THE DEVELOPMENT OF THE VASCULAR SYSTEM 



247 



adult. During development they gradually migrate caudally to their normal 

 positions. 



Other branches of the aorta develop in connection with the urinary and 

 genital organs. Several lateral branches supply the mesonephroi, but when 

 the latter atrophy and disappear the vessels also disappear. A periaortic 

 plexus of vessels, with many branches from the aorta, supplies the develop- 

 ing kidneys until these organs reach their definitive position, when one of 

 the branches on each side enlarges to become the renal artery. The de- 

 veloping genital glands are likewise supplied by several branches from the 

 aorta. Later the majority of these vessels disappear, one pair only per- 

 sisting as the internal spermatic arteries which differ in accordance with the 



Coeliac artery 



Sup. mesenteric 

 (vitelline) artery 



Umbilical artery 



Aorta 



Duodenum 



Inf. mesenteric artery 

 Int. iliac artery 



FIG. 226. Diagram of the visceral arteries in a human embryo of 12.5 mm. Tandler. 

 Numerals indicate segmental arteries. 



sex of the individual. In both sexes they are at first very short; in the 

 female, as the ovaries move farther into the pelvic region, they become 

 considerably elongated to form the ovarian arteries; in the male, with the 

 descent of the testes, they become very much elongated to form the testicular 

 arteries. 



The fifth (or fourth?) pair of segmental lumbar arteries primarily gives 

 rise to the vessels which supply the lower extremities, viz., the iliac arteries. 

 These then would be serially homologous to the subclavians. But certain 

 changes occur in this region, which are due to the relations of the umbilical 

 arteries. The latter, as has already been noted, arise as paired branches of 

 the aorta in the lumbar region, pass ventrally through the genital cord 

 (Chap. XV) and then follow the allantois (urachus) to the umbilical cord. 



