458 



ON THE ORIGIN AND EARLY DEVELOPMENT 



by microscopic sections, that the early lymphatic system in chicks of this stage is 

 connected in certain regions with the venous system. No valves are present at 

 this time. Although there is evidence from study of parallel sections that an 

 absorptive function has already started in these vessels, such function is compara- 

 tively slight and not sufficient to overcome the lateral pressure of the veins. That 

 the blood enters the lymphatic system from the veins can be demonstrated in a 

 number of ways. 



(1) The amount of blood present in the superficial lymphatic system can be 

 increased in the living chick by changing the position of the embryo so as to allow 

 the force of gravity to affect its various parts. For example, the posterior lym- 



Area shown in figure 3. 



FIG. 2. Chick of 5 days 7\ 

 hours, measuring 14.5 mm. 

 before fixation. The lymph- 

 he.irt plexus was injected with 

 India ink. The small square 

 marked Vindicates the region 

 reconstructed and shown in 

 figure 3. X 3.7. 



FIG. 3. Drawing of a portion of 

 the superficial plexus from the 

 region beyond the injection 

 in the same embryo shown in 

 figure 2 (the region marked 

 with a small square). The 

 plexus was reconstructed 

 with the aid of the oil-im- 

 mersion lens from sections 

 cut parallel to the surface. 

 X indicates mitotie figure in 

 lymphatic endothelial cells. 

 X 300. (Magnification of 

 original drawing, 1 to 800.) 



phatics, which spread out over the surface of the tail and over the posterior border 

 of the pelvis, lie, for the most part, anterior to the coccygeal veins with which they 

 are connected. By pulling on shreds of the amnion the tail can be raised so that 

 this region is uppermost, and the embryo can be held in this position for some min- 



