Charles R. Bardeen 269 
region which corresponds with the site of entrance of the chief nerve of 
that muscle. In Plate IT, Fig. 3, several nerves and muscles are shown. 
The nerve to the gracilis muscle shows especially clearly. From this 
region the gracilis muscle may be traced in successive sections toward the 
pelvis and toward the tibia. The entrance of the inferior gluteal nerve 
into the gluteus maximus muscle also shows well in the figure. The two 
parts of the adductor magnus muscle, the obturator and sciatic portions, 
are shown near the site of entrance of the respective nerves. The semi- 
tendinosus muscle and the two heads of the biceps are shown cut at some 
distance from the site of entrance of nerves. About the two divisions of 
the sciatic nerve there is some dense tissue which probably does not, 
however, represent muscle tissue. 
It is to be noted that during these earlier stages of muscle differentia- 
tion the muscle anlages are often connected at one extremity, less fre- 
quently at both extremities, with the skeletal anlages to which the muscle . 
is subsequently attached. The tendons of the muscles are developed in 
continuity with the anlages of the muscles. As a rule the differentiation 
of the longer tendons begins in the vicinity of the muscle bellies and 
gradually extends toward the skeletal attachments. 
In a considerably older embryo, CXLYV, length 33 mm. (Plate II, 
Fig. 4), differentiation of the muscles is much further advanced. Not 
only the muscles but also the fasciculi are separated by a large amount 
of connective tissue. This shows especially well in the gluteus maximus 
muscle. The main branches of the nerves of the muscle may be followed 
in the larger intramuscular septa, the smaller branches in the smaller 
intramuscular septa. I have elsewhere described the intramuscular 
growth of nerves in the mammals (Bardeen, 00 and 03). It is of 
interest to note that after muscle differentiation is well under way there 
is relatively a much greater amount of connective tissue in the muscu- 
lature of the embryo than in that of the adult. 
After the stage of development exhibited by Embryo CIX the condi- 
tions within the limb become so complex that they can be better fol- 
lowed by tracing through the development of specific groups of nerves 
and muscles than by attempting to picture all the details of each suc- 
cessive stage of differentiation of the whole limb. In order, however, that 
the relations of specific groups of nerves and muscles to the general 
structural condition of the limb may be followed we shall first briefly 
describe the relations of the peripheral nervous system to the skeleton 
at two important stages of development. 
