The Lymphatic System in Human Embryos. 



67 



valves, namely the coronal. This will be readily seen in Fig. 10, 

 which shows that the valve is made by a long projection of the 

 lymphatic duct into the angle of the internal jugular vein with the 

 cephalic vein. Imaginary cross sections through this figure will show 

 that the place of the valve would be represented by a small duct in 

 the angle between two veins and this is exactly what is seen in Fig. 

 5 for this embryo. A study of Fig. 10 will also show that there 

 could be nothing distinctive of the actual opening of the lymphatic 

 to the vein in cross sections, for they would consist simply of a 

 double layer of endothelium between the veins. In like manner 

 sagittal sections are still more difficult than transverse ones for locat- 

 ing the valves and indeed only in an occasional, fortunate section can 

 it be accurately done. 



A. N Valv/e 



Fig. 5. Transverse section through the jugular sacs of a human embryo, 

 10.5 mm. long. Mall collection, No. 109, to show the left valve, x 28. A., 

 artery ; N., nerve ; Oe., oesophagus ; P., pericardium ; S. 1, j., vena jugularis ; 

 V. j. i., vena jugularis interna. 



To return to the embryo 10.5 mm. long, I think that the valve is 

 present, for, as is seen in Fig. 5, there is a small duct in the angle 

 between two veins and the duct connects with the sac as traced in 

 serial sections. Secondly, the lateral vein in this section is the 

 cephalic, see Fig. 4, and is therefore in the exact position of the 

 undoubted valves seen later in Figs. 10 and 13. Whether this valve 

 is open or not it is impossible to say. The sacs are both empty, and 

 in the earlier stages where there are no valves they are often, though 

 not always, full of blood. They measure .7 x .28 mm., showing a con- 

 siderable increase over the two preceding specimens. This embryo 



