268 The Nerves and Muscles of the Leg 
nerves and become differentiated as the muscle branches are given off. 
The fact that Harrison, 04, has shown that in the tadpole muscle differ- 
entiation may take place when no nerves are developed makes this pos- 
sibility highly improbable. 2d.—The ingrowth of the nerves and the 
development of muscle branches may cause a “ precipitation” of pre- 
muscle tissue about these branches. This likewise is rendered improbable 
by Harrison’s experiments. 3d.—Muscle differentiation begins in specific 
regions. Under normal conditions this differentiation begins simulta- 
neously with the ingrowth of the nerves into the limb. Muscle branches 
extend into the differentiating musculature, owing perhaps to some 
specific attraction exerted upon the growing nerves. This seems on the 
whole to be the most probable course of development. The considerable 
variation shown in the origin and distribution of the nerves to the muscles 
renders it not improbable that their ingrowth is due in part to some 
special attraction exerted by the developing musculature upon the grow- 
ing nerves, and variously responded to by the latter. 
The paths opened up for the growth of the nerves to the muscles are, 
however, at first not as a rule in regions in which muscle tissue is to be 
differentiated, but in intermuscular areas. Thus the chief nerve trunks 
usually grow along paths which lie between main muscle groups. As 
the muscles of these various groups become differentiated the main nerve 
trunks of each muscle group are distributed in the septa which separate 
the individual muscles and finally after a nerve has entered the muscle 
for which it is destined it is usually distributed at first in the coarser 
intramuscular septa. During the early stages of development, however, 
the true muscle tissue cannot be sharply distinguished from the tissue 
which is to make up the skeletal framework of the muscle. For this 
reason it often appears as though the nerve to a muscle plunged at once 
into the midst of muscular tissue. 
At a slightly later stage of development than that of Embryo CIX the 
differentiation of muscular tissue from the skeletal framework of the 
musculature is much better marked than in that embryo. Thus in Em- 
bryo CXLIV, length 14 mm., the individual muscles of the thigh may 
many of them be clearly distinguished (Plate II, Fig. 3). It may be 
seen in this embryo that although muscle differentiation in a given muscle 
is most clearly marked in the region where the respective nerve has come 
in contact with or has entered the muscle, the differentiation is not hm- 
ited to this area but extends for a considerable distance toward the 
skeletal areas to which the muscle is to be attached. It is probable, how- 
ever, that the differentiation of a given muscle begins as a rule in a 
