AORTIC-ARCH SYSTEM IN THE HUMAN EMBRY. 87 



SUBCLAVIAN ARTERY. 



The development of the subclavian artery in its early stages is a convincing 

 illustration of the capacity of the blood-stream to take over and remold into a unit 

 various vascular channels as the need arises. Though it shows great plasticity and 

 inconstancy at this time, yet later, when it is more mature, it meets with surprising 

 success in maintaining itself when exposed to stress due to the shifting of the sur- 

 rounding organs upon one another. 



In the development of the early arterial supply to the limb-bud there are con- 

 siderable differences between the bird and the mammal, although at such an early 

 period one might with reason anticipate a close correspondence. Rabl (1907), 

 who has studied the condition in the duck by graphic reconstructions, describes a 

 period in which there is an increase in the number of small arteries lying in succes- 

 sive intersomitic spaces and passing from the aorta to the plexus of the early limb- 

 bud. Evans (1909a) confirms this in the chick by injections, and figures as many as 

 four vessels passing to the bud plexus along the planes of separation of the body 

 segments. He finds that at a still earlier period, before the 34-somite stage, there 

 are already distinct arteries of supply to the plexus from the aorta, but they are 

 not segmental. The vessels, both of the non-segmental and the segmental types, 

 are referred to as subclavians, but most of them have so little to do with the devel- 

 opment of that artery that it will be best to designate them merely as limb-bud 

 arteries. 



In the human embryo, Keibel (Keibel and Elze, 1908) described two limb-bud 

 arteries from successive segments at the time of the first appearance of the vessel. 

 Evans (1908) also found a similar case, though not from the same two segments. 

 Goppert (1909) figures, from graphic reconstructions of the mouse embryo, both 

 segmental and non-segmental limb-bud arteries. He also claims that the seg- 

 mental type can be traced as independent channels well within the bud and dis- 

 cusses then- changes. Since a segmental arrangement here implies a segmentation 

 of the substance of the limb, there is little probability that he is correct. Woollard, 

 in this volume of the Contributions to Embryology, gives very complete illustrations 

 of both arterial and venous vessels of the early limb-bud from injections of pig 

 embryos. He does not find, at any time, more than one distinct artery to the limb- 

 bud. It is a branch of a segmental or, more precisely, intersegmental artery and 

 passes to the limb through the intersegmental space. There are short twig-like 

 vessels in other intersegmental spaces which come off from the segmental artery 

 and quickly go over into a plexus lying largely in the intersegmental spaces and con- 

 necting them with the limb-bud plexus. As the embryo enlarges, these secondary 

 connections do not increase in proportion to the single limb-bud or primitive sub- 

 clavian artery and soon become negligible. Woollard does not find the non- 

 segmental limb-bud arteries of Goppert. His study throws doubt upon the exist- 

 ence of both multiple segmental and earlier non-segmental limb-bud arteries in 

 mammals, though the possibility of a certain amount of individuality in this regard 

 in the different species must not be lost sight of. 



