denser chromatin patleru is somewhat more dif- 

 ferentiated than the cell in figure 346. 



Erytlnoblasts have been seen in the circulating 

 blood of the adult (figs. 9 and 121), in the cir- 

 culating blood of the early embryo (figs. 224 

 and 233-235), in the later embryo (figs. 254- 

 256), and in embryo spleen (figs. 329 and 330) . 

 To all of these Sabin (1920) applied the term 

 "megaloblast," but it has been claimed that the 

 stem cells of all erythrocytes at all ages are not 

 alike and also that those of normal blood are not 

 identical with those of anemia (Jones, 1943). 

 Anemias have not been studied in the chicken 

 but from the other observations it is evident that 

 all erythroblasts do not look alike cytologically. 

 The differences are due in part to the fact that 

 the erythroblast is not a static cell covering one 

 point of time in development, but is a cell that 

 during the differentiation process can often be 

 divided into early and late phases, both of which 

 are capable of mitosis and the production of 

 progeny. 



Another factor that modifies the cytology of the 

 erythroblast is the degree of urgency for hemo- 

 globin. In the first generation of erythrocytes 

 it is evident that a high rate of hemoglobin ac- 

 quisition has been given priority over cytologic 

 difl^erentiation. With each generation of erythro- 

 blasts there is a gradual change to the definitive 

 condition where the taking on of hemoglobin 

 becomes an integral part of cell differentiation. 



As this study shows, the erythroblasts under 

 these various conditions have a different appear- 

 ance cytologically. Thus the names primary 

 erythroblast, embryo erythroblast, and definitive 

 erythroblast have been used. It is conceivable 

 that under the stress of a severe drain on the 

 erj^hrocytes in the adult these cells might be 

 pushed back to the same demand for new hemo- 

 globin that occurred in the first embryonic gene- 

 ration. Under these circumstances it might he 

 exjjected that they would look alike, but under 

 normal conditions the definitive erythroblast of 

 the bone marrow does not look like the primary 

 erythroblast, and thus there is less confusion if 

 specific terms are used whenever possible. 



Beyond the erythroblast there are three poly- 

 chromatic stages that cover the period of dif- 

 ferentiation from a cell with a small amount of 

 hemoglobin up to one with its full capacity. 

 These three stages are, of course, arbitrary ones, 

 and in setting them up an effort has been made 



to di\'ide tlie range into equal parts. In spite 

 of full awareness that there were three stages in 

 this range, and after considerable deliberation 

 had been given to the criteria of color and cellular 

 morphology, there were times when immature 

 erythrocytes on a particular slide were not named 

 as they should liave been. It was felt that the 

 difficulty might be due to a tendency, inherent 

 in human nature, to divide any continuous varia- 

 bility into smaller units. If such a tendency 

 were to influence an investigator when he had a 

 slide under examination, and if the slide did not 

 have the full range of polychromatic erythrocytes 

 in the fields examined, the investigator might be 

 impelled to divide what range there was into three 

 parts. Repeated surrender to this tendency — one 

 slide to the next, or one day to the next — might 

 result in inconsistent determinations. After the 

 drawings for the Atlas were completed and ar- 

 ranged, a imiform scheme was set up and with 

 the illustrations in hand there has been no recent 

 difficulty in calling cells of the same type by the 

 same name. 



Early polychromatic stages of the bone mar- 

 row are represented by figures 348 and 349, and 

 both show shadows of nucleoli against the lower 

 nuclear margins. The first of these two cells is 

 not far removed from the blast stage and the 

 second has a cytoplasmic coloration approaching 

 the mid-polychromatic erythrocyte. In these 

 cells mitochondrial spaces are still visible; yet 

 in some examples of early polychromatic eryth- 

 rocytes the textural quality of the cytosome may 

 be as uniformly homogeneous as in later stages 

 of development. 



Mid-polychromatic erythrocytes are repre- 

 sented l)y two drawings (figs. 350 and 351) ; the 

 one in the first drawing is the younger. In these 

 particular examples, the nucleus is still large rel- 

 ative to the size of the cell. In some mid-poly- 

 chromatic erythroc}'tes the nuclei may show 

 greater chromatin condensation than in figures 

 350 and 351. They may be more like the one 

 shown in figure 352. This figure shows a cell 

 lying at the borderline between the mid- and late 

 stages of erythrocyte development. The cell has 

 been placed in the late stage group because the 

 baso]iliilic substance has almost disappeared and 

 tlie acidophilic materials in the stroma are begin- 

 ning to dominate. The cell represented by figure 

 353 is clearly more differentiated than the pre- 

 ceding one and not as far along in its develop- 



192 



