302 - The Nerves and Muscles of the Leg 
velopment from an anlage common to it and the iliacus muscle indicates that 
it should be placed with the intrinsic musculature of the limb. 
The phylogenetic development of the psoas minor, on the other hand, is 
somewhat uncertain. It may be derived from the trunk musculature. This 
is perhaps indicated by its frequent innervation through a branch from the 
genitofemoral nerve, while the psoas major is innervated by branches which 
arise from the femoral nerve or its roots. Its embryonic origin should be 
studied in some of those forms in which the adult muscle is highly developed. 
In mammals with an ilium triangular in cross-section the iliacus lies ex- 
ternally, a position which corresponds with the situation in which its anlage 
appears in the human embryo (Lubsen). In those forms in which the 
iliac blade is developed the muscle comes to have an internal position. 
The variations of the iliopsoas in man are chiefly those of a greater or less 
independence of the two muscles composing it and a greater or less special- 
ization of fasciculi in either. There are also slight variations in the origin 
and attachment of the muscles. The very inconstant psoas minor varies 
chiefly in the extent of its development. The inferior insertion may take 
place into the iliac fascia, the inguinal ligament, the femur between the 
small trochanter and the head, or together with the iliopsoas into the small 
trochanter (Le Double). Fasciculi may unite the psoas major and the psoas 
minor. These variations may indicate a common origin of the two muscles. 
Pectineus (Plate VI, Figs. 1 and 2). In an embryo 11-mm. long the 
anlage of the pectineus is not distinct. it may be represented in those 
portions of the iliopsoas and the obturator muscle anlages which he 
nearest the region in which the pectineus will be developed. Grafenberg, 
04, describes in the region immediately distal to the superior pubic 
ramus a union of a branch of the obturator nerve with a branch of the 
femoral before the muscle anlage of the pectineus appears. This I have 
not found in the embryos of a corresponding stage which I have ex- 
amined. Grafenberg describes the pectineus anlage when it first appears 
as fused proximally with the iliopsoas anlage. It is probable that the 
superficial portion of the pectineus is thus at one stage usually fused 
with the iliopsoas anlage. In the youngest embryo in which I have 
found it distinct it is, however, separated by a small interval from the 
iliopsoas muscle mass (Plate II, Fig. 3) and seems more closely associ- 
ated with the anlage of the adductor longus. 
In this 14 mm. embryo (Plate VI, Fig. 1) the anlage of the muscle is 
closely applied to the pubic blastema and can be followed from the body 
of the pubis to the blastema of the femur. Into it a nerve branch may 
be traced from the femoral nerve. Dorsal to the obturator nerve in the 
obturator foramen is a mass of tissue closely associated with the anlage 
of the obturator externus on the one side and with that of the pectineus 
on the other. No definite nerve branch can be traced into it from the 
