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 



A 



av. 



Sa 



FIG. 290. FIG. 291. 



FIG. 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; ra, 

 radix aortae; sa, subclavian. 



FIG. 291. A, side view of developing anterior arteries oiLacerta, after van Bemmeln; 

 the vertebral artery not developed behind; 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 

 aortas; 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 



