomes practically 
completed by hyperplastic development of the intact portion. 
The above is not always the case, however, and inasmuch as there 
is considerable variability in the degree of development of the 
shoulder-girdle in the heterotopic position, restricted function 
of the limb may be a result of marked deficiencies in the girdle. 
Intimately associated with the development of the shoulder- 
girdle is the development of the shoulder muscles. These, how- 
ever, may develop and differentiate independently of one another 
so that with a well-developed girdle there may be muscular 
deficiencies. 
The lack of interdependence of muscle and skeletal differen- 
tiation has previously been shown. Braus (’06 a) showed this 
to be true in the development of the pectoral fin of the Elasmo- 
branch embryo. Personal observations on the development of 
the shoulder-girdle of Amblystoma have shown that in the 
absence of skeletal differentiation, shoulder muscles may develop. 
It is obvious, therefore, that restricted function may result from 
muscular deficiencies in addition to those of the girdle. 
Even though skeletal and muscular differentiation may be com- 
plete, restricted function of the limb might be due to the failure 
of certain of the individual muscles to receive innervation, so 
that this offers a third factor conditioning the degree of function. 
