240 Growth and Histogenesis of Nerves 
of nerve and muscle fibres at a comparatively late period of muscle devel- 
opment.’ 
As the mass of premuscle tissue in a given region becomes differen- 
tiated into the individual muscles characteristic of that region, paths 
for nerve growth are offered in the loose, vascular mesenchyme which 
separates the various muscles. The nerves extend out rapidly toward 
the various muscles and the various parts of each muscle which they are 
to supply. When the area to be supplied by a given nerve or set of 
nerves is considerable, the 
nerves, as they are spread 
out, may branch and give 
rise to coarse plexuses. Fig. 
7 shows such a plexus in pro- 
pnt PEG aes wut 
cess of formation in the area 
between the internal oblique 
and the transversalis mus- 
cles of a pig-embryo 4 cm. 
long. From the intermus- 
cular nerves branches are 
Fia.7. A portion of the nerve-plexus formed on aie : Ske 5 
thevsurface of the'transsersus abdominis muscle of given Off which enter the 
Se a aay muscle substance and make 
their way, rapidly ramifying, toward the middle of each bundle of 
muscle fibres constituting the muscle (see Fig. 8). In simple muscles, 
like the segment of the rectus above mentioned, there is little or no 
plexus formation during the period of intramuscular distribution. But 
in complex muscles where the bundles of muscle fibres interdigitate, 
plexus formation is active during this period. As in the skin, so here, 
this plexus formation seems to be due to an attraction which causes 
branches to grow from several sources toward a given area. 
In the course of development of the muscles new fibres may be differ- 
entiated in one or more directions, and toward these the intrinsic nerves 
of the muscle extend to be distributed. This may be beautifully fol- 
4In many of the lower vertebrates there is good evidence that the motor roots of 
the spinal nerves become associated at an early period with the musculature of the 
myotomes. But in these vertebrates the myotome musculature is functional. In 
mammalian embryos, on the other hand, I have been able to find no good evidence of 
union of motor-root fibres with the myotome cells. Branches from the spinal nerves 
do not begin to enter the dorsal and the thoraco-abdominal musculature until the 
latter begins to be differentiated from the myotomes. This I have previously shown 
in describing the development of the thoraco-abdominal musculature of the pig. 
The myotomes are probably no more functional in mammals than are the branchial 
clefts. 
