286 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
the pro- and mesonephroi, while there is usually but a single pair of 
renal arteries to supply the metanephroi (true kidneys) of the amni- 
otes. The arteries to the gonads may be included under the single 
head of genital arteries, though they are usually subdivided into the 
spermatic and ovarian arteries according to the sex. Like the neph- 
ridial, the genital arteries are 
more numerous in the lower 
and are reduced in number in 
the higher forms. 
The somatic arteries are 
more numerous and are meta- 
merically arranged. In _ the 
da 
FIG. 290. Fic. 291. 
Fic. 290.—Diagram of early relations of vertebral arteries in an amniote. av, vertebral 
artery; da, dorsal aorta; ec, ic, external and internal carotids; pa, pulmonary artery; 7a, 
radix aorte; sa, subclavian. 
Fic. 291.—A, side view of developing anterior arteries of Lacerta, after van Bemmeln; 
the vertebral artery not developed hehind; B, ventral view of the relations of the arteries at 
the base of the vertebrate brain. av, vertebral artery; b, basilar artery; cw, circle of Willis; 
da, dorsal aorta; ec, ic, external and internal carotids; pa, pulmonary artery; ra, radix 
aorte; sa, segmental arteries; sc, subclavian; 2-6, aortic arches. 
early stages they are given off in pairs from the radices and the 
dorsal aorta, an artery on either side, extending laterally between 
each two successive myotomes (fig. 275). Many of these remain 
in a slightly modified condition and are called intercostal arteries 
(including lumbar and sacral arteries, etc., according to position). 
These usually become connected on either side (fig. 290), near their 
