68 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



dorso-medial angle of the common jugular trunk. The thoracic 

 duct approach (12), occupying the same relative position as in the 

 preceding figure, lies between this tributary and the thyro-cer- 

 vical artery, dorsal to the main vein. Section 12 of slide x (fig. 

 61) shows the distal reduced end of the approach in this same 

 situation. The subclavian approach of the jugular lymph sac 

 (14) is reduced to a narrow longitudinal cleft on the dorso-lateral 

 aspect of the main vein. This reduction is coincident with the 

 derivation from the dorsal aspect of the subclavian approach of 

 the primitive ulnar veno-lymphatic (31). This channel arches 

 dorsad over the Vlth spinal nerve (30), and appears in this sec- 

 tion in its typical relation to this nerve. Finally in the succeed- 

 ing section, 13 of slide ix, (fig. 62), the thoracic duct approach 

 ends blindly. The structures shown in the section dorsal to 

 the common jugular trunk are now encountered in the following 

 order: mesally the terminal of one of the branches of the dorso- 

 medial venous plexus (16); next the thyro-cervical artery (24); 

 then the Vlth segmental nerve (30), and, dorsal of this, the 

 primitive ulnar veno-lymphatic (31). 



The series of sections of this embryo shown here are interest- 

 ing not only in respect to the behavior of the thoracic duct 

 approach, but also in reference to the mutual relations of this 

 structure and the terminals of the dorso-medial tributary venous 

 plexus. These relations are to be discussed subsequently in con- 

 sidering, as whole, the evidence upon which some investigators 

 have based their conception of direct venous origin of the lym- 

 phatic system by fusion of a number of separate elements de- 

 tached from the main venous trunks (cf. infra p. 153.) 



The variable appearance of the thoracic duct approach in the 

 younger stages is perhaps best illustrated by the exaggerated 

 development presented in a 13 mm. embryo (series 92), in which 

 the process occupies the last thirteen sections of slide x, and the 

 first fourteen sections of slide ix. Selected microphotographs 

 of this series are shown in figs. 63 to 78. The embryo presents 

 the largest and most dilated thoracic duct approach of any in my 

 collection. Fig. 63 gives a topographical view of the region 

 involved. The succeeding sections are cut down and confined 



