214 



EDWARD A. BOYDEN 



surface to the non-vascular triangle, so that as they swing in 

 they first meet in the median line under the point where the 

 epitrichial ridge first appears, and thereafter progressively an- 

 terior to this point. In an embryo of 15.5 mm, (fig. 2) they are 

 just reaching the mid-line for the first time. In an embryo of 

 18.5 mm. (injected specimen 6 days, hours), they have moved 



Fig. 2 Blood vessels in the pectoraljbody-wall of a 15.5 mm. chick. Camera 

 lucida drawing of injected embryo ji (6 days, hours) X 15 diam. Note the non- 

 vascular area in the center and the capillary plexus above it which has grown 

 ■down from the cervical region, p.o., opercular fold; v.u.s., left umbilical vein. 



Fig. 3 Blood vessels in the pectoral wall of a 20.6 mm. chick. Camera lucida 

 drawing of injected embryo ka (7 days, 2 hours) X 15 diam. Note the disappear- 

 ance of the non-vascular area and the direction of the blood vessels as compared 

 ^with those in figure 2. u., umbilicus; x., median line, on either side of which are 

 the right and left sets of marginal veins. 



in upon the mid-ventral fine as far up as the point where the epi- 

 trichial ridge meets the surface ridges, (the region of the truncus 

 aorticus). Finally in an embryo of 20.6 mm. (fig. 3), the area 

 over the heart is completely filled with parallel longitudinal veins 

 extending from the neck to the umbilicus. Thus in forty-eight 

 hours the original marginal veins of each side have described an 

 arc of 45°. 



