66 H. D. SENIOR 
In passing from the medial to the lateral side of the sacro- 
pudendal plexus, the axial artery runs between the main plexus 
and a branch from its lower part. The branch referred to has 
been removed from the reconstructions illustrated in figures 1 
to 6, since it partially covers the artery when viewed from the 
medial side. 
From the lateral aspect of the concavity of the umbilical 
artery, some distance proximal to the origin of the axial artery, 
there now arises a new vessel, the a. iliaca externa. This vessel 
takes a cephalic direction, nearly parallel to the aorta, coursing 
medially to the origin of the obturator nerve. At this stage 
the wall of the external iliac artery is thin and its course slightly 
tortuous. The artery is not connected with the dorsal segmental 
arterial series or with any other artery. The further history of 
the external iliac is rather remarkable. It soon becomes quite 
straight and acquires a wall of great thickness, but remains un- 
branched until a stage of (approximately) 12 mm. The a. iliaca 
externa appears as a thick-walled straight artery in the well- 
known pig embryo of 12 mm. 
The place of origin of the a. iliaca externa marks the permanent 
subdivision of the ‘dorsal root of the umbilical artery into two 
parts. The proximal part becomes the adult a. iliaca com- 
munis while the distal part represents the a. hypogastrica and a 
short proximal section of its umbilical branch. 
At this stage the marginal vein is fully formed; its caudal 
root, represented by the v. ischiadica, runs in close contact with 
the proximal part of the corresponding artery. 
c. Stage of 12 mm. M. E. C., no. H. 16, figs. 3 and 9 C 
Condensation of the mesenchymal skeleton is now well ad- 
vanced. It is not sufficiently definite in the tarsal region, how- 
- ever, for the determination of the course taken by the connection 
between the axial artery and the dorsal rete of the foot. The 
surfaces of the growing nerves are considerably roughened by 
the beginning outgrowth of numerous branches, many of which 
can be identified. 
