linin network. Another cell taken from the same 

 slide (fig. 256) is smaller but is essentially at 

 the same stage of development as the larger cell 

 except for a slightly more condensed pattern of 

 nuclear chromatin. 



Mitosis continues. The cells in figures 257 

 and 258 were found in the circulating blood of 

 two embryos, both inculjated for 9 days 18 hours. 

 The clumps of basophilic cytoplasm are uni- 

 formly distributed. In the process of division 

 the cell loses all its identifying marks and it can 

 only be suggested that these two cells are either 

 late erythroblasts or early polychromatic eryth- 

 rocytes. 



The next three cells illustrated (figs. 259-261) 

 probably should be included with examples of 

 artifacts. They are presented with the idea 

 of showing how blast cells appear when they are 

 improperly stained. The cytoplasm in each cell 

 stained well and showed the mitochondrial spaces 

 but, because the nucleus was colored only faintly, 

 the boundary between nucleus and cytosome was 

 not definite and the vacuoles of nucleoplasm and 

 cytoplasm merged. It is interesting to note again 

 that when the surface reticulum is lightly colored 

 the internal structures become visible; thus the 

 existence of a nucleolus is revealed in figures 259 

 and 260, but in 261, where the reticulum takes 

 the stain more energetically, the nucleolus is only 

 vaguely indicated as lying to the left side of the 

 nucleus. 



The failure of the stain to penetrate the nu- 

 cleus in figures 259-261 is limited to the large 

 blast cells. In more difi^erentiated cells such 

 as figures 262-264, taken from the same slide, 

 the surface chromatin of the nucleus is intensely 

 colored. The difference is not due to cell size 

 since the larger and smaller are equally flattened ; 

 it seems more reasonable to suppose that there 

 is a difference in the physical character of the 

 nuclear membrane. 



The cytoplasm in figures 262 and 263 shows 

 the same intense, dark blue that is shown in the 

 blast-cell stage; it is still a narrow rim extending 

 only slightly beyond the nucleus, and mitochon- 

 drial spaces are still present. The nucleus re- 

 tains a nucleolus that is conspicuous even in 

 figure 264; yet differentiation has occured, as 

 clearly evidenced by the clumping of the chro- 

 matin pattern and the taking up of hemoglobin. 

 This sequence of differentiation is reminiscent 

 of the precocious development occurring in the 



first generation of erythrocytes, where chromatin 

 clumping and hemoglobin acquisition developed 

 ahead of cellular differentiation. 



In the generation succeeding the primary one, 

 the precocious development of these two factors 

 does not get as far ahead of general cellular dif- 

 ferentiation as before and, with each succeeding 

 generation, there is a gradual approach to the 

 condition found in the adult bone marrow, where 

 cellular differentiation and hemoglobin ac- 

 cumulation keep pace with each other. Thus, 

 all three cells within this group have been desig- 

 nated as early polychromatic erythrocytes, and 

 in figure 264 the tinctorial quality of the cyto- 

 some clearly indicates that hemoglobin is present. 

 Figure 265 has been included under the same 

 category. In this case, however, all the features 

 that are used to measure cytologic differentiation 

 have shifted to indicate a more differentiated 

 cell: the nucleus has become slightly eccentric, 

 the cytosomal rim around the nucleus is greater 

 in proportion to nuclear size than in the preceding 

 cells, the mitochondrial spaces have become 

 small and the chromatin clumps have grown in 

 size and density and in their closeness to each 

 other. 



The mid-polychromatic eiythrocytes present 

 no difficulty in identification and several ex- 

 amples are pointed out in figure 227, cells 13—15. 

 Figure 266 is a cell at this stage, drawn at a 

 higher magnification. There is, of course, a 

 range of color in the cytoplasm of various cells 

 at this stage as there is for the one before and the 

 one following, and so some mid-polychromatic 

 eiythrocytes may appear gray and others nearly 

 blue. 



The next stage of development is the late poly- 

 chromatic erythrocyte (figs. 267-269). The 

 cytoplasm is more homogeneous than it was in the 

 previous stage, and at the beginning of this 

 phase it still contains considerable basophilic 

 material. Many of the cells in figure 228 are at 

 this stage of development. As hemoglobin ac- 

 cumulates, the cell elongates and the ratio of 

 cytosome to nucleus increases. The nucleus also 

 becomes slightly elongated and the chromatin 

 pattern takes on the characteristics of the lepto- 

 chromatic type of nucleus found in mature cells. 



The mature embryo erythrocyte often con- 

 tinues its hemoglobin synthesis until it reaches 

 a tinctorial level equal to that of the primary 

 generation of erythrocytes (figs. 269 and 271). 



129 



