370 The Nerves and Muscles of the Leg 
At the center a core of scleroblastema, on each side of this a thick layer 
of myoblastema, at the periphery of the limb-bud a thinner layer of 
dermoblastema are differentiated. This differentiation begins near the 
anlage of the hip joint and extends proximally and distally. 
The myoblastema represents the anlage of the muscles and of the 
skeletal framework of the musculature, including the fascie and the 
tendons. 
The spinal nerves which grow into the limb-bud fuse to form a plexus 
and from this the nerves of the limb arise. At the time these nerves ex- 
tend distally and give off branches the myoblastema becomes differentiated 
into anlages for specific grcups of muscles and each of these anlages be- 
comes further differentiated into the anlages of the specific muscles which 
compose the group. The main nerve trunks grow as a rule in regions 
which lie between the anlages of muscle groups, the main branches to 
each of the groups between the anlages of the muscles which constitute 
the group, and the intramuscular branches in the intramuscular septa 
which appear between the differentiating bundles of muscle fibres. Finally 
the terminal branches for the individual muscle fibres are given off. The 
site of entry of a nerve marks the region of earliest differentiation in the 
muscle. In many instances, at least, the distribution of the nerve in 
an adult muscle indicates the course of development of that muscle. 
(Nussbaum, 94). 
The development of muscles from the muscle anlages consists es- 
sentially of a differentiation of the, at first, apparently nearly homogene- 
ous tissue of the anlage, into muscle cells and into the connective tissue 
framework which serves to hold these in place and harness them to the 
structures on which they are to act. The adult architecture of a muscle 
must be understood before its development can be intelligently followed. 
In the simplest muscles in the adult the muscle fibres are bound by the 
endomysium into bundles which are inserted at each end of the muscle 
into a tendon. Asa rule neither the muscle-fibres nor individual bundles 
of fibres extend the entire distance from tendon to tendon.” 'The fibre 
bundles anastomose in such a way that they form a long-meshed net- 
work, such as that diagrammatically represented in Fig. 7 b. 
The muscle-fibres either take a nearly parallel course from one tendon 
to the other, Fig. 7 c, or they diverge from one tendon toward the other, 
Fig. 7 d. In the majority of simple muscles the distance from tendon 
°° In some short mammalian muscles, like the segments of the rectus ab- 
dominis of the mouse, the muscle-fibers run from tendon to tendon. On the 
segmental musculature of elasmobranches and urodeles, see Bardeen, 03. 
