308 Herbert M. Evans. 



pletely and grown out as a loose plexus over the outer surface of 

 the myotomes. 



Fig. 13, from the embryo of 116 hours (^o. 16) show the fur- 

 ther growth and elaboration of these changes. The common trunks 

 of the subclavian and dorsal segmental vessels, are themselves being 

 shifted toward the mid-dorsal line, soon to arise from a single com- 

 mon trunk. The segmental arteries and veins have each two main 

 sj'stems of branches which alternately supply and drain the cord 

 at successive points around its circumference. Penetrating arte- 

 ries extend from the ventral arterial tract into the cord substance 

 at the boundary zone of the neuroblasts and ependyma. They are 

 drained by delicate transverse venules. 



The subclavian arteries have two small branches before supplying 

 the limb proper, a dorsal branch which supplies the outer capillary 

 plexus over the myotomes and a ventral twig to the Wolffian duct. 



The subclavians are large vessels and control the blood supply 

 to the limb. They must be considered now at the height of func- 

 tional activity, and with this stage in the history of the primary 

 subclavian the present account closes. 



IV. Observations ox the Conditioxs Present at Similar 

 Stages ix Embryos of the Duck. 

 Rabl's research on the development of the subclavian artery was 

 conducted entirely on ducks. In it he failed to find stages earlier 

 than the period of strictly segmental subclavians. It was conse- 

 quently of some importance that these forms be investigated to 

 see if the early subclavian capillary plexus which was present in 

 chicks was not of fundamental value and hence present here also. 

 Of a series of ten of these duck embryos I shall describe carefully 

 only two typical ones, one, an embryo of thirty-three somites in 

 the stage of an irregular subclavian capillary plexus, and the other. 



Fig. 13. — Cross section of chick of one Iiundred and sixteen liours incubation, 

 in region of fore limbs. My., myotome; Dor. Vein, dorsal branch of the 

 segmental vein; Post. Sp. Art., branches of the segmental artery whicli con- 

 tribute to the formation of the posterior spinal artery; Pen. Art., penetrating 

 artery ; P. C. V., posterior cardinal vein ; Se. V., Seitenrumpfvene, thoraco- 

 epigastric veins; Rad. Art., radicular artery. 



