SUMMARY AND CONCLUSIONS 167 



sections in successive stages which will present approximately the 

 same topographical picture, allowing for the necessary changes 

 consequent on increasing development. The area to which I thus 

 desire to call detailed attention is the mesenchymal tissue, situated 

 on the right side of the thoracic aorta, between this vessel and the 

 mesal aspect of the right azygos vein, ventral to one of the right 

 dorsal somatic (intercostal) branches of the aorta, and under cover 

 of the dorsal surface of the oesophagus. 



These limitations, as is well known, define the position of this 

 portion of the main thoracic duct of the adult, and consequently 

 the development of this structure, however produced, must run 

 its course in this situation. The successive stages, which in my 

 opinion definitely and conclusively prove the independent extra- 

 intimal anlage of this portion of the right thoracic duct, may be 

 grouped as follows, and referred to selected illustrations above de- 

 scribed in the body of the text in detail : 



A. Venous stage. (Embryos from 10 mm. to 13 mm.). Illus- 

 trations in body of text: figs. 194 to 203. 



The sections show an extensive ventral and ventromedial azy- 

 gos tributary plexus. 



B Stage of replacement of ventral azygos venous plexus by inde- 

 pendently developed perivenous or extraintimal lymphatic anlages 

 which accompany and surround the branches of the antecedent 

 venous network, and form the first anlages of the azygos segment of 

 the thoracic ducts. (Embryos from 13 to 14 mm.) Illustrations 

 in body of text: figs. 204 to 213; 247 to 251. 



1. The earliest anlages of the future thoracic ducts are seen in 

 certain sections of the thoracic region in embryos between 12 and 

 13 mm. crown-rump measure. I have found these early traces 

 almost exclusively on the right side, the sinistral position of the 

 aorta appearing to delay their development on the left side. They 

 make their first appearance as delicate endothelial-lined extremely 

 sparse and widely separated independent mesenchymal clefts, 

 at some distance ventral to the main right azygos trunk and near 

 the right lateral wall of the aorta. 



At first they are not clearly related to the co-existing ventro- 

 medial tributaries of the azygos. But they soon extend to involve 



