160 ADAM M. MILLER 



that the flat cells forming their endothelial walls are differentiated 

 in situ from the mesenchymal cells are based upon prolonged and 

 thorough studies, with high magnification, of serial sections of 

 chick embryos in practically perfect states of preservation. The 

 writer is therefore forced to ally himself unequivocally with the 

 advocates of the view that the thoracic duct originates independ- 

 ently of the veins and lymph sacs. 



Studies of subsequent stages also show that the numerous 

 isolated lacunae, or rudiments of the thoracic duct, enlarge still 

 further, principally in a longitudinal direction, and coalesce with 

 one another to form a plexus of lymph channels which lies ven- 

 tral to the aorta. Other similar isolated lymphatics develop along 

 the dorso-lateral aspect of the aorta and in the region dorsal to 

 the aortic roots and arches, and then coalesce to fonii plexuses. 

 Eventually all the plexuses intercommunicate. 



In the meantime a connection is established between the large 

 ventral plexus and a branch of each jugular lymph sac known as 

 the thoracic duct 'approach,' All the components of the tho- 

 racic duct system thus drain into the jugular lymph sacs. Com- 

 munications, or taps, are established between the lymph sacs and 

 the great veins, and the thoracic duct then drains into the hemal 

 vascular system, the lymph sacs serving as portals of entry. 



In the region of the developing thoracic duct, namely, along 

 the aorta and dorsal aortic roots, and also in the region dorsal to 

 the aortic arches, a great number of blood cells arise. The gene- 

 sis of these cells is indicated by certain changes in some of the 

 irregular elements of the mesenchymal syncytium, comprising a 

 marked increase in the basophilia of the cytoplasm, a rounding 

 of the cell body and a separation from the general mesenchymal 

 reticulum. The resulting cells thus lie free in the interstitial 

 spaces, and structurally are similar to the large mononuclear 

 cells (lymphocytes of Dantschakoff) in the area vasculosa of the 

 blastoderm. They increase in number both by mitosis and by 

 constant differentiation from the mesenchymal syncytium. 



Many, at least, of these basophilic cells are transformed 

 into erythrocytes through the addition of hemoglobin to the 

 cytoplasm and certain nuclear modifications comprising the 



