520 S. R. DETWILER 
at this time. It was successfully transplanted in several cases 
from embryos with low medullary folds, but no experiments 
have been made upon still younger embryos. 
The region of mesoderm extirpated from the embryos in the 
stage of high medullary folds (text fig. 2) not only included the 
limb cells, but, in many cases, the entire girdle rudiment, so 
that there was complete absence of the girdle on the operated 
side (fig. 17). This figure is a section taken from the same 
embryo (EK. Tr. Ext. 12) which is shown in text figure 2. The 
size of the region extirpated in this case corresponds to that of a 
three-somite wound in an embryo of the tail-bud stage, an 
extirpation which, in the later stage, removes only the central 
portion of the girdle rudiment so that isolated pieces of the 
girdle develop from the unremoved tissue (fig. 10). It is seen 
by comparing the two conditions resulting from extirpations of 
the same size that in the embryo with high medullary folds the 
girdle rudiment occupies a smaller area than in an embryo of the 
tail-bud stage. 
The excised areas in the earlier stage always includes the rudi- 
ment of the pronephros. In the absence of the pronephros on 
the operated side a compensatory hypertrophy of this structure 
on the intact side is always found (fig. 17). This is in accord 
with Miss Howland’s (’16) observations. Hypertrophy appears 
to be greater when the rudiment is removed from these early 
embryos than when the partially developed structure is removed 
from older embryos. 
While in a number of these larvae, without the appendage, 
the entire girdle was lacking, in others a ‘girdle’ was found to be 
present (fig. 18). Although such girdles are present as continu- 
ous cartilaginous bands, nevertheless the central portion lacks 
those characteristics which typify the normal structure. This 
portion probably represents a hyperplastic growth of those 
parts which developed from the unremoved girdle cells which 
were located near the periphery of the wound. The completeness 
of these girdles therefore cannot be regarded as a manifestation 
of true restitution processes, for the completeness is only one of 
quantity and not of quality. 
