416 ELEANOR LINTON CLARK 



7 and 7| days the lymph-flow is all posterior, draining into the 

 tail veins through the lymph heart. 



Figure 5 represents the injected superficial lymphatics of this 

 stage. The increased size of the channels and their moderately 

 straight course is noticeable. The position of the main channels 

 varies in chicks of the same stage: the main channel for the 

 pelvis may be situated ventrally or dorsally with reference to 

 the rest of the surrounding plexus, or two or even three main 

 channels may develop and persist for two or three days. Fre- 

 quently the two sides of the same embryo show variations of 

 pattern, one having a ventral duct as its main channel and the 

 other a dorsal or median one. Similarly the channel selected 

 through the plexus connecting the supra-scapular lymphatics 

 with those of the side may cling closely to the shoulder, or it may 

 follow a more gradual, dorsal course, or, occasionally, channels 

 in both positions may persist. Whatever the location of the 

 ducts and whate\'er the variation in the path taken by the 

 injected granules, the general direction of the lymph-flow, in 

 normal healthy chicks of the same stage: is always the same. 



At this stage, the pulsations of the lymph heart are still in- 

 separable from the body movements and occur only at the time 

 of the periodic spasms, but they are much more vigorous than 

 in the younger chicks and the course of the granules through 

 the lymph heart is more direct. Associated with this and also 

 with the increase in the area drained by it, injected specimens 

 show a change in the form of the lymph heart. Although it is 

 still in the form of a plexus, the various channels have enlarged 

 and many of the small capillaries have disappeared so that the 

 heart has become niore compact and regular in outline. 



Injections also show similar changes in the deep plexus under 

 the shoulder. Although no muscle develops around this plexus, 

 various channels enlarge and many vessels disappear so that 

 the plexus occupies a less extensive area than in the embryos of 

 5 and 6 days. Connections are still present between this deep 

 plexus and the superficial lymphatics, although the lymph from 

 the surface all flows toward the posterior venous connections. 

 In the axillary region, a large vessel is still present, and from 



