shown in figures 309-318. The nucleus is im- 

 mense and has a uniformly delicate granulation. 

 The cytosome carries clear vacuoles ranging 

 from small to very large. There is much to 

 suggest that this cell is a yolk sac or endodermal 

 macrophage. How so large a cell can flow 

 through capillary channels is not understood. 

 Cells of similar size but of different structure 

 were found in blood taken from the heart, but as 

 already mentioned diese were present in only a 

 few embryos and absent from most of them, and 

 were found most commonly at about 10 days of 

 incidjation. This age ( 10 days) agrees with the 

 span from 7 to 12 days of incubation when 

 Sugiyama (1926) found macrophages in the 

 endnyo blood. 



The cell shown in figure 310 is definitely a 

 macrophage althougli the cytosome contains but 

 a few inclusions, which are represented by 

 several magenta granules and numerous vacu- 

 oles. The luicleus was pressed down on some of 

 the vacuoles when the cell dried; these vacuoles 

 made the nucleus appear to be vacuolated also, 

 but actually it was not. An earlier less dif- 

 ferentiated stage is represented by the three cells 

 clumped together in figure 309. The nuclear 

 structure is identical with that shown in figure 

 310, but differentiation of the cytoplasm toward 

 the phagocytic condition of the macrophage has 

 not progressed as far as in the latter cell. The 

 nuclear structure of both of these cells is similar 

 to that observed in macrophages seen so abun- 

 dantly in blood spots (Lucas, 1946). In blood 

 spots they appeared to be derived from fibro- 

 blasts and the same type of cell with long uni- 



polar or Itipolar processes was found also in the 

 embryo (figs. 314-316). Whether these pre- 

 cursor cells are reticular cells or fibroblasts needs 

 to be determined, but at least it can be stated 

 that in the embryo and in blood spots of eggs 

 the precursor cell is not a lymphocyte. In the 

 circulating blood of the adult, lymphocytes dif- 

 ferentiate into a reactive type cell, and it has 

 been suggested that this may be leading toward 

 a macrophage, but never has a series of stages 

 jjeen observed spanning the gap from the reac- 

 tive lymphocyte to the type of macrophage shown 

 in figures 310, 313, or 318. 



In the same slides that contained the cells of 

 figure 309 were numerous protoplasmic spheres 

 (fig. 311 A to D). Although differing in size 

 they all stained in the same way and resembled 

 closely in structure the cytoplasm seen in figure 

 309. These cytoplasmic spheres were vacuo- 

 lated like the cytoplasm of macrophages and 

 probably were pinched oft from cells of this 

 type — at least no other cell visible in the blood 

 at this time seemed large enough to have pro- 

 duced them. The cytoplasm of the macrophages 

 has a structure identical with that found in the 

 pinched-oft fragments. 



Later in the study, cells grouped in a mass were 

 discovered (fig. 312). In nuclear and cytoplas- 

 mic structure they were identical with the small 

 group of three discovered earlier (fig. 309). 

 Many large macrophages had been observed up 

 to this time but the bulkiness of this group pre- 

 cluded the assumption that it could get through 

 the smaller circulatory channels. It is entii'ely 



Figures 276-287. — Early developmental stages of the embryo tlu-ombocytes. 2,470 X. 



276 Early thromboblast, loljulated stage. Embryo incu- 

 bated 2 days 17 hours. 



277 Either a late thromboblast or a large early thrombo- 

 cyte. Duplicate slide from same embryo as pre- 

 ceding figure. Treated with Ralph's benzidine and 

 stained with M. G. G. No trace of hemoglobin in 

 cytosome. 



278 Medium embryo thrombocyte. Same slide as pre- 

 ceding figure. 



279 Thromboblast in the telephase of mitotic division. 

 Embryo incubated 2 days 18 hours. 



Figures 280-286: Drawn from the same slide. Embryo in- 

 cubated 4 days. 



280 I,arge embrj-onic thromboblast. 



134 



FiGTRES 281-283: Embryo thromboblasts showing incom- 

 plete staining of the nucleus. 



281 This type of empty nucleus often seen in blast cells 

 following Wright's stain, less often with M. G. G. 



282 A partial staining of the nucleus. 



283 Nuclear content well stained but nuclear boundary 

 not well delineated. 



284 Embryo thromboblast that shows the punctate char- 

 acter of the chromatin granules. 



285 Large embryo thrombocyte. Cytoplasm still shows 

 the structure of the thromboblast. Nucleolus present. 



286 I^arge embryo thrombocyte. Rarely fixed in this 

 round form. 



287 T^arge embryo thrombocyte that shows precocious 

 development to the oval form. Same age as pre- 

 ceding one. 



