164 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



majority of the spaces develop, as above described, extraintimally 

 along or around avenule,but at the same time a considerable num- 

 ber of these spaces develop as independent mesenchymal clefts 

 not directly related to a degenerating embryonic vein. The em- 

 bryonic history of this portion of the mammalian lymphatic sys- 

 tem, up to the junction of the completed preazygos segments of 

 the thoracic ducts, including their ventral mediastinal tributary, 

 with the thoracic duct approaches of the left and right jugular 

 lymph sacs, is, as above stated, clearly shown in cat embryos of 

 between 13 and 15.5 mm. and need not be repeated here. 



Comparison of the 13 and 14 mm. stages with slightly older 

 embryos (15 mm.) definitely proves the correctness of the view 

 just expressed. In the course of further development the thoracic 

 duct approach, turning ventro-mesad into the neuro-venous inter- 

 val between sympathetic nerve and common jugular vein in 

 proceeding latero-mesad across the dorsal aspect of the latter 

 vessel, necessarily intersects the line of the dorso-medial venous 

 tributaries. Its blind terminal must exchange the position which 

 it occupies in the earlier (13-14 mm.) stages, lateral to the dorsal 

 somatic venous branch, for one situated on its mesal aspect, if 

 the vessels are functional venous tributaries and not, as has been 

 held, venous anlages of the future thoracic duct in this region. In 

 other words, they should continue in the later stages side by side 

 with the thoracic duct more fully developed, if they have not be- 

 come detached from the main vein to form by fusion the lymphatic 

 channel. This is seen to be the case, for example, in the sections 

 of the 15 mm. cat embryo (series 245) shown in figs. 79, 80, 81 and 

 82. The thoracic duct approach (12} here has extended further 

 meso-ventrad into the recess between sympathetic nerve (1) and 

 common jugular vein (26) and now lies on the mesal aspect of the 

 ventro-medial venous tributary (16) which opens into the mam 

 vein in fig. 82. Fig. 90 shows clearly in the dorsal view of the 

 reconstruction of this embryo the lymphatic channel (12-35) 

 intersecting at an acute angle the coexisting dorsomedial tributary 

 (16-46). 



