292 Herbert M. Evans. 



It will be seen that the embryos included in the table presented 

 comprise only those in the first three periods or stages of the devel- 

 opment of the subclavian artery 



Embryos 1 and 2 show some early steps in the downgrowth of 

 the primary body-wall plexus, the system forming the latter umbil- 

 ical vein. 



Embryo 1, with twenty-four somites, shows this mesh of cajiilla- 

 ries extended caudally in the somatopleure to the level of the ninth 

 inter-somitic space. 



In embryo 2, with thirty somites, these capillaries have reached 

 a point opposite the fourteenth interspace. In addition, from the 

 aorta itself several capillaries have now grown out, one of which 

 has joined the main jilexus at a point near the thirteenth inter- 

 space and its dorsal segmental vessels. These apparently unim- 

 portant endothelial sprouts are but the first of a considerable series 

 to grow from the lateral aortic wall, and though there is as yet no 

 external indication of a limb bud, they are probably to be regarded 

 as the first limb capillaries. 



In the next succeeding stage, embryo 3, of thirty-two somites, a 

 slight swelling of the somatopleure constitutes an infinite, yet un- 

 doubted, limb bud, and we find a considerable row of these aortic 

 or rather subclavian capillaries. A glance shows that these vessels 

 are not segmentally arranged (Figs. 2a and 2b). 



In such injected specimens one may look down on the prepara- 

 tion as a whole and examine carefully any area. 'No reconstruc- 

 tion is necessary; no doubt about relations exists, for the entire 

 picture is spread out before one. The dorsal segmental vessels 

 stand out sharply and it is easy to determine the relation of these 

 to the new subclavian capillaries. It is impossible to say that the 

 latter vessels are determined in position by the former for the ivhole 

 appearance giveri is of a profuse irregular outgrouifh of capillaries 

 which form a simple plexus. On the right side, as the figure shows, 



Fig. 1. — Chick embryo of 30 somites (embryo 2 of table). Showing 

 downgrowth of primary wall plexus, X 53i/4. Ant. Card. V.. anterior cardinal 

 vein. Ext. Jug. V.. external or inferior jugular vein (linguo-facial vein). 

 Post Card. V.. posterior cardinal vein. P. b. w. p., primary body wall plexus. 



