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Or 
Charles R. Bardeen 
the first to extend distally into the anlage of the great trochanter, but 
proximally it extends only to the acetabular portion of the ilium. From 
here the muscles extend over the lateral surface of the iliac. ala and finally 
reach the iliac attachments characteristic of the adult. The ascending 
branch of the superior gluteal nerve takes a course at first distal to the 
transverse branch, but as the gluteus medius grows toward the iliac crest 
the ascending branch is carried proximally across the transverse. 
The gluteus medius and minimus muscles correspond with the ilio-femoralis 
of urodeles and reptiles, a muscle supplied by branches from both the femoral 
and sciatic nerves. In the monotremes the “ gluteus medius”’ is a thin muscle 
which arises from the sacro-caudal vertebre and is supplied by a branch of 
the peroneal nerve while the gluteus minimus and scansorius are represented 
by a mass of muscle which arises from the fascia lumbo-dorsalis, the lumbar 
and sacral vertebre and the ilium and is innervated by branches of both the 
femoral and peroneal nerves. In all higher forms the gluteus medius- 
minimus musculature is innervated by branches which arise directly or indi- 
rectly from the peroneal portion of the sacral plexus (Westling, cited by 
Leche). It seems not unlikely that in the urodeles, reptiles and mono- 
tremes elements of the ilio-psoas musculature of higher forms are included 
in the ilio-femoral musculature. The more superficial and posterior part of 
the sciatic portion of the ilio-femoral anlage has given rise in the higher 
mammals to the gluteus medius and piriformis, the deeper and more anterior 
portion to the gluteus minimus and scansorius. The degree of separation of 
these various elements varies greatly in different mammals. 
The variations of the two muscles which have been found in man are 
chiefly those of a greater differentiation than usual of individual muscles 
from the common anlage (i. e., M. scansorius) or a partial or complete fusion 
of the muscles with one another or with the piriformis. 
The piriformis.—This is differentiated from tissue at first closely as- 
sociated with the gluteus medius and minimus (Plate VIII, Fig. 4). 
According to Grifenberg the muscle anlage can from its first appearance 
be traced to the sacrum. While it is true that a dense mass of cells sur- 
rounding the sciatic nerve and its roots of origin can be followed back 
to the sacrum this condensed tissue is not, I believe, to be looked upon 
as the anlage of the piriformis, although the two are not at first sharply 
to be distinguished. Differentiation of the muscle is first clearly marked 
in the region between the sacral plexus and the anlage of the great 
trochanter. From here the developing muscle may be followed in older 
embryos toward its sacral attachment. In embryo XXII (Plate VIII, 
Fig. 5) the sacral attachment has not yet been reached. The region in 
which the nerve enters the adult muscle corresponds with the area in 
which muscle differentiation is first seen. As pointed out above, the 
differentiation of the muscle at a period preceding the fusion of the 
