414 ROSS G. HARRISON 
Amniota, with possible exceptions among the Reptilia (van 
Bemmelen, ’89, Mollier, 95) develop according to the latter 
type. 
The first experimental work aiming at an analysis of these 
factors was done by Byrnes (’98 a), who found that the normal 
development of the posterior extremity in both Amblystoma 
and Rana is independent of the presence of the muscle plates of 
the limb region. More recently this same point has been dem- 
onstrated with greater precision by Lewis (10), who has shown 
that while extirpation of muscle plates in the limb region of 
Amblystoma produces very definite defects in the lateral and 
ventral musculature, it has no effect upon the development of 
the limb itself. 
It was found by Langnecker (Harrison, ’04) that the develop- 
ment of the skeletal and muscular elements of the limbs of the 
frog would take place even when the nerves of the limb were 
excluded by operation on the embryo. This was afterwards 
confirmed by Braus (’05), using different methods of experi- 
mentation.” 
There remain, then, as possible factors determining the dif- 
ferentiation of the limb bud from the body wall in general, a) 
the potencies of the ectoderm cells of the region; b) the potencies 
of the somatopleuric mesoderm cells, and c) the position of these 
elements with respect to the organism as a whole, or perhaps 
better with respect to its immediate surroundings, including, in 
the case of the fore limb, the pronephros and the branchial 
region,’ which have close topographical relations to the limb 
rudiment. The blood vessels are not included in this enumera- 
tion, for while it is obvious that they are necessary for the nour- 
ishment and growth of the limb, no specific action on their part 
can be assumed. Braus (’03, ’04, 09) found that the limb bud 
of the anuran larva constitutes a self-differentiating system 
2 Braus (’06 a) also*’showed that in the elasmobranch fin certain skeletal ele- 
ments, the cartilaginous rays, would develop independently of the muscle buds. 
° Braus has suggested (’06 b, p. 545), on the basis of experiments upon Rana 
embryos, that the gill region may exert a formative influence upon the regene- 
ration of the fore limb, though no evidence is adduced to show whether this region . 
has an influence upon the normal development 9f the limb. 
