138 ADAM M. MILLER 



undoubtedly they should be considered as collections of intra- 

 embryonic developing blood cells. 



These clumps of cells, as noted by Sala, develop in the vicinity 

 of the aortic arches, especially the sixth, and along the dorsal 

 aortic roots and the aorta as far as the exit of the superior mesen- 

 teric artery. The early stages in the formation can be clearly 

 seen in embryos of 100 to 110 hours. 



Some of the stellate elements of the mesenchymal reticulum 

 (syncytium) become differentiated from their neighbors. Their 

 processes are retracted and separated from the general reticulum, 

 the cells thus becoming rounded (figs. 1 and 2). The cytoplasm 

 acquires a more strongly basophilic character, increases in amount 

 and becomes more homogeneous than that of the true mesen- 

 chyme. The nuclei contain a relatively small amount of chro- 

 matin and one or two, usually two, distinct nucleoli. Mitotic 

 figures are occasionally seen (fig. 2). While the cell contours are 

 generally regular, there are sufficient irregularities to denote an 

 ameboid character. These cells appear to be identical with the 

 large mononuclear cells which Dantschakoff describes as differen- 

 tiating from the blood islands in the area vasculosa of the blasto- 

 derm and later in the capillaries of the yolk sac, and which she 

 calls lymphocytes. 



The cells here under consideration increase in number not 

 only by their own proliferation but also by continued differen- 

 tiation from the mesenchymal syncytium (fig. 2). For the most 

 part they lie in compact groups, which gradually increase in 

 size as the cells increase in number, but some cells usually appear 

 in the vicinity of the groups (figs. 4, 5, 7, 8 and 10, 16) and not 

 infrequently at some distance from them. After their differentif,- 

 tion and separation from the mesenchymal syncytium all the 

 cells, both members of the groups and isolated, lie free in the spaces 

 among the stellate elements of the mesenchymal tissue (figs. 3 

 and 4). No blood vessels or lymphatics are present in the imme- 

 diate vicinity of the cell groups when the latter first develop and 

 consequently the cells are extravascular (figs, 1, 2, 3 and 4). 

 It will be shown later that after the lymphatics develop in this 

 region the cells for the most part are included within them and 

 thus become intravascular elements. 



