DEVELOPMENT OF THE THORACIC DUCT 97 



components. On the right side the corresponding structures end 

 blindly. 



In the following section (41, fig. 116) the left venule and lym- 

 phatic also terminate almost entirely in the indifferent mesoder- 

 mal tissue, only a small remnant (4, 5) persisting between trachea 

 and left vagus. This disappears entirely in the following section. 



The succeeding stage (14 mm.) shows the same lymphatic 

 anlages surrounding and replacing the atrophying venules of the 

 ventral mediastinal plexus in still higher development. It ap- 

 pears from the study of numerous embryos of this length, that 

 the average 14 mm. stage represents the point in the lymphatic 

 ontogenesis of this region at which the pictures are most strik- 

 ing. The central kernel of the decadent vein is still large and, 

 in places, as yet incompletely separated from the definite venous 

 channels. At the same time the perivenous lymphatic space has 

 acquired an increased lumen and hence the two combined struc- 

 tures occupy a relatively large field. 



Thus figs. 117 to 122 show transverse sections of the ventral 

 thoracic region in a 14 mm. cat embryo (series 212, slide x, sec- 

 tions 4, 5, 6, 7, 10 and 11, X 225). In figs. 117 and 118 the 

 developing lymphatic anlages (5) of the broncho-mediastinal 

 trunk can fce readily recognized and present the same relation 

 to the atrophying venules, which they surround, as in the sections 

 of the preceding series just described and figured. In contra- 

 distinction to these structures the components of the permanent 

 functional venous plexus of this region (32) form well-differen- 

 tiated vascular channels. 



In fig. 119 (series 212, slide x, section 6) the scattered lym- 

 phatic anlages occupying in the two preceding sections the space 

 between the left pulmonary artery (10) and the left vagus (22), 

 with its accompanying ventral mediastinal vein (32), are gathered 

 together into a more extensive elongated space (5). In the 

 interior of this is seen the endothelial bag of the empty and 

 atrophying venule (4) which the lymphatic is about to replace. 

 The vein remnant is almost completely surrounded by the lym- 

 phatic space and only connected to the latter's endothelial wall 

 by a bridge at one point. Other areas of similar lymphatic devel- 



