The Lymphatic System in Human Embryos. 



73 



Fig. 10. It is in exactly the position found in the embryo 11 mm. 

 long (Fig. 7) and in No. 317 which is 12.5 mm., which are cut in 

 coronal sections, but in the two earlier embryos I could not make 

 out the break in the endothelium. The extent of the lymph sac in 

 the section is 1.5 mm. and there is a slight extension along the prim- 

 itive ulnar vein. 



The next embryo of the series (No. 74) measures 16 mm. In 

 Dr. Mall's catalogue it is placed after those measuring 17 mm., for 



Fig. 10. Coronal section through the arm bud of a human embryo, 17 mm. 

 long, Mall collection, No. 296, to show the open valve of the jugular lymph 

 sac in relation to the veins, x about 26. S. 1. j., saccus lymphaticus jugu- 

 laris; V. c, vena cephalica ; Y. j. i., vena jugularis interna; V. u. (p.). vena 

 ulnaris (primitiva). 



it is undoubtedly further developed. This embryo, in which the sec- 

 tions are 50 microns thick, is a very satisfactory one for determining 

 the sacs, for the veins are unusually distended with blood and the 

 lymph sacs are filled with a serum w^hich takes a definite stain. 

 The sacs extend a distance of 1.8 mm. along the internal jugular 

 vein. There is no sac on the primitive ulnar vein and there are 



