DEVELOPMENT OF THE THORACIC DUCT 109 



recent detailed investigations of lymphatic development in birds 

 and reptiles have fully sustained the interpretation given in the 

 publication quoted. 



b. The darkly stained mass of cells seen in the interval between 

 16 and 17 in both sections is the result of obliteration of early 

 embryonic vascular channels, which have lost their connection 

 with the vessels, and whose endothelial walls are apparently in 

 the process of reverting to indifferent mesodermal cells. Many 

 of these degenerated vascular aggregations are formed in embryos 

 between 13 and 15 mm. crown-rump measure. 



Some of the elements of this early embryonic prevertebral 

 venous plexus are secondarily replaced by perivenous or extrainti- 

 mal lymphatic spaces in exactly the same way as is observed in 

 the development of the ventral mediastinal duct. The resulting, 

 originally separate, extraintimal lymphatic anlages, having re- 

 placed the venule along and around which- they primarily devel- 

 oped, unite with each other and form the preazygos segment 

 of the thoracic duct, between the thoracic duct approach of the 

 jugular lymph sac and the level of the aortic arch, at which the 

 azygos portion of the thoracic duct begins. 



Fig. 162 shows a transverse section of the left side of the upper 

 thoracic region in a 14 mm. cat embryo (series 210, slide IX, 

 section 26, X 200). The prevertebral tributary plexus (17) turns 

 mesad of the sympathetic nerve (1) to enter the dorso-medial 

 angle of the left innominate vein (40). Dorsad of the main vein 

 is the ascending portion of the left subclavian artery (33). Be- 

 tween aorta (7) and innominate vein (40) are 'other elements of 

 the mediastinal venous plexus, in the interval between the sympa- 

 thetic nerve (1) and the vagus (22), with ventral branches (32) in 

 front of the latter. 



Between the oesophagus and the prevertebral plexus (17) is 

 an area in which venous elements (4), formerly connected with 

 the same, are undergoing replacement by the extraintimal lym- 

 phatic anlages (5) of the preazygos segment of the thoracic duct. 

 These structures cannot be distinctly made out in fig. 162, but 

 they are shown in a higher magnification in detail in fig. 163, 

 which includes the region of the same section dorsal to the oesoph- 



