108 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



vian comes off just above the bifurcation. Before the interruption, as was earlier 

 explained, the right fourth arch and the paired aorta distal to it had already been 

 reduced to a mere channel of supply to the subclavian. The interruption of the 

 right aorta distal to the subclavian is the final step in giving these two segments 

 over as the proximal end of the right subclavian. 



The left subclavian continues its movement up the aorta and arch until, in 

 the 17-mm. embryo, in which the aortic arch is complete, it is but a little way from 

 the summit. 



Basilar Artery. 



Paired longitudinal arteries develop along the lower surface of the brain 

 and are continuous at their anterior ends with the aorta? in the human embryo just 

 after the establishment of the fourth arch. They were still incomplete in a 4-mm. 

 embryo in which the fourth arch had just formed. Only the first cervical and second 

 occipital (hypoglossal) arteries have been seen connected with the caudal end of 

 these paired longitudinal arteries, but this part probably arises as anastomoses 

 between all segmental arteries cranial to the second cervical, followed by a loss of 

 the connection of these vessels with the aorta. The paired neural arteries were 

 traced caudally into paired longitudinal arterial tracts of the cord. 



The contiguous walls of the paired longitudinal neural arteries approach, 

 as in the case of the primitive aortas, merely by enlargement and not by actual 

 movement of the vessels toward each other. Cross anastomoses develop from 

 enlarged capillaries; and in the more cranial part of the region destined to be occu- 

 pied by the future basilar artery, successive segments, taken irregularly from one 

 or the other neural artery with cross anastomoses, are remolded into the basilar. 

 Near the caudal end there is apparently a fusion of the two neural arteries to form 

 the basilar. By the time the pulmonary arch is established, the formation of the 

 basilar is well under way. 



Vertebral Artery. 



There would never be a vertebral artery did not the aorta shift caudally. Its 

 movement is responsible for the proximal ends of the segmentals, back to the 

 seventh cervical, becoming stretched, decreased in diameter, and bent obliquely 

 on the aorta. The more distal part of the segmentals also takes on a slope which is 

 less abrupt and due to the shifting of the nerve-tube on the digestive tract and 

 adjacent structures. It may be that this, too, is unfavorable to their maintenance. 

 The seventh segmental, being larger than the more cranial vessels, due to its sub- 

 clavian branch and because it lies in a region where there is as yet little caudal 

 movement of the aorta, does not become oblique or constricted in diameter and 

 does not degenerate. 



Anastomoses develop between the successive cervical segmental arteries in the 

 9-mm. embryo. These pass caudally from one vessel and connect with the more 

 distal part of the next succeeding member of the series. A channel is thus devel- 

 oped from alternating anastomoses and segments of segmental arteries. Of the 

 two, the arteries contribute the most. The resulting vessel is tortuous in both 



