DEVELOPMENT OF THE THORACIC DUCT 93 



5. Subthymic plexus, passing ventral to the thymus anlage 

 (fig. 101, 20). This forms, if retained and further developed in 

 the adult, the occasionally observed "prethymic" left innominate 

 vein '^*^\ 



Any number of good examples of all these conditions are afforded 

 by every 13 or 14 mm. embryo. Thus in a 14 mm. embryo 

 (series 210, slide viii, section 41, X 225), fig. 102, direct com- 

 parison of the schema shown in fig. 101 with the section will, 

 without further description, make the arrangement of the ele- 

 ments composing this plexus {16, 17, 18, 19, 20) clear. (Com- 

 pare also fig. 43 [series 189, slide vii, section 10, 13.5 mm.] 

 and fig. 30 [series 210, slide viii, section 39, 14 mm.]) 



The ventral divisions of this plexus extend in the earlier em- 

 bryonic stages (11-12 mm.) as a well developed capillary reticu- 

 lum into the upper thoracic region around and between oesopha- 

 gus, trachea, precardinal veins, vagi and pulmonary arteries. 



Beginning in some 12 mm. embryos, but more constantly and 

 characteristically in the 13 mm. stage, certain components of this 

 ventral mediastinal venous plexus appear partly surrounded by 

 independently developed extraintimal lymphatic spaces, which 

 form the anlages of the future broncho-mediastinal lymphatic 

 trunk. 



Thus, if the ventral portion of this venous plexus is followed 

 caudad into the upper thoracic region, into the area of the develop- 

 ing broncho-mediastinal trunk marked X in fig. 100, the following 

 observations can be made in stages of the proper length, and 

 adequately fixed and stained: 



1. In embryos between 11 and 12 mm. only venous capillaries 

 are found. Thus figures 103 and 104 show transverse sections 

 of this part of the upper thoracic region in a 12 mm. embryo 

 (series 217, slide viii, sections 21 and 29, X 200). The field, 

 caudal to oesophagus and aorta (7), contains in the middle line 

 the trachea {9), the vagi {22), and further ventrad, the pulmonary 



'* Geo. S. Huntington : ' 'Contribution to the topographical anatomy of the thorax 

 in the foetus at term and the new-born child." Rep. Soc. N. Y. Lying-in- 

 Hospital, 1897, p. 343, fig. xxxi. 



