AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 109 



sagittal and frontal planes. Due to the confined space in which the artery runs, it 

 is not entirely straightened out even in a 52-mm. fetus. Each anastomosis arches 

 laterally where it lies free of the corresponding nerve. That between the first and 

 second segmental arteries extends more laterally than the others, so that it passes 

 external to the suboccipital nerve. The part of the first cervical segmental artery 

 distal to the anastomoses, up to its ending in the longitudinal neural artery, also 

 becomes part of the vertebral. It was not decided whether the caudal end of the 

 longitudinal neural also supplies material for the distal end of the vertebral or 

 whether they all go into the basilar. 



There are a number of considerations pointing to a cranial source of the blood 

 passing through the entire basilar and the cranial part of the cervical cord at the 

 time the vertebral is forming. It is probable, though less certain, that at an 

 earlier period the longitudinal neural arteries also were supplied nearly to their 

 caudal ends by the current passing forward into the neurals from the cranial end 

 of the aorta?. Where the vertebral is forming, the plexus on the cranial part of 

 the cervical cord is turgid and the segmental arteries have a characteristically 

 swollen appearance at their distal ends. They taper proximally to enter the aorta 

 by a slender channel or to terminate in a plexus at this end. The current formerly 

 borne by the cervical segmentals is apparently rather abruptly thrown into the 

 plexus and the distal ends of the segmentals, due to the interruption of their prox- 

 imal ends. As it is of higher pressure than usually carried by them, their walls 

 are stretched. They are relieved by the establishment of the vertebral artery. 



The subclavian artery, because it does not succumb to the unfavorable con- 

 ditions which cause the disappearance of the more cranial segmental arteries, and 

 because it has a more direct connection with the main arterial stream than the 

 cranial ends of the longitudinal neurals, falls heir to the anastomotic chain, thus 

 making the vertebral its branch. The course of development of the human ver- 

 tebral is in accord with the claim of De Vriese that in vertebrate phylogeny the brain 

 is first supplied by the internal carotids and that only later does its caudal part 

 come to be supplied by the vertebral. 



DESCRIPTION OF PLATES. 



Plate 1. 



Fios. 29 and .30. Ventral and lateral views of the cranial portion of the arterial system of a 22-somite embryo. The 

 first arch is at its maximum development and the dorsal and ventral outgrowths, which are to aid in 

 the formation of the second, are just appearing. Embryo No. 205.3, length 3 mm. 



Figs. 31 and 32. Ventral and lateral views of an embryo in which the first arch has gone, the second arch is much 

 reduced in diameter, and the third arch well developed. Dorsal and ventral outgrowths for the fourth 

 and probably for the pulmonary arch are present. Embryo No. 836, length 4 mm. 



Plate 2. 



Figs. 33 and 34. Ventral and lateral views of a 5-inm. embryo (No. 1380). The third and fourth arches are in a 

 condition of maximum development and dorsal and ventral sprouts of the pulmonary arch have nearly 

 met. The primitive pulmonary arteries are already of considerable length. 



Figs. 35 and 36. Ventral and lateral views of an 11-mm. embryo (No. 1121). The pulmonary arches are complete 

 and the right is already regressing. The third arch is now bent cranially at its dorsal end and its stream 

 is about to become deflected in that direction. 



Plate 3. 



Figs. 37 to 39. A 14-mm. embryo (No. 940) in which the last indications of the aortic-arch system are just disap- 

 pearing and a very primitive condition of the larger arteries derived from them is already recognizable. 

 Figure 37, lateral view; figure 38 ventrolateral view; figure 39, ventral view. 



Fig. 40. Ventral view of an 18-mm. embryo (No. 1390). The arterial evolution has preceded so far that the adult 

 vessels are easily recognizable. 



