DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 119 



A comparison of the series just studied (series 143) with another 

 15.5 mm. embryo (series 141) shows identical conditions. Thus 

 figs. 184 and 185 show transverse sections through the upper part 

 of the left thoracic region of the embryo. In fig. 184 the preazy- 

 gos segment of the thoracic duct and the broncho-mediastinal 

 trunk (38) are approaching each other in the interval between the 

 innominate vein (50} and the subclavian artery (33} laterad, and 

 trachea (9} and oesophagus (8} mesad. This quite agrees with 

 the conditions seen in series 143, in figures 172 amd 173 (sections 

 16 and 14 of slide xi). In fig. 185 (slide x, section 21 of series 

 141) the two lymphatic anlages have joined (38), repeating practi- 

 cally conditions which series 143 shows in sections 11 and 12 of 

 slide xi, figs. 174 and 175. The permanent functional peri- 

 tracheal venous plexus (18) appears clearly differentiated from the 

 lymphatic channels in both sections. 



The earlier stages in the development of this junctional area 

 just studied in the 15.5 mm. stages (series 143 and 141) are found 

 in a 13.5 mm. cat embryo, series 223. Two sections of the left upper 

 thoracic region are shown in figs. 186 and 187 (series 223, slide 

 viii, sections 39 and 40, X 225). The sections pass through the 

 left innominate vein (40), the left subclavian artery (33), at the 

 level of origin of the vertebral (41) and the costocervical arteries 

 (43), and the innominate artery (45}. 



Both sections show the radicles of the peritracheal mediastinal 

 venous plexus, and the beginning replacement of certain elements 

 of the same (4) by extraintimal perivenous lymphatic spaces (5), 

 which form the anlage of the future junctional channel between the 

 broncho-mediastinal lymphatic trunk and the preazygos segment 

 of the thoracic duct. These spaces are situated dorsad to the aorta 

 and the innominate artery (45), in the interval between the tra- 

 chea (9) mesad, the oesophagus (8) dorso-mesad, and the inno- 

 minate vein (40) and sympathetic nerve (/) laterad. 



The developing perivenous lymphatic spaces seen here in this 

 earlier 13.5 mm. embryo correspond to the finished lymphatic 

 channel segments of the later 15.5 mm. stages, in which the pre- 

 azygos segment of the thoracic duct meets and joins with the inde- 

 pendently developed broncho-mediastinal trunk, as just seen in 

 series 14,3 and 141 (figs. 172 to 185). 



