VEINS AND LYMPHATICS IN TRAGULUS 205 



directly with the veins but proceeds caudad to form three other 

 processes, namely, the lateral process or subcutaneous duct, 

 the dorsal descending process or thoracic duct approach (fig. 

 7, 26a) and the ventral descending or broncho-mediastinal ap- 

 proach. The ventral and dorsal descending processes become 

 divergent immediately above the cephalic vein; the ventral proc- 

 ess passes caudad on the ventro-lateral aspect of the jugular 

 vein to join the broncho-mediatinal systemic channels on the 

 right side. On the left this connection is not completed. The 

 dorsal descending process extends a short distance caudad, dorso- 

 lateral to the thyreo-cervical artery to end blindly. 



Organization of the systemic lymphatic drainage lines. The 

 evidence concerning the development of the lymph sac in Tra- 

 gulus shows that this structure is derived from the venous system. 

 In its early stages the sac is wholly independent of the systemic 

 lymphatics. Subsequently it joins with the systemic lymphatic 

 channels, and thus serves as the connecting link between these 

 channels and the venous system. In the material studied it 

 was impossible to discern any genetic relation between the devel- 

 oping systemic lymphatics and the veins. Such regions as gave 

 the earliest pictures of the organization of the systemic lym- 

 phatics revealed these elements as independent mesenchymal 

 spaces, at first presenting the form of a plexus. By a process 

 of expansion and confluence this plexus comes to form definite 

 channels. In the 20 mm. embryo development has advanced 

 too far to offer anything that is conclusive as to the actual histo- 

 genesis of these lymph spaces. The process referred to as con- 

 fluence and expansion is not carried on with the same degree 

 of rapidity or effectiveness in all regions of the embryo; it confines 

 its activities to certain selected districts which remain, for some 

 time, separate. The systemic lymphatic channels arise from 

 three major segments which, because of their relations to the 

 venous system, may be termed the azygos, pre-azygos and post- 

 azygos segments, corresponding to similar segments in the cat 

 as recently described by Huntington ('10). 



1. The azygos segment {jig. 7, 28). It is in this segment that 

 the systemic lymphatic channels have attained their greatest 

 development. Longitudinally the segment reaches caudad from 



