130 BYRNES. [Vou. XIV. 
injury to the myotomes has also involved the somatopleure 
immediately below them. 
In such cases, however, the reduction in the size of the 
limbs is due to interference with metabolic processes, and is 
not due to the exclusion from the limbs of masses of cells that 
are destined to provide them with any given tissue. Evidence 
that the limbs are not dependent on the myotomes for any 
such large proportion of their tissues as is represented by 
the muscles is given in Pl. XII, Figs. 42-47. All these cases, 
besides others which have not been figured, go to show that 
the limbs develop alike on both sides of the body, and nor- 
mally, even though the muscle-structures have been very largely 
destroyed. 
The regions in which the different tissues are going to 
develop become clearly outlined in the limbs long before 
differentiation into muscle and cartilage actually begins. The 
peripheral regions of the limb-rudiments give rise to muscles 
and contain many more nuclei than the central or cartilaginous 
region, and hence stain much more intensely than the rest of 
the section. Comparing the darker peripheral part of the limb- 
rudiment on the right (injured) side of the embryo with the 
limb on the normal (left) side, the different regions are found 
to exactly correspond, showing that there are neither quantita- 
tive nor qualitative distinctions between the two limbs. Even 
in Amblystoma, in which an actual reduction in the size of the 
limb on the right side of the embryo often occurs, owing to the 
necessary extent of the injury, the peripheral parts of the limb- 
rudiment are blocked out into the areas in which the various 
muscles of the upper limb are going to develop; and these regions 
correspond precisely to those in the limb on the normal side of 
the body. These facts show that even in those cases where 
there is a quantitative difference in the two limbs the difference 
is only one of size and not one of kzzds of tzssue present in the 
limbs. 
Should the constant regeneration of the abdominal muscle- 
rudiment be urged as an objection to the validity of the conclu- 
sions drawn from the experiments, it must be remembered that, 
although the muscle does regenerate, it nevertheless regenerates 
