506 S. R. DETWILER 
derived from tissue other than that which gives rise to the limb 
and the remainder of the girdle. Sections of older embryos 
show that dorsal to the pronephros scattered mesenchyme cells 
gradually appear and become continuous, external to the pro- 
nephros, with the limb-forming cells. It is evident that this 
mesenchyme, which later forms the suprascapula, is segregated 
from the outer or cutis layer of the somites in this region and 
that the suprascapula is formed in situ. Such a conclusion is 
strengthened by the fact that after complete removal of the 
limb rudiment and the pronephros, the suprascapula develops in 
its normal place, provided the third, fourth, and fifth somites 
are undisturbed. ; 
Not only do these experiments show that the suprascapula is 
already determined at the time of the operation and that it is 
formed in situ, but they demonstrate as well the impotency of 
the unremoved girdle tissue to replace the missing part. 
3. Removal of the dorsal zone rudiment of the girdle and the 
limb mesoderm. ‘This series of experiments consisted of the re- 
moval of the area a-e X 1-5 (text fig. 1). This included the 
outer portion of the ventral halves of the somites, the pronephros, 
and the limb mesoderm. The wounds were cleaned and covered. 
The removal of this area suppresses development of the supra- 
scapula, scapula, and the free extremity, and only the ventral 
half of the girdle develops, no glenoid cavity being formed in 
any of these cases (figs. 9 and 26 and table 3). The formation 
in situ of the procoracoid and coracoid is evidence that they, too, 
are already determined at the time of the operation and are not 
dependent for differentiation on the remainder of the girdle and 
the limb, the rudiments of which were removed in this type of 
experiment. Although the myotomes proper were left intact, 
several larvae showed slight defects in the ventro-lateral mus- 
culature. This is no doubt due to a partial injury of the ventral 
portions which furnish the muscle buds. 
The rudiments of practically all the shoulder musculature are 
included in these extirpations. In one case a few partially de- 
veloped muscle fibers were present just external to the unre- 
moved girdle elements. They probably represent the m. supra- 
