514 S. R. DETWILER 
ventral zone shows again their lack of equipotentiality for this 
structure. 
Experiments thus far described were made upon embryos at 
about the time of the appearance of the tail bud when the limb 
tissue is in the form of a definite limb bud. The relation of the 
girdle rudiment to that of the limb has been shown in these 
experiments. They have further served as a means of showing 
in two different ways—extirpation and transplantation—that 
not only is the girdle rudiment already determined at this stage, 
but that the rudiments of the separate components have already 
become localized. 
A further discussion of these experiments will be taken up 
after the experiments in part 2 have been considered. 
2. LIMB EXPERIMENTS ON EMBRYOS IN THE STAGE OF HIGH 
MEDULLARY FOLDS (STAGE 18) BEARING UPON THE 
DEVELOPMENT OF THE SHOULDER GIRDLE 
A. Extirpation experiments 
While the limb rudiment is present as a definite somatopleural 
thickening in the embryos used for all experiments hitherto de- 
scribed, its definiteness is gradually lost as we study successively 
earlier stages. In the stage of open medullary folds, no somato- 
pleural thickening is present and the axial and the lateral meso- 
derm have not yet been separated. Therefore at this stage the 
limb rudiment is present as a region of mesoderm without vis- 
ible local characteristics. Embryos at this stage also lack the 
surface markings, viz., somites and pronephros, which indicate 
the position of the limb rudiment in the older stage and which 
are used as landmarks in experimentation. Consequently, it is 
impossible to make exact determinations of the area of limb- 
forming cells. 
Location of the limb mesoderm in the absence of surface mark- 
ings was determined by a fairly accurate method, already de- 
scribed by the author (Detwiler, ’17), a brief description of 
which may be repeated here. 
After removing the embryos from the capsule a circular cut 
