272 The Nerves and Muscles of the Leg 
ened at its distal border into an embryonic inguinal ligament (lig. ing.). 
This latter extends from an anterior mesenchymatous process of the ilium 
toward the pubis. Ventrally it becomes continuous with the blastema 
of the pubic crest. Beneath the external oblique lies the internal oblique 
muscle. Distally this is connected by a mesenchymatous membrane with 
the inguinal ligament. In the figure merely the costal and inguinal por- 
tions of the muscle are shown. 
The transversus, muscle is differentiated immediately beneath the 
peritoneal membrane. It is not clear whether the material of the trans- 
versus musculature is derived from the ccelomic ning or from the 
myotomes. If from the latter the tissue wanders along the peritoneum 
from the region of the ribs. 
At this early stage the anlage of the processus vaginalis may be seen 
in the form of a thickened mass of tissue which is continued from the 
plica gubernatrix through the internal oblique muscle and the aponeuro- 
sis of the external oblique above the inguinal hgament to the junction of 
the thigh with the trunk. Here it spreads out into processes which ex- 
tend on the one side toward the mid line of the body, on the other toward 
the femur. 
Between the transversus musculature and the internal oblique run the 
main trunks of the thoracico-abdominal nerves. The ilio-hypogastric and 
inguinal nerves pierce the internal oblique muscle and the aponeurosis 
of the external oblique much as in the adult. The iliac branch of the 
iho-hypogastric, however, pierces the oblique muscles in a region anterior 
to its relative adult position. This is also the case in Embryo XXII, 
length 20 mm., Plate V, Fig. 1. Beyond the region of the inguinal nerve 
the coelomic wall, backed by a thickened membrane representing the trans- 
versalis fascia, curves medially while the oblique musculature takes a 
somewhat lateral direction toward the inguinal ligament. Between the 
two is a space in which lie the femoral nerve, its proximal branches and 
the anlage of the ilo-psoas muscle. The genital branch of the genito- 
femoral nerve follows along the ccelomic wall almost parallel with the 
hypogastric and inguinal nerves but converging toward the latter. ‘The 
point “ X ” in the figure represents a region where later the peritoneal 
wall will be pushed laterally over the ilo-psoas muscle so as to cover 
this and be brought in contact with the iliac crest. The lumbo-inguinal 
nerve passes out beneath the inguinal ligament in the vicinity of the 
femoral artery. It probably represents a lateral branch of the genito- 
femoral considered as the ventral division of a typical spinal nerve. 
