AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 89 



ing around and through the plexus as resulting from anastomoses between a series 

 of limb-bud arteries and their brachial continuations in the limb-bud. Since 

 the evidence of a limb-bud artery in more than one space is the exception in man 

 and was not found to occur in the pig, we must regard such accessory channels as 

 the result of a chance enlargement of a part of the general plexus. 



At the beginning of the post-branchial period the primitive subclavian has 

 thoroughly incorporated the territory derived from the stem of the earlier segmental 

 from which it arose, and thus the latter has entirely lost its identity (figs. 11, 12). 

 The serial position of this vessel can at first be told by its relation to the body seg- 

 ments. After its identity is lost in the subclavian, its place in the segmental series 

 may be inferred from the vertebra into which the vertebral artery enters. A series 

 of 15 embryos were examined to learn how constantly it arises from the seventh 

 cervical segmental. The specimens ranged from 4 to 24 millimeters in length, and 

 in the youngest the fourth arch had just been completed. In 2 of the younger ones 

 the subclavian comes off from the sixth cervical segmental artery, while in the 

 others it comes off from the seventh. In 1 embryo of the post-branchial period 

 both vertebrals enter the transverse foramen of the rudimentary fifth cervical 

 vertebra (plate 3, figs. 37 to 39) ; in another the right vertebral enters the fifth and 

 theleft the sixth. In the other 4, both vertebrals enter the sixth transverse process. 

 The frequency of variation from the origin of the primitive subclavian from the 

 seventh segmental noted here and of the corresponding entrance of the vertebral 

 into the sixth vertebra is far greater than is encountered in adult life. Since 2 of 

 the embryos were in a very early limb-bud stage, when its supply is scarcely more 

 than a plexus, it may be that, had death not occurred, a readjustment might have 

 soon taken place by the enlargement of a twig in the interspace usually occupied 

 by the seventh segmental superseding the aberrant subclavian. In 1 of these 

 embryos there was an artery extending nearly to the limb-bud in the usual position. 

 Whether or not this is the true explanation of the occurrence of these early aberrant 

 vessels, the other embryo, which is in the early post-branchial phase with the 

 vertebral already formed, was apparent^ in the course of a return to a usual type 

 in an entirely different manner. The vertebral has a double connection with the 

 subclavian by way of both the sixth and the seventh segmental arteries. The 

 position of the subclavian shows that it arises from the sixth segmental artery. 

 On the left side the subclavian slopes cranially as it leaves the aorta. It is evidently 

 being carried along by the shifting of the aorta, which just at this time has become 

 accelerated. It may be that this would have resulted in a breaking of the con- 

 nection of the vertebral with the subclavian by way of the sixth segmental artery 

 and an enlargement of the latter by way of the seventh, thus restoring the usual 

 condition. Since the right subclavian will also after a time be drawn caudally, 

 a like readjustment of the vertebral on this side would have been possible. 



The changing relation of the primitive subclavian to the aortic bifurcation is 

 significant in its development. Were the right vessel to come off below, it would 

 arise from the descending aorta in the adult. For a time after its appearance it 

 was always found to arise below the bifurcation. The distance of the origin from 

 the bifurcation is not great; but because of the irregularities in the position of the 



