SHOULDER GIRDLE AND ANTERIOR LIMB 52D 
of the corresponding portions of the rudiment, are sufficient 
evidence to show that the system cannot be regarded as being 
equipotential at the stage when the limb mesoderm is in the 
form of a definite limb bud. 
Experiments upon embryos in the stage of open medullary folds 
show that the girdle rudiment occupied a smaller area than in 
embryos of the tail-bud stage and that it was frequently entirely 
removed along with the limb cells so that no girdle developed. 
In other cases where it was not entirely removed a complete 
cartilaginous band was formed which, upon superficial examina- 
tion, resembled a normal girdle (fig. 18). This cannot be re- 
garded necessarily as a complete girdle, however, for it is lack- 
ing in certain components which typify the normal structure. 
The central portion of a girdle of this type no doubt represents 
a hyperplastic growth of the more distal portions whose rudi- 
ments were not removed with the excised area, and the com- 
pleteness of this structure must be regarded as one of quantity 
rather than one of quality. From the incompleteness of quality 
of a structure of this type, it is seen that even at thestage of 
open medullary folds the system is not capable of qualitative 
restitution. This is strengthened by the fact that in several 
cases the girdles which had developed from implanted meso- 
derm at this stage were composed of only the ventral zone, in- 
dicating that only the corresponding portion of the rudiment 
was transplanted. 
It has been shown that girdles which develop from trans- 
planted areas in stage 18 were considerably larger than those 
which formed from areas of equal size in stage 29 (compare figs. 
20 and 21y. This condition might be used as a point in favor 
of equipotentiality of the system in the early stage, were it not 
known that in the former case all or nearly all of the rudiment 
was transplanted, while in the latter case only the central 
portion. 
Braus (’09, p. 277) claimed that, between his stage 2; when 
the limb bud first becomes visible, and the stage when chondrifi- 
cation of the humerus begins, an area of transplanted mesoderm 
taken from a younger embryo did not develop into a girdle any 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 25, NO. 2 
