62 H. D. SENIOR 
The limb preserves its primitive position which, with the 
exception of progressively increasing flexion of the knee, remains 
unchanged throughout the period of development under con- 
sideration. The flexor aspect of the embryonic thigh and leg 
and the plantar aspect of the sole are directed medially; the 
great toe is preaxial or cephalic. The term posterior for the 
adult has, therefore, the same significance as the term medial for 
the embryo; so have, respectively, the terms anterior and lateral, 
medial and pre-axial, lateral, and post-axial. For the parts 
above the hip-joint there can be no possibility of confusion, since 
like terms of orientation serve equally well for the postnatal and 
embryonic periods. 
Il. THE ARTERIAL SYSTEM OF THE LOWER LIMB IN PROGRESSIVE 
STAGES OF DEVELOPMENT 
a. stage of 6mm. C. 1. EK. Cy no. 1075, figs, and OA: 
In the lumbar region the nerve roots are not recognizable. The 
ganglia of the lumbar and sacral regions appear as segmentally 
disposed swellings upon the continuous neural crest. Distal to 
the second lumbar segment the postcardinal vein becomes plexi- 
form. The medial part of the plexus receives the segmental 
veins, the lateral part represents the still indefinite marginal 
vein. 
The dorsal segmental arteries, with the exception of the fifth 
lumbar and second sacral, pass directly to the spinal cord with- 
out branching. The umbilical arteries, the ventral roots of 
which are still very large, arise opposite the intervals between 
the third and fourth lumbar segmentals. 
The secondary, or dorsal, roots of the umbilical arteries are 
present, but are smaller than the original or ventral roots of that 
vessel. Each dorsal root, in the embryo under consideration, 
arises from the union of two arteries. 
The chief share in the formation of the dorsal root of the a. 
umbilicalis is taken by a vessel which arises from the fifth 
lumbar segmental artery a short distance beyond its root. The 
vessel in question is joined almost perpendicularly near its 
origin by a smaller one which springs from the aorta in the 
interval between the fourth and fifth lumbar segmental arteries. 
