whole cell falls wilhiii the depth of focus of even 

 the oil immersion lens. This is readily con- 

 firmed hy looking at any dried hlood cell of 

 average size. 



The cytosome is flattened in the same way as 

 the nucleus, and the disk-shaped nucleus is cov- 

 ered by a thin layer of cytoplasm above and 

 another below. The stainable liodies of the cyto- 

 plasm may thus be superimposed on the nuclear 

 structure; likewise stainable particles in the fluid 

 around the cell may come to lie upon the flat- 

 tened cell, and it is impossible, because of the 

 shallow depth of focus, to tell whether they are 

 inside or outside the cell. 



There are numerous illustrations of the points 

 that have been presented thus far. Figures 70, 

 7], and 72 show examples of substances outside 

 the cell that appear to be inside; in figures 70 

 and 71, parts of snuidged nuclei from other cells 

 fell upon the cytosome and produced stained 

 bodies. In figure 72, stained serum granules 

 fell upon the cell, and had not other granules 

 around them been stained also, thev might have 

 l)een considered as lying within the cell instead 

 of on it. In other instances, granules having an 

 identical appearance mav lie inside the cell. 

 Considei'able judgment may be required to de- 

 termine what is artifact and what is real. 



Figures 103, 105-107, and 110 show cyto- 

 somal inclusions that appear to l)e in the luiclei; 

 thev stain intensely, and the nuclei in some cases 

 are lighter than normal, so that the particles stand 

 out in contrast to the Ijackground. Cytoplasmic 

 inclusions such as pale straining azurophilic 

 bodies of the monoc}1e usually are not visible 

 when they overlie a darkly stained nucleus (figs. 

 132-134) but they may appear to be located 

 inside the nucleus if they stain intensely I fig. 

 135). 



There is need at this point to explain more 

 fully what has been said regarding the desira- 

 bility of viewing both the outside and the inside 

 of the nucleus. It was stated that in the flattened, 

 dried cell one sees primarily the surface of the 

 nucleus rather than its interior. It is the coarse- 

 ness of this network at the surface that in part 

 determines the extent to which oljjects within 

 become visible. It is the same experience that 

 one has in attempting to view an object through 

 a very fine screen, as against a coarse wire net; 

 the former almost completely blocks the view of 

 things bevond and attention is automatically 



focused on the structure of the screen itself; the 

 latter offers verv little obstruction to a clear view 

 of the objects beyond: in fact, attention must be 

 shifted in order to see the wire net. Examples 

 will bring out these differences as they occur in 

 avian blood cells. 



Erythroblasts and thromboblasts are charac- 

 terized by the presence of nucleoli; whereas, 

 granuloblasts, lymphoblasts, and the most imma- 

 ture monocytes that we have been able to find, 

 do not appear to have nucleoli, owing in part at 

 least to the difference in screen effect as deter- 

 mined by the coarseness of the chromatin reticu- 

 lum. Whether nucleoli actually exist in all blast 

 cells is not the point for consideration at this 

 time. Granuloblasts (figs. 366 and 367) have 

 a close screenlike pattern of chromatin reticulum 

 and. if a nucleolus exists, it cannot be seen. The 

 same is true for the lymphoblast (fig. 334) . So 

 far, all efforts to demonstrate a luicleolus in the 

 granuloblast and lymphoblast of chicken blood 

 have failed. Yet. in a few cells, a vague bluish 

 image seemed to be present below this network 

 (fig. 382). For practical purposes of cell-type 

 identificalion, it makes no dift'erence whether a 

 nucleolus is actually present or not, but it is im- 

 portant and would have a Ijearing on the accept- 

 ance of one hematopoietic theory over another. 

 In the box-turtle the inmiature heterophil pos- 

 sesses a blue-staining plasmosome nucleolus, ac- 

 cording to Ryerson (1943). In sectioned chick 

 material studied by Dantschakoff (19081)) a 

 nucleolus was present inside the nucleus of the 

 granuloblast. 



The blast stages of both erythrocytes and 

 thrombocytes show nucleoli ( figs. 345, 346, 357, 

 and 358 ) : the former is shown more clearlv than 

 the latter, which is to be expected, since the latter 

 has a denser chromatin layer at the nuclear sur- 

 face. Yet. at best, these plasmosome nucleoli are 

 never sufficiently sharply defined to show clearly 

 the boundary of this body. There seem to be 

 two reasons for this — (1) the masking effect of 

 overlying surface chromatin particles which has 

 already been discussed, and (2) the penetration 

 of the stain. 



Normally, when the cell dries it does not rup- 

 ture; instead, the cell and nuclear membranes 

 remain intact and merely flatten out like balloons 

 partlv filled with water, and membrane resist- 

 ance to penetration is far more effective in a dried 

 cell, even though the cell is compressed to a thick- 



