larger area around the nucleus than is charac- 

 teristic of blast cells but is considerably less than 

 found in the mature plasmacyte, and this par- 

 ticular cell has been called an early immature 

 plasmacyte. The cytosome shows a mixture of 

 mitochondrial spaces and the spherical vacuoles 

 that are typical for the plasmacyte. both imma- 

 ture and mature. The bluish color taken by the 

 cytoplasm has little or no red in it; thus it has an 

 azure quality usually not found in other cell 

 lines. The nucleus, even at tliis stage of imma- 

 turity, stains intensely; the chromatin is uni- 

 formly distributed but is composed of blocks 

 larger than commonly found at this early stage of 

 diiferentiation. This was true also of the im- 

 mature plasmacyte pictured in the spleen (cell 1. 

 fig. 331). 



The next four cells (figs. 392-395) are classed 

 as late immature plasmacytes . They are almost 

 as large as the early immature plasmacyte but 

 proportional size of the nucleus has decreased. 

 The sequence in this progressive change is in- 

 dicated by the arrangement of figures. The most 

 differentiated cell of the group is figure 395, in 

 which a small nucleus lies at one side of a large 

 cell having a strongly basophilic cytoplasm. 

 During this process the chromatin aggregates into 

 large clumps, but the clumps do not fill the entire 

 nucleus. In figure 393 the cytosome is filled 

 with small vacuoles; in the others there is a range 

 in size from small to large. 



The mature plasmacyte (figs. 396 and 397) is 

 smaller than the immature stages. The nucleus 

 still holds an eccentric position in the cell. The 

 Hof, which was present in some of the immature 

 cells, usually persists. Thus, the Hof is present 

 in figure 396 and absent from 397. Vacuoles 

 characteristic of plasmacytes are still present in 

 the cytosome. A drawing was made of a cell 

 identified as a mature plasma cell in the circulat- 

 ing blood, but since the cell occurred only once 

 among many slides examined, the drawing has 

 been omitted. It did not have a vacuolated cyto- 

 plasm, and this raised some doubt that it was a 

 plasmacyte. 



OSTEOCLASTS 



Osteoclasts can be found in most bone marrow 

 smears. They are readily recognized by their 



large size (figs. 398-399) . Cells of this size are 

 multinucleated, as shown in figure 399. Search 

 was made for a mononucleated osteoclast and it 

 was thought that such a condition had been found 

 in figure 398, but beneath the pink-stained ma- 

 terial in the center of the cells were faint traces 

 of other nuclei. Small mononucleated cells like 

 those found in the bone marrow of the embryo 

 (figs. 319, 326, and 327) probably exist in adult 

 bone marrow, also, but were not seen. 



The cytoplasm of osteoclasts in smears seems 

 to merge into the surrounding serum and only 

 rarely can one identify with any accuracy the 

 exact boundary all the way around the cell; often 

 it appears as shown at the left end of figure 399. 

 The cytoplasm forms a network and in it are 

 vacuoles of various sizes and granules of various 

 sorts; most of the latter — for example, the rods 

 and magenta rings from heterophils — fell on the 

 surface of the cell when die smear was made. 

 Sectioned material usually gives an indication 

 that the osteoclast has polarity — one end pressed 

 against the spicule of bone and the other free in 

 the marrow cavity. The appearance of the cell 

 in figure 399 would suggest that the left side of 

 the cell has been adjacent to the bone and the 

 basophilic right end toward the cavity. 



DIFFERENTIAL COUNTS ON BONE 

 MARROW 



Differential counts have been made on the cells 

 of bone marrow in tlie chicken at 3 ages before 

 and at 4 ages after hatcliing. One hundred cells 

 were counted from each of five chicks at each age. 

 The average was based on 500 cells. This num- 

 ber is small for bone-marrow studies but, even 

 in this preliminary survey, certain ratios and 

 trends are indicated. Late polychromatic and 

 mature erythrocytes are abundant at all the ages 

 covered in table 10, but it is questionable whether 

 the fluctuations in the two cell types have sig- 

 nificance; for example, where there were 16.8 

 and 29.2 percent at 285 hours, 49.6 and 18.4 

 percent at 347 hours, and 6.0 and 29.4 percent 

 at 481 hours. Stages younger than the late poly- 

 chromatic erythrocyte are always scarce. 



Thrombocytes are present at all ages and are 

 distributed fairly uniformly among the various 

 stages of development except for the thrombo- 



198 



