nient as the ceil in figure 354. Accompanying 

 ihe increase in hemoglobin is a slight elongation 

 of the cell and an increased nuclear condensation. 

 Tiie tawny color, so often oljserved in late poly- 

 chromatic erythrocytes just before maturity, may 

 1)6 due to the diffusion of basophilic granular 

 material associated with the reticulocyte stage of 

 development. 



Differentiated cells with a full complement of 

 hemoglobin and a small nucleus with a condensed 

 chromatin pattern of the mature cell, yet with a 

 round, rather than an oval shape, may be found 

 ill the bone marrow of the adult chicken (fig. 

 355). When such cells were observed in the 

 embryo, tliey were interpreted as representing an 

 atypical condition associated perhaps with forced 

 development, and probably the same can be said 

 for sucli cells when they appear at older ages. A 

 mature, normal, typical erythrocyte is illustrated 

 by figure 356 and is identical with those seen in 

 circulating blood, even to the tone of cytoplasmic 

 color, in spite of the fact that the cells in the bone 

 marrow were stained with May-Griinwald Giemsa 

 and those of circulating blood with Wright's. 



Thromboblasts were not recognized in the bone 

 marrow until after the developing stages had 

 been worked out in the circulating blood of the 

 embryo. One reason was that they resembled 

 the erythroblasts. In fact, one cell in this series 

 (fig. 357) was originally placed among the eryth- 

 roblasts, but its punctate nuclear stmcture soon 

 indicated that it was in the wrong series. Two 

 stem cells of the adult bone marrow, the granu- 

 loblast and the thromboblast, have narrow cyto- 

 plasmic rims of cytoplasm around their nuclei. 

 The cytosomes take an intense violet color, but 

 these two cell types are readily distinguishable 

 l)y their difference in nuclear structure. 



The erythrocyte was divided into five levels of 

 development and tlie same number of sida- 

 divisions has been proposed for the thrombocyte, 

 but for the thrombocyte series these are based on 

 structural and size changes without the assistance 

 of significant tinctorial changes as in the erythro- 

 cytes; thus it has been somewhat difficult to estab- 

 lish terminal criteria for each stage of develop- 

 ment of tlie thrombocyte. The thromboblasts 

 are large cells (figs. 357 and 358) with a densely 

 stained cytosome that has numerous vacuolar and 

 mitochondrial spaces. 



The appearance of definitive thromboblasts of 

 the bone marrow is not much different from that 



of the embryo thromboblast (figs. 280-284). 

 In both, the nucleolus may or may not be visible, 

 depending upon the extent to which the plasmo- 

 some is masked by the overlying chromatin. The 

 frayed appearance of the peripheral margin of 

 the cytoplasm seen in figure 362 may also occur 

 in the blast stage. 



The term "early immature thrombocyte" is a 

 rather awkward one; perhaps other investigators, 

 after they have reexamined the problem, will 

 think of a shorter name. In the early immature 

 thrombocyte, tiiere is some clumping of the nu- 

 clear chromatin and often the structure of the 

 nucleus will appear similar to that of the par- 

 tially autolysed cell where the chromatin clumps 

 have vague indefinite boundaries like those 

 shown in figure 363. Figures 358 to 362 were 

 made from cells of the bone marrow of the 6-day- 

 old chick rather than from the adult because there 

 were a large number of early stages available 

 for observation. This made it possible to ar- 

 range them in their proper developmental 

 sequence. 



The mid-immature thrombocyte has already ac- 

 quired the characteristics that definitely identify 

 it as a thrombocyte — namely, the shift in nuclear 

 staining from violet to purple, the appearance of 

 definitive granules (fig. 363) and the elongation 

 of shape as in the immature embryo thrombo- 

 cyte. The cell at this stage of development has 

 a deep basophilic cytosome, which, during dif- 

 ferentiation to the late immature stage, fades to 

 a lightly stained cytoplasmic framework (fig. 

 364). The cell lias not yet assumed its mature 

 oval shape. The nucleus is still larger than in 

 the mature stage; it is still round, ratlier than 

 oval. Cells of this type are often found in the 

 circulating blood, as shown in outline (fig. 88 i). 

 Late immature and mature throml)ocytes may be 

 found in bone marrow but they are not common; 

 usually they have undergone at least partial dis- 

 integration. The cell shown in figure 365 is iden- 

 tical in appearance with that in figure 73 — a 

 thrombocyte from circulating blood. 



GRANULOCYTES 



The classification of granulocyte stages of de- 

 velopment has been outlined in table 2, which 

 gives 6 steps for the heterophil, 5 for the eosino- 



193 



