ing but not from every chick. None are shown 

 in figure 230. 



Figure 246 was inchided because amitosis is 

 said to occur in human, amphibian, and bird 

 blood cells after reproduction by mitosis has 

 ended. Constricted cells, cells with indented 

 nuclei, and cells with two nuclei can often be 

 found, but according to E. B. Wilson (1925) 

 (quotation on p. 31), amitosis involves con- 

 striction of botli cytoplasm and nucleus. Of the 

 hundreds of slides examined, this is the only cell 

 wliich entirely fulfilled the criteria of amitosis 

 and, when a cell having this appearance occurs 

 so rarely, additional evidence is needed before 

 we can consider that amitosis is a normal means 

 of increasing cell number. 



Later emhryonic erythrocyte generations 



The term "later embryonic erytlnocyle genera- 

 tions"' is used in the legends because it fully dif- 

 ferentiates between the cells to which it refers 

 and the cells of the primary generation; but it 

 is a cumbersome term, and in this section the cells 

 will be referred to simply as erythrocytes. 



Cells that produce the later genei-ations of em- 

 jjryonic erythrocytes undergo a spurt of multi- 

 plication within the luniina of the venous retic- 

 ulum. According to Dantschakoff (1908b), 

 this occurs on the fourth day of incubation, and 

 the venous reticulum appears at the time the yolk 

 sac membranes grow into the yolk substance. 



Cells of this series appear first about the fiftli 

 day, as observed by Fennel (1947) and by others 

 who have followed the changing blood picture in 

 the endnyo. Dawson (1936a) reports that stem 

 cells of this series appear on tlie fourth day. It 

 has been our observation that up to 96 hours in- 

 cujjation the only cells present are tlie primary 

 erythrocytes and the embryonic thrombocytes, 

 and that the blast cells seen on the fourth day be- 

 long to the latter series. These cjuickly differen- 

 tiate into tluombocytes, and die blast cells seen 

 on the fifth and sixth days are the precursors 

 of the later erythrocytes. Cytologically there is 

 no real difference between the tluomboblasts of 

 4 days and the erythroblasts of 6 days (compare 

 figs. 255 and 280). At 4 days incubation the 

 structural series leads to the thrombocytes, but 

 at 5 days and later it leads to the erythrocytes. 

 Further testing might show that the same stem 



cell serves both lines. A common intravascular 

 origin has been suggested for these cell lines. 

 (See Sugiyama, 1926, for review of the litera- 

 ture.) The cells shown in figure 226, 17 and in 

 figure 227, 26 cannot be classified with certainty; 

 either cell may belong to either of the lines men- 

 tioned. Blast cells of both lines contain nucleoli. 

 The l)last cells of the two lines can be sep- 

 arated fairly accurately later — during incuba- 

 tion and, in the adult, when bone marrow pro- 

 ihices these cells. 



The erythroblast has a more abundant, more 

 lightly stained cytoplasm than the thromboblast, 

 in which the cytoplasm is a darkly stained rim 

 aroinid the nucleus. In die erythroblast the 

 chromatin pattern is reticulate, whereas in the 

 thrombocyte it is more particulate and punctate. 



Anything subsecjuent to the blast stage is easy 

 to identify; for example, in figure 227, cells 

 13-15 are mid-polychromatic eiythrocytes and 

 cells 16-18 are late polychromatic erythrocytes. 

 Many of the latter type (cells 19-24) reveal 

 cytosomal artifacts. Early polychromatic eryth- 

 rocytes are scarce; one is shown in mitosis 

 (cell 25) and even this approaches the color of 

 the mid-erythrocyte. Differentiation proceeds 

 slowly and even by the nintli day there are only 

 relatively few mature cells; at least there are only 

 a few that give a good vigorous color. Most of 

 them are late polychromatic erythrocytes. By 

 the 13th day mature cells are considerably more 

 abundant, but at hatching or within the first 24 

 hours after hatching, the dominant cell is the ma- 

 ture erythrocyte, with a few scattered immature 

 cells (fig. 230). 



The steps involved in the differentiation proc- 

 ess of the erythrocyte line as well as some typical 

 artifacts of the mid- and late polychromatic 

 erythrocytes are shown in figures 254-275. The 

 least differentiated cell observed is illustrated in 

 figure 254. It has a slight amoeboid shape and 

 a smooth-textured cytoplasm, wliicli has a few 

 small spaces. The nucleus is a reticulum with 

 fine meshes, and individual chromatin granules 

 are indistinct. A nucleolus is very faintly vis- 

 ible at the upper side of die nucleus. 



A type of blast cell much more commonly 

 found is that shown in figure 255, which is round. 

 The cytoplasm with its mitochondrial spaces is a 

 narrow, strongly basophilic rim around the nu- 

 cleus. The latter is a coarse reticulum, still 

 without sharp distinction between chromatin and 



128 



