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Florence R. Sabin. 



shows, one further point of interest, namely a possible beginning of 

 the thoracic duct in the shape of a duct running over toward the 

 aorta as shown in Fig. 6. The entire question of a thoracic duct 

 will be discussed later, on page 77. 



The next embryo of the series (No. 353) is 11 mm. in length. 

 This embryo is cut in coronal sections which proves to be the best 

 plain not only for seeing the valves but for understanding all of 

 the cervical lymphatics. This embryo is represented in a series of 

 three figures, 7, 8 and 9, two of them sections and the third a diagram 



Fig. 6. Transverse section through the left jugular sac to show the possible 

 beginning of a vessel growing down to form the upper part of the thoracic 

 duct. X 40. A., aorta; N. X., N. vagus; Oe., oesophagus; S. 1. j., saccus 

 lymphaticus jugularis ; V. j. i., vena jugularis interna. 



from the same series. In Figs. 7 and 8 will be seen the extension 

 of the lymphatic plexus along the external border of the jugular 

 vein. These two figures show a number of important things, first 

 in connection with the veins, they show the relations of the primitive 

 ulnar and the cephalic to the jugular vein; for the lymphatics they 

 show the relation of the lymphatics to the cephalic vein and in general 

 to the arm bud. These relations are all brought together in the 

 diagram of Fig. 9. Here it will be seen that this plexus which 

 appears isolated in Fig. 7 is really continuous. The plexus is 

 actually much more complex than is shown in Fig. 9. Measuring 



