The Lymphatic Sy&tcm in Human Embryos. 69 



from the valve, which is in the angle between the cephalic and internal 

 jugular veins, it extends 1.2 mm, along the jugular vein. By a 

 comparison with the reconstruction of the preceding stage, Fig. 4, 

 I think that the sections shown in Fig. 7 and 8 suggest that the sac 

 is extending along the jugular vein by means of a plexus of veins. 



The next point of interest is the location of the valve. In Figs. 

 7 and 8, it will be seen that the beginning cephalic vein is easily 

 recognized by its position opposite the upper curve of the arm bud. 

 The lymphatic sac runs deep into the angle between the cephalic 

 and the internal jugular veins. Fig. 7, but in no section is there 

 any break in the endothelium of the sac, which leads one to think 

 that the valve may not yet be open and that this fact may account 

 for the complete filling of the lymphatic sacs with blood. The in- 

 ternal jugular vein is only partially filled with blood. The blood 

 of the vein itself was omitted in the drawing. 



In this embryo there is an extension of the jugular sac along the 

 primitive ulnar and lateral thoracic veins. This extension forms 

 the subclavian sac which gives rise to the lymphatics of the arm. 

 Fig. 9. This is especially interesting in connection with F. T. 

 Lewis's observations on the subclavian sac in rabbits where it begins 

 as an isolated sac. I was able to confirm Lewis' observations on 

 his specimens of rabbit embryos, but feel sure that in human embryos 

 the sac in the arm bud is an extension of the jugular sac. The sac 

 along the ulnar veins measures .8 mm. beginning from the valve. 

 This makes 2 mm. the total extent of the lymphatics in this embryo. 



By putting together Figs. 7 and 8, relating them by the position 

 of the cephalic veins, it will be noted that following along the ex- 

 ternal border of the internal jugular vein there are a series of 

 branches which we might call segmental, some of them, as for 

 example above the lymphatics in Fig. 7 or between the lymphatics 

 and the primitive ulnar vein in Fig. 8, are obviously small veins, 

 others like the cephalic and primitive ulnar are large veins, while still 

 others are being transformed into lymphatics. This suggests the 

 process of transformation of the various branches of the internal 

 jugular vein from the original simple segmental type into the adult 

 system. In these transformations some of the branches become 



