THE VASCULAR SYSTEM 335 



become completed and arch III is forming from ventral and 

 dorsal buds, and in c, arches I, II, and III are complete, with 

 a ventral bud forming for arch IV. Arches IV and VI are 

 both formed in d, with several " islands " in the former, which 

 probably have no especial significance. Arch I has broken, 

 leaving dorsal and ventral stumps. 



From the dorsal aorta appear segmental arteries (s) which 

 soon disappear; of these the lowest is the hypoglossal (hy). 

 Below this are other segmental arteries, of which the first 

 cervical (ec) is figured here. Stage e shows but little change 

 save in proportions and the loss of segmental arteries. A 

 pulmonary artery appears, arising from arch VI. The human 

 embryo at about this stage shows a well-developed arch V. 

 The segmental arteries of the head have disappeared. In f 

 arch II has also broken through, leaving stumps, and of these 

 the ventral becomes closely associated with that of arch I, and 

 both are borne by the anterior end of the ventral aorta (av), 

 a part destined to play an important role later on. Arch III 

 has become large; arch IV is very large, and a rudiment of 

 arch V has appeared. The dorsal stump of I has divided 

 into two branches, the maxillaris (ms), which goes to the de- 

 veloping upper jaw, and the mandibularis (md), which be- 

 comes distributed to the lower jaw. 



In stage g the maxillary artery just mentioned has divided 

 again into a supra-orbital (o) and an infra-orbital (i), thus 

 giving three terminal branches of the dorsal stump of arch I. 

 From the free end of the ventral aorta (av} appears a branch 

 that goes to the tongue-anlage, the lingualis (/). The point 

 especially to be noted here is that of the two buds, O) and 

 (y) r which arise from the dorsal stumps of arches I and II, 

 respectively, and grow toward one another. The formation of 

 the arteria vertebralis cerebralis (vc) by the union of the hypo- 

 glossal and first cervical arteries with one from much further 

 forward is also to be noticed, but is without special interest. 

 In stage h the buds (^r) and (y) have united the dorsal buds 

 of I and II, and the significance of this step is seen by compar- 

 ing this with stage t, for here the portion connecting the com- 



