88 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



The earliest vessels to the limb-buds that I was able to recognize in the human 

 embryo (4 to 6 mm. in length) were found in specimens in which the fourth arch 

 was present but the sixth was not complete. The anterior limb-buds were as yet 

 but slight elevations from the general body- wall ; they had not been penetrated by 

 the outgrowing nerves, and they contained a nearly homogeneous vascular plexus. 

 One definite artery was present for each forelimb bud, and this lay in an interseg- 

 mental space. Enlarged channels could frequently be traced a greater or less 

 distance from the segmental arteries in other intersegmental spaces toward the 

 plexus of the limb-bud. Their appearance in sections favored the interpretation 

 that they usually broke up into a plexus at a greater or less distance from the limb- 

 bud and were thus similar to the twigs described by Woollard. It seems not un- 

 likely, in some instances, that there were also one or more secondary channels 

 traceable as definite vessels to the plexus, though it was not possible to prove this 

 from the study of sections on account of their small, almost capillary caliber. The 

 second subclavians in human embryos described by Keibel and Evans are probably 

 of a similar nature, but the existence of such vessels can be proved only in sections 

 cut very favorably and probably will not be established without the use of in toto 

 preparations of complete injections. 



In slightly older embryos it was impossible to trace the primitive subclavian 

 into the limb itself where it continued as the primitive brachial artery. At this 

 time the subclavian is found to be coming off from an outpocketing of the aorta, 

 which at the same time gives rise to a dorsal segmental branch. Later, this con- 

 nection elongates into a definite vessel (the stem of the primitive segmental artery) 

 in the manner described by Rabl (1907) and Sabin (1917) in other forms. 



In a model of a 5-mm. embryo the advance in differentiation of the limb-bud 

 is marked by the entrance of the spinal nerves into its base and is reflected in the 

 vascular system. A venous marginal sinus draining into the umbilical vein, as 

 described by Evans (1909a, 19096) in the pig and chick, is now well defined. The 

 segmental branches of the post-caval vein can be followed between the spinal nerves 

 to their origin in the brachial plexus. They are accompanied by branches of the 

 segmental arteries. The primitive subclavian is a branch of the seventh cervical 

 segmental artery. Within the body it lies between the sixth and seventh nerves 

 and passes over the dorsal surface of the plexus and soon breaks up into capillaries. 

 The model of the limb-bud of a 7-mm. embryo shows the limb considerably 

 elongated and containing an axially placed nerve mass which is already giving off 

 branches. The primitive subclavian has now become surrounded by the brachial 

 plexus. This seems to be due to the growth o neurons across its dorsal surface 

 to complete a canal about it, not to any development of a new arterial channel 

 through the plexus. The brachial artery is divided into three terminals. At 

 14 mm. the primary branches of the nerves and arteries are well developed in the 

 arm. Not only the radial, ulnar, and interosseus are present, but digital branches 

 as well. Their development was not followed. 



The model of a 5-mm. embryo, in which a primitive subclavian was well 

 developed, showed a slender channel passing from subclavian to brachial artery 

 over the dorsal surface of the plexus. Goppert (1909) explained various loops pass- 



