238 Growth and Histogenesis of Nerves 
development be shifted extensively with respect to neighboring parts. 
Grosser and Fréhlch have called attention to this phenomenon in the 
dorsal and thoraco-abdominal regions of man. 
Having thus briefly considered the mode of distribution of cutaneous 
nerves during development, we may take up the development of the 
nerves distributed to the voluntary motor-apparatus. In order that a 
description of this process may be clear, it is necessary to consider briefly 
the general features of the development of the musculature. 
The muscles are differentiated from a mass of premuscle tissue which 
is variously derived in different parts of the body in the mammals. 
Thus the dorsal and thoraco-abdominal musculature arises from tissue 
derived from the myotomes, and the musculature of the leg from the 
mesenchyme of the limb-bud. . 
At the time when differentiation begins in this premuscle tissue two 
distinct groups of cells may be distinguished, myoblasts and embryonic 
connective-tissue cells. The former in part multiply rapidly by indirect 
division and in part become elongated into spindle-shaped muscle-fibres. 
These muscle-fibres are usually grouped into bundles, new fibres being 
constantly added at the periphery of-the bundles by elongation of myo- 
blasts. Meanwhile, the skeletal tissue of the muscle becomes differen- 
tiated from the connective-tissue cells. The latter grow into and break 
up the primitive bundles of muscle fibres into smaller bundles. Each 
muscle fibre finally becomes surrounded by a certain amount of con- 
nective tissue, but usually groups of fibres become surrounded by a 
denser connective tissue than that surrounding any individual fibre. 
In the simplest muscles the muscle fibres are parallel, of about equal 
length, and are attached at each end to a tendon running transverse to 
their course. In mammals these conditions may be seen in an anterior 
segment of the rectus abdominis muscle of a small rodent. 
In most muscles the arrangement of the bundles of muscle fibres is 
far more complex. They run in various directions, interdigitate, and 
are so complexly combined that it is difficult to get an accurate idea of 
the internal architecture. Very often each muscle fibre is inner- 
vated about midway between its extremities. Im the segment of the 
rectus mentioned above, gold-chloride specimens show a band of motor- 
endings running across the muscle midway between the two transverse 
tendors which limit it. In Fig. 6 there is represented a portion of the 
nerve-plexus distributed to the WV. transversus abdominis of a guinea-pig 
embryo 84 cm. long. A few bundles of muscle-fibres are shown in out- 
