94 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



previous longitudinal arteries for one to be confident that it is entirely made up 

 from segments taken over from the paired vessels. The presence of small islands 

 in the center of its caudal portion is also strong evidence that this portion was 

 formed by coalescence of the two arteries. The dorsal aorta, which was found 

 unquestionably to coalesce, had similar islands. The fact that the small islands 

 are also in line with the large anterior island at the hypophysis, which undoubtedly 

 has on either side unchanged segments of the original paired longitudinal vessels, 

 also speaks for this method of formation. 



The longitudinal neural arteries are described as approaching each other 

 before the formation of the basilar. The distance between their axes was compared 

 in a series of 7 embryos, from the time of their appearance to the establishment of 

 the basilar, to find whether this actually takes place, since a lateral movement of 

 vessels as immature as these seems highly improbable. The measurements were 

 made on models and Edinger projections and the values thus obtained divided by 

 the magnification. The interval between the axes of the vessels was -found to 

 remain constant. It was therefore only their adjacent walls that approached, 

 due to the result of their increase in diameter just as found in the fusion of the 

 paired aortae. 



In the early sixth-arch stage the earlier history of the basilar is still indicated 

 by the presence of occasional islands and an irregular, dorso-ventrally compressed 

 form. In a 12-mm. embryo, with subclavian well established, this condition had 

 passed. Tardiness in fusion was shown in a 14-mm. embryo by the persistence 

 of the paired condition for a considerable distance back of the isthmus. It is in 

 this manner that partly double adult basilars are formed. 



The connection of first and second occipital segmental arteries and of more 

 cranial branches from the dorsal aorta with the longitudinal neurals is to be ex- 

 pected at an earlier period than is represented in our series. It was found that at 

 the time of formation of the basilar artery the hypoglossals, as well as the first 

 cervical segmentals, connected the dorsal aorta and longitudinal arteries, though 

 this is later than they have previously been observed. Since, in two instances, 

 the hypoglossal vessels were very slender and were lost in a capillary plexus for a 

 short distance, they were evidently just about to lose their identity. The first 

 cervical segmentals were of considerable size and clearly served to supply the blood 

 to the paired longitudinal arteries in their late stage and consequently are the 

 chief caudal connection of the basilar. 



The continuity of the paired longitudinal neural vessels of the cranial region 

 with the longitudinal tracts of the plexus on the spinal cord was readily distinguish- 

 able before the time of formation of the basilar artery. It could not be seen from 

 the examination of cross-sections that a pair of definite vessels superseded the 

 tracts on the cord either before or after the establishment of the basilar. 



VERTEBRAL ARTERY. 



The formation of the vertebral artery is the most perfect example of the 

 evolution of a longitudinal arterial channel from the segments of a series of trans- 

 versely running arteries and the anastomoses between them. While the internal 



