64 



Florence K. Sabin. 



diagrammatic form in Fig. 1, there is a group of vessels lying 

 external to the internal jugular vein near its junction with the 

 primitive ulnar vein. The figure can be interpreted by reference 

 to Fig. 4. These vessels are completely filled with blood and yet I 

 cannot find any openings from them into the veins, or into each other, 

 and thus interpret them after a study of corresponding stages in 

 other mammalian embryos as a plexus of veins which have separated 

 from the jugular vein preparatory to the formation of the anterior or 

 jug-ular lymph sac. These vessels are small, the largest measuring 



-V.u.(p.) 



Fig. 1. Reconstruction of the plexus of small veins lateral to the V. 

 jugularis interna in a human embrj^o, 8 mm. long, (crown rump). Mall 

 collection, No. 397. X about 50. The plexus of veins is shaded. D. C, 

 ductus Cuvier ; V. j. i., vena jugularis interna ; V. u. p., vena ulnaris primitiva. 



Fig. 2. Reconstruction of the small veins lateral to the right V. jugularis 

 interna in a human embryo 9 mm. long. Mall collection. No. 163. X about 50. 

 Of the veins, six are shaded, indicating that they are full of blood, while 

 the two with heavy outlines are nearly empty. Lettering same as Fig. 10. 



approximately .3 mm. in the antero-posterior diameter, by .19 mm. 

 laterally. 



The next specimen in the series is an embryo (E'o. 163) measuring 

 9 mm. This specimen shows a similar condition but with certain 

 differences. In the first place the plexus of isolated vessels occupies 

 a slightly different place, as seen in Fig. 2. They lie farther ventral- 

 Avard, extending over the body wall external to the heart. Most of 

 these vessels are well filled with blood, while two are nearly empty. 

 This embryo differs also in having an asymmetrical development, the 



