THORACIC DUCT DEVELOPMENT IN THE PIG 463 



to shrink into seemingly isolated segments so that they can be 

 pursued only with great difficulty, as described by Clark, but he 

 does deny, supported by the decisive evidence of the injected 

 series 23a and reinforced by all of the transition stages, that the 

 discontinuous anlagen observed by him and invariably found to 

 be concomitants in the formation of a large lymphatic trunk like 

 the thoracic duct are artifacts, produced by the preserving or 

 fixing reagents. 



V. RESUME OF OBSERVATIONS AND CONCLUSIONS 



1. Derived from the supracardinal or azygos system of veins, 

 a series of venous channels, called veno-lymphatics, are formed 

 in the pathway finally occupied by the thoracic duct, and at the 

 culmination of their development they exist as plexuses of vessels 

 abundantly connected with the parent veins. 



2. The actual genesis of the thoracic duct is initiated by the 

 appearance of blind mesenchymal lymphatic spaces either around 

 or not immediately in contact with the venous derivatives, or 

 veno-lymphatics, which become detached from their venous 

 trunks and break up into degenerating segments. The lympha- 

 tic spaces or anlagen arise by the local disintegration of the fibrils 

 of the tissue reticulum and the fusion of the interstitial lacunae, 

 and they enlarge and elongate in a similar manner. If they are 

 of the nature of extra-intimal spaces the endothelium of the evan- 

 escent abandoned veno-lymphatics, which they replace, collapses 

 as the result perhaps of the increasing influence of the lymph 

 pressure on its external surface after its release from the blood 

 pressure. During their inception and growth the walls of the 

 discontinuous thoracic duct anlagen are composed of the ordinary 

 unmodified mesenchymal cells. That such lymphatic anlagen 

 are not artifacts is shown by their definite position and period of 

 formation and the determinate sequence from their first appear- 

 ance as mesenchymal vacuoles, through the phase of their growth 

 and elongation, to their final fusion into a continuous channel. 

 In the production of the most posterior portion of the thoracic 

 duct, or cisterna chyli, veno-lymphatic channels by fusion with 



