THORACIC DUCT IN THE CHICK 159 



V. SUMMARY 



Prior to the appearance of lymphatics in the region of the future 

 thoracic duct, namely, along the aorta and dorsal aortic roots, 

 the mesenchyme comprises a syncytium of irregular strands with 

 correspondingly irregular interstitial spaces. The tissue is non- 

 vascular. 



The initial change in conditions is manifested in the appear- 

 ance of distinct lacunae in the mesenchymal tissue along the 

 ventro-lateral aspect of the aorta at the level of the celiac artery. 

 The lacunae are bounded by unmodified protoplasmic elements 

 of the mesenchymal syncytium, and open freely into the adjacent 

 intercellular — interstitial — spaces. Obviously the lacunae repre- 

 sent enlarged intercellular spaces, and the inference is justifiable 

 that they are filled with the intercellular fluid. 



In a slightly advanced stage of development in general there is 

 a greater number of lacunae in the same region in the embryo 

 and also an increase in size and a difference in the appearance of 

 some of the lacunae. The increase in size certainly depends in 

 part upon actual dilatation and in part upon addition of more of 

 the adjacent mesenchymal intercellular spaces, for every possible 

 gradation can be seen between the smallest and the largest. The 

 difference in appearance is observed to be due to the presence 

 of flat cells which form a distinct boundary or wall, although 

 not usually complete. Morphologically these cells are equiva- 

 lent to endothelial cells. Inasmuch as they shade by invisible 

 gradations into the unmodified mesenchymal cells bordering upon 

 the rest of the lacuna, we conclude that they are derived directly 

 from the indifferent mesenchymal cells. The differentiation, we 

 may also infer, is due in part to pressure and friction incident 

 to the flow of the tissue fluid. Another factor in the differ- 

 entiation may also be pressure and friction incident to the to 

 and fro motion of blood cells in the tissue spaces and lacunae in 

 response to the heart-beat, a phenomenon observed in living 

 blastoderms. 



Studies of later stages show these isolated lacunae to be the 

 rudiments of the thoracic duct, and the conclusions that they are 

 direct derivatives of the mesenchymal intercellular spaces and 



