THORACIC DUCT IN THE CHICK 137 



III. HISTOGENESIS 



Sala, in his account of the development of the thoracic duct in 

 the chick (10), states that the anlagen of this lymph channel 

 appear as isolated mesenchymal spaces which become clothed 

 with endothelial cells derived from the mesenchyme, and which 

 subsequently coalesce to form continuous vessels. He thus 

 implies a denial that the lymphatics in this region arise from 

 veins and that the lymphatic endothelium is derived from hemal 

 vascular endothelium. He also calls attention to accumulations 

 of mesenchymal cells which contrast clearly with the surrounding 

 tissue. They appear to consist in large part of elements ex- 

 hibiting all the characteristics of young connective tissue cells, 

 with roundish forms, no processes and large intensely colorable 

 nuclei. Among these elements appear red corpuscles in larger or 

 smaller numbers. The accumulations or clumps of cells develop 

 first on the mesial aspect of the superior vena cava, and then 

 extend caudad to the level of the celiac artery. Without describing 

 the histogenesis of these masses of cells, Sala states further that 

 within them in large part the anlagen of the thoracic duct are 

 excavated or 'hollowed out.' The details of the 'hollowing' process 

 are not given. 



In the main I can confirm the results of Sala's observations so 

 far as he has carried them. As for the isolated mesenchymal 

 spaces which he describes as the anlagen of the thoracic duct 

 my study of their histogenesis in the closely graded series of chick 

 embryos leads to the conclusion that they do arise independently 

 of the veins and that their endothelial lining is derived from the 

 indifferent mesenchymal cells bordering upon them. The details 

 of the process I shall attempt to demonstrate in the following 

 pages. After tracing the development of the accumulations of 

 cells in the mesenchyme and their subsequent history in relation 

 to the developing thoracic duct, it seems to the writer that a 

 different and greater significance can be attached to them than 

 was given by Sala. Since the cells composing them correspond 

 so closely in their history to the developing blood cells as described 

 by Dantschakoff (17) in the extraembryonic area of the chick, 



