DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 313 



5. Entrance of these developing blood cells, after resolution 

 of the islands, into the above described spaces and channels^ 

 and the incorporation of the free cells in the haemal circulation. 



B. Development of the non-haemophoric systemic lymphatic vessels. 



The development of intercellular mesenchymal spaces, their 

 confluence to form a connected system of channels, the modifi- 

 cation of the mesodermal parietal cells as lymphatic endothelium, 

 and the establishment of secondary connections between this 

 lymphatic system and the veins. All these genetic processes 

 are in principle identical with the primary stages of the blood- 

 vascular development, prior to the inclusion of the free blood 

 cells in the haemal channels {A, 1, 2 and 3). 



C. Development of the haemophoric or veno-lymphatic portion of 



the lymphatic system 



This proceeds on lines identical with the haemal vascular 

 development up to the point where the free blood cells are added 

 to the circulating venous stream. 



After this evacuation the early haemophoric lymphatic chan- 

 nels or sacs revert to the lymphatic system proper in whole or 

 in part. The distribution of the haemophoric division of the 

 general Ijonphatic system varies in the three amniote classes. 

 In the mammalian and reptilian embryo the haemophoric lym- 

 phatic area of the anterior portion of the body is chiefly repre- 

 sented by the elements of the jugular lymphsac and by the 

 primitive lymphatic drainage of the anterior limb-bud (primitive 

 ulnar lymphatic). 



In the avian embryo, addition to the jugular lymphsac, the 

 entire axial lymphatic line of the thoracic ducts is haemophoric 

 in the primitive stages. 



In all three classes the jugular lymphsacs, developed from 

 early haemophoric lymphatic anlages, serve, after performing 

 this function, as the anterior links or junctional segments between 

 the veins and the vastly more extensive complex of the non- 



