DEVELOPMENT OF THE JUGULAR LYMPH SAC 



483 



Fig. 5 From a photograph of a transvere section through the upper thoracic 

 region of a chick embryo of six days. 1, Precardinal vein; ^a, vertebral vein; 

 4h, branch of vertebral vein; 5, veno-lymphatic (prelymphatic) plexus; 6, aorta; 

 7, oesophagus; 8, notochord; 9, spinal nerve; 9h, vagus nerve; 10, bronchi; 11, pul- 

 monary artery; 12, vertebral artery. 



This relatively earlier loss of communication between the veno- 

 lymphatics and the venous system in the chick is probably due 

 to different rates of metabolism in the two classes of animals. 

 Following the dissolution of the connection between veno-lym- 

 phatics and veins, the plexus of channels in question may now be 

 termed prelymphatics, in accordance with the usage of Hunting- 

 ton and McClure. 



The evolution of the complicated and extensive veno-lymphatic 

 plexus of the six day chick (figs. 4 and 5) from the simpler dorso- 

 lateral group of venous channels and islands of the 5-day-and- 10- 

 hour embryo (fig. 3) is shown to occur in intervening stages by 

 progressive growth in size and number of the vascular elements 

 involved. The dorso-lateral venous channels of the earlier stages 



