DEVELOPMENT OF THE THORACIC DUCT 57 



which the majority of embryos conform and into which even the 

 aberrant types fit. As a matter of fact, the regularity with which 

 the thoracic duct anlages first appear in the majority of the 

 individuals of the proper stage, and the uniform and constant 

 character of the resulting histological pictures, is one of the most 

 striking and significant features of the entire process. Prior to 

 the 11 mm. stage no definite anlage of the anterior segments of 

 the future ducts is observable. 



I believe that the adult thoracic ducts of the cat are developed 

 by fusion of three distinct and separate regional segments. Each 

 of these segments is in turn formed by confluence of a number of 

 originally discrete and independent anlages, which develop inde- 

 pendently of the venous system as extra-intimal or perivenous 

 mesenchymal spaces in the sense previously defined. (l> 2> 21> 

 22) - These spaces are applied to, or surround, the walls of 

 the embryonic veins of the lower cervical and of the dorsal 

 mediastinal region. The three main divisions, thus developed 

 independently of the venous system, unite with each other to 

 form the channels of the left and right thoracic ducts, and these 

 usually gain their point of entrance into the systemic veins by 

 uniting with a process of the jugular lymph sacs (" thoracic duct 

 approach") derived from their dorsal aspect, just cephalad to 

 the common jugular approach. Exceptionally the main (left) 

 thoracic duct, by combining with the ventral mediastinal trunk 

 presently to be described, unites (fig. 98) with the ventral process 

 of the "subclavian approach," prolonged from the jugular lymph 

 sac over the ventral surface of the jugulo-subclavian confluence, 

 and thus establishes its connection with the venous system (cf. 

 pp. 84 89, figs. 89 to 99). 



The ontogenetic history of the ducts may therefore (fig. 29) 

 be considered under four headings, viz.: 



1. THE THORACIC DUCT APPROACH of the jugular lymph sac, 

 forming the terminal of the adult duct on each side (fig. 29, 1, blue) . 



2. THE PREAZYGOS SEGMENT (fig. 29, II). This includes two 

 distinct and separate channels: 



A. The broncho-mediastinal or ventral mediastinal lymphatic 

 trunk (fig. 29, 37, green), which drains the ventral mediastinum 



