Charles R. Bardeen Dveat 
I. ANTERIOR BORDER NERVHS. 
a. Development. 
When the nerves begin to enter the limb-bud this lies, as pointed out 
above, usually opposite the five lumbar and first sacral nerves (Plate I, 
Fig. 1). The posterior margin of the developing body-wall and the 
anterior margin of the lmb-bud usually overlap opposite the 21st seg- 
ment. The nerves arising from the 21st spinal (1st lumbar) nerve are 
therefore true border nerves, being in part distributed to the abdominal 
wall and in part to the limb. The 20th and 22d spinal nerves (12th 
thoracic and 2d lumbar) also usually contribute to a greater or less ex- 
tent to both regions, the 20th contributing to the cutaneous supply of the 
leg, the 22d slightly to the extreme margin of the abdominal musculature. 
In Embryo CIX, length 11 mm. (Plate III, Figs. 1, 2 and 3), the 
border nerves are beginning to extend toward the skin. At this stage the 
oblique and the rectus muscles of the abdomen are beginning to be differ- 
entiated. The transversus muscle has not yet appeared. Between the 
ventro-anterior margin of the pubis and the ventro-caudal angle of the 
differentiating abdominal musculature a slight thickening of the 
mesenchyme represents the beginning of the tendon of the rectus and of 
the inguinal ligament. A considerable interval exists between the distal 
margin of the abdominal musculature and the anlage of the iliac crest. 
The musculature hes near the peritoneal cavity, while the crest is in the 
mesenchyme lateral to this cavity. Between body cavity and crest lies 
the femoral nerve with its branches (Fig. 3). From the first and second 
lumbar nerves the iliohypogastric and inguinal and the genital branch * 
of the genito-femoral extend ventrally between the ccelomic wall and the 
distal margin of the developing abdominal musculature. From the com- 
mon trunk of the ilLohypogastric and inguinal nerves a lateral branch, 
the “iliac,” extends toward the skin in an area considerably anterior to 
the ilium. The lumbo-inguinal nerve and the lateral and anterior cuta- 
neous branches of the femoral extend toward the anterior margin of the 
limb-bud. 
In aslightly older embryo, CXLIV, length 14 mm. (Plate VI, Fig. 1) 
differentiation of the abdominal musculature has proceeded much fur- 
ther. The external oblique muscle is a thin sheet, somewhat wrinkled in 
the specimen. In the figure merely its origin from the lower ribs is 
shown. It extends distally into a sheet of mesenchyme which is thick- 
1The term “ genital’ nerve is here used in preference to ‘“‘spermaticus 
externus.” 
