92 AORTIC- ARCH SYSTEM IN THE HUMAN EMBRYO. 



injections of the cord of the chick, described them as definite channels, but he did 

 not find so great a differentiation in a number of mammals which he studied. His 

 figure for the sheep shows them as rather large and approximately straight vessels 

 in a thick plexus. 



Evans (1909o) published a very full series of figures from injections of the 

 cord of the pig, showing the paired condition and successive stages of fusion. Here 

 we have to do as frequently with two or three vessels side by side as with a single 

 enlarged vessel of the plexus. Taken together, these findings seem to imply 

 specific differences in mammals in respect to their being supplied with continuous 

 arteries running under the nerve-tube or by longitudinal tracts made up of 

 irregularly succeeding segments of vessels which are in some places double or 

 even triple. 



The caudal connections of the paired longitudinal neural arteries in man were 

 described by Zimmerman (1889) as the hypoglossal and first cervical segmental 

 arteries. Since Evans (1912), by tabulating the segmental arteries in man in 

 order of their appearance, finds the first occipital to appear first and the other 

 segmentals in the order of position, there can be little doubt, in the light of the 

 observations of De Vriese and Sabin on the development of the longitudinal 

 neural arteries in other forms, that all the cranial members of the segmental series, 

 as far back as the first cervical, contribute by anastomosis to the formation of the 

 longitudinal neural arteries. 



De Vriese (1905), in her study of the rabbit embryo, has given the only descrip- 

 tion of the formation of the basilar artery from the paired longitudinal neurals. 

 She states that the neurals first form strong transverse anastomoses and that the 

 segment of the right or the left tract between two successive anastomoses disap- 

 pears, so that the basilar is made up of successive segments taken irregularly from 

 the right or left tracts. Sterzi (1904) earlier described and pictured this same 

 process in the formation of the anterior spinal artery of the cord in the chick. 



The preceding discussion of the literature shows that the longitudinal neural 

 artery in mammals varies from a zone of enlarged vessels in a plexus to a single 

 continuous channel. Its nature and relation to the development of the basilar in 

 man have not been described. 



Two models were made covering the time of formation of the basilar, and 

 sections of earlier embryos, before the 22-somite stage, were studied. In a beauti- 

 fully preserved 4-mm. embryo, in which the third arch had just become complete, 

 it was possible to distinguish longitudinal vessels in the region of the future basilar 

 artery. At this time there were already paired arteries continuous cranially 

 with the internal carotids and caudally traceable to the posterior third of the 

 hindbrain, where they were lost among capillaries. They were not much larger 

 in diameter than the vessels of the surrounding plexus and showed their probable 

 origin from it by numerous lateral branches and a slight tortuosity of course. 

 They were most closely approximated in the medullary region, where they were 

 separated by a distance equal to about sLx times their diameter. In a still earlier 

 embryo, which had 22 somites, no vessels could be identified in the plexus under 



