DEVELOPMENT OF THE THORACIC DUCT 91 



The Development of the Broncho-Mediastinal Trunk and of the 

 Preazygos Segment of the Thoracic Duct 



The area in which the development of the broncho-mediastinal 

 and preazygos segments of the thoracic duct occurs is shown in 

 fig. 100, which gives a topographical view of a transection of the 

 upper thoracic region in a 12 mm. embryo (series 78, slide IX, 

 section 5, X 50). 



The section includes the entire coelom ventrally, and the 

 structures between it and the notochord dorsally. The right (3) 

 and left (6} precaval veins occupy the lateral limits of the dorsal 

 region. In the interval between them are seen the oesophagus 

 (8), the trachea (9), the vagi (22) and the dorsal aortic arches 

 (7). Dorsal to the main veins on each side are the strands of the 

 brachial plexus (not labelled in the figure), and dorsal to the 

 nerves is the primitive ulnar veno-lymphatic (SI). Further 

 mesad follow the subclavian arteries (33), the entrance of the 

 dorso-medial venous tributaries (16) into the main veins, and the 

 sympathetic strands (1). The letter Y in the figure, between 

 the notochord and the dorsal mediastinal structures named, 

 indicates the center of the area in which the development of the 

 preazygos segment of the thoracic duct will proceed in the suc- 

 ceeding stages. 



The ventral portion of the field contains the coelom, with right 

 (48) and left (49) auricles, right ventricle (50), ascending aorta (7) 

 and ascending trunk of main pulmonary artery (10): Dorsal to 

 this are seen the cross-sections of the right and left pulmonary 

 arteries (10) descending to the lungs. Between them ventrally, 

 and trachea (9), aortae (7), vagi (22) and praecaval veins (3, 6) 

 dorsally, is the area marked by the letter X, in which the develop- 

 ment of the broncho-mediastinal trunk will proceed. 



A . The development of the broncho-mediastinal or ventral medias- 

 tinal lymphatic channels, which drain subsequently caudo-sinistrad 

 into the left thoracic duct, or cephalo-sinistrad into the subclavian 

 approach of the jugular lymph sac, or into both. 



In the earlier stages (embryos of 11-14 mm.), an extensive 

 ventral venous capillary network develops between the main 



