312 GEO. S. HUNTINGTON 



imposes on the developing thoracic ducts the additional func- 

 tion of conveying the newly developed blood cells to the venous 

 circulation. 



It will thus be seen that the development of the thoracic ducts 

 in all three amniote classes follows precisely the same main 

 fundamental principle, viz., the formation of lymphatic channels 

 by confluence of numerous originally separate intercellular mesen- 

 chymal clefts and spaces. 



The reptile presents this genetic process in its simplest form, 

 in a region in which systemic venous development is reduced to 

 a minimum. In both the bird and the mammal the development 

 of the thoracic duct becomes complicated by a direct or indirect 

 relation of the lymphatic anlages to the adjacent elements of the 

 haemal (venous) vascular system. In the bird the relation of 

 the lymphatic anlages to the haemopoetic axial mesenchyme is 

 direct, the avian thoracic ducts becoming for a time functionally 

 haemophoric. 



In the mammalian embryo the relation of the developing 

 thoracic ducts to the axial venous system is indirect, the lym- 

 phatic anlages replacing the temporary ventro-medial haemo- 

 phoric azygos tributaries topographically, but never themselves 

 assuming the haemophoric function. 



In all three classes of amniote embryos the final result of the 

 genetic processes outlined above is the same, viz., the establish- 

 ment of a periaortic, or paraaortic, lymphatic channel, the amni- 

 ote thoracic duct. In a wider interpretation it becomes evident 

 that all the diversified phenomena of vascular ontogeny, haemal 

 as well as lymphatic, are focussed in the small field which any 

 extra-embryonic vascular area presents. These phenomena 

 comprise : 



A. Blood-vascular development 



1. Development of intercellular mesenchymal spaces. 



2. Confluence of the same to form connected channels. 



3. Modification of the mesodermal cell lining the spaces and 

 channels as the vascular endothelial cell. 



4. Aggregation of mesenchymal cells into vascular strands and 

 islands in which active haemopoesis takes place. 



