SHOULDER GIRDLE AND ANTERIOR LIMB 503 
No description of the shoulder muscles of Amblystoma could 
be found in the literature. The musculature, however, so far as 
has been studied, closely resembles that of Salamandra macu- 
lata, a European tailed Amphibian described by Firbringer (op. 
cit.). In referring to the musculature, Firbringer’s nomencla- 
ture will be employed. 
3. EXPERIMENTAL 
The experiments were carried out upon embryos in two dif- 
ferent stages: a) the so-called tail-bud stage, and b) the stage of 
open medullary folds. 
Fig. 1 Camera-lucida drawing of an embryo of Amblystoma in the tail-bud 
stage. The larger of the two circles represents the typical limb dise. X 15. 
pn, = pronephros. 
1. EXPERIMENTS ON EMBRYOS IN THE TAIL-BUD STAGE (STAGE 29) 
A. Extirpation expervments 
1. Removal of the suprascapula rudiment. As has already 
been pointed out (Harrison, 715), the fore-limb rudiment of an 
embryo in the tail-bud stage consists of a somatopleural thick- 
ening just ventral to the pronephros, centering in the region of 
the fourth myotome and extending over into that of the third and 
fifth. The formation of a suprascapula following the extirpation 
of the limb rudiment in this stage shows that its rudiment is not 
included with the limb mesoderm. Although there is no visible 
suprascapula rudiment, nevertheless, extirpation of the region 
a-e X 1-3 (text fig. 1), including the outer or cutis layer of the 
