122 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



caudally as the median dorsal aorta. The fifth (sixth ?) aortic arches (cf. p. 99) 

 are connected with the pulmonary trunk, and from them arise small pulmonary 

 arteries to the lungs. Dorsal intersegmental arteries arise, six pairs from the de- 

 scending aortae, others from the dorsal aorta. From the seventh pair, which arise 

 just where the descending aortas fuse, the subclavian arteries pass ofi^ to the 

 upper Umb buds and the vertebral arteries to the head. The latter are formed 





Fig. 126 A. — Reconstruction of a 12 mm. pig embryo to show the veins and heart from the left side. 

 For names of parts see Fig. 126 B on opposite page (F. T. Lewis). X 9. 



by a longitudinal anastomosis between the first seven pairs of intersegmental 

 arteries on each side, after which the stems of the first six pairs atrophy. 



Ventro-lateral arteries from the dorsal aorta supply the mesonephros and 

 genital ridge (Fig. 123). Ventral arteries form the coeliac artery to the stomach 

 region, the vitelline or superior mesenteric artery to the small intestine, and the 

 inferior mesenteric artery to the large intestine. 



