144 ADAM M. MILLER 



endothelial cells. There is probably also some proliferation of 

 the endothelial cells, although in the study of the sections mitotic 

 figures were not seen. 



The coalescence of the originally unconnected lymphatics 

 results in a network or plexus of channels (cf. figs. 21, 22 and 24). 

 This is constantly being augmented by the coalescence of other 

 independently formed spaces and channels with one another 

 and with the previouly established plexus. Most of the vessels 

 composing the plexus lie longitudinally in the embryo along the 

 line of the aorta and dorsal aortic roots, and out of this plexus is 

 e\'entually crystallized the main drainage lines of the thoracic 

 duct. The establishment of these lines is best considered, how- 

 ever, in the section on morphogenesis. 



The correlation of groups of developing blood cells in this 

 region, the formation of which has already been described, and 

 the lymph spaces and channels constituting the anlagen of the 

 thoracic duct now remains to be discussed. As stated earlier in 

 this paper, the blood cells that are differentiated from the mesen- 

 chymal syncytium, whether they are arranged in groups or isolated, 

 lie free in the tissue spaces. In case the tissue spaces enlarge and 

 coalesce in the region where the blood cells are situated the latter 

 are then allowed to become free also in the larger space resulting 

 from the enlargement and coalescence. The larger space become^ 

 lined with endothelium, in the manner previously described, to form 

 a definite vessel or channel. The blood cells, therefore, which 

 were originally free in the tissue spaces are included in an}" of the 

 lymphatics developing in that particular locality (fig. 10, 16, 17). 



In that manner some of the blood cells become intravascular 

 elements during the earliest stages of lymphatic development. 

 Occasionally an entire group of cells is included in a lymph 

 channel; in other cases only part of a group or a few scattered 

 cells. It is true also that a great many lymphatics develop in 

 the mesenchyme quite apart from the blood cells (figs. 15, 16 and 

 17). 



In the earlier stages of lymph vessel formation there are great 

 numbers of blood cells, in various degrees of differentiation, in 

 the mesenchymal tissue spaces in the vicinity of or more or less 



