THORACIC DUCT IN THE CHICK 147 



ries. They are isolated, for absolutely not any divect connection 

 with any other vessel can be traced even with high powers of 

 magnification. They open freely into the surrounding inter- 

 cellular spaces, as show in figures 12, 13 and 14, which are photo- 

 graphic reproductions of three successive sections of the embryo. 



The spaces fit the previous description, given in the section 

 on histogenesis, of accumulations of an intercellular fluid in the 

 mesenchyme. The objection that they may be shrinkage spaces 

 is nullified by the fact that the preservation of the tissue is 

 practically perfect, and that in this and in other stages the more 

 nearly perfect the preservation the more clearly defined are the 

 spaces. Furthermore, in cases of poorer preservation where there 

 are obvious shrinkage spaces in the mesenchyme the boundaries 

 of such spaces are almost invariably ragged and irregular and 

 do not anywhere exhibit a smooth endothelial lining. 



The masses of developing blood cells in this particular embryo 

 (six days and sixteen hours) are perhaps unusually extensive 

 (fig. 20, 16). They extend in irregular groups from about the 

 level of the superior mesenteric artery forward along the ventral 

 and ventro-lateral aspect of the aorta (1) to the level of the celiac 

 artery (5), with a tendency to cluster around the last named 

 vessel (cf. fig. 7, 16). They then divide into two general lines, 

 one on each side, which bend laterad and extend forward along 

 the mesial aspect of the ducts of Cuvier (12) and precardial veins 

 (10), ending rather abruptly in a large mass which hes at the level 

 of the sixth aortic arches (3) and extends across the mesial line 

 ventral to the dorsal aortic roots (2). 



The three spaces representing the first anlagen of the thoracic 

 duct (17) bear no particular relation to the large groups of devel- 

 oping blood cells (16), although a few isolated blood cells lie in 

 the tissue spaces around the larger rudimentary lymph spaces. 



Chick embryo of six days and twenty-one hours, llf. 7nm. (Columbia 

 Collection, series no. ^65). Reconstruction, dextro-ventral view. 

 Figure 21. In this embryo there is a considerable increase in the 

 size and number of lymph spaces and channels which constitute 

 the early anlagen of the thoracic duct (17). They are situated 

 for the most part in the mesenchymal tissue ventral and ventro- 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 15, NO. 2 



