though, as pointed out earlier, it is present in the 

 primordial embryonic cell from which the lym- 

 phoblast is derived. Even if it can be demon- 

 strated by sections that a nucleolus is present, 

 this would not invalidate the usefuhiess of the 

 smear method as a means of differentiating be- 

 tween the early stages of various cell lines, and 

 for practical purposes it can be stated that, in 

 smears, erythroblasts and thromboblasts have nu- 

 cleoli and that lymphoblasts and granuloblasts 

 do not. (For discussion of monoblasts, see 

 Lucas, 1959.) 



The cells in figures 335 and 336 are still suf- 

 ficiently large to be designated as immature 

 lymphocytes, and the immature cell (fig. 122) 

 found in circulating blood closely resembles 

 figure 336. Other cells somewhat more ad- 

 vanced in development in which the chromatin 

 shows early stages of clumping have already 

 been discussed in connection witli figures 331 and 

 332, and the medium and small mature lympho- 

 cytes (figs. 337 and 338) will be mentioned 

 again in connection with figure 333. 



The general appearance of a smear from the 

 thymus 35 days after hatching is practically in- 

 distinguishable from a smear of the spleen at 

 the same age. The medium to small lymphocyte 

 is the dominant cell of both organs, but the de- 

 velopmental stages of the monocytes have not 

 been identified in the thymus. Cell 3 (fig. 333) 

 is a rather small lymphoblast similar to figure 

 335; cells that are not much smaller reached a 

 stage of nuclear clumping equal to the immature 

 Ijnnphocyte (cells 4 and 5 and the cell below 

 17). There are many medium and small mature 

 lymphocytes, some of which are indicated by 

 cells 6—12. In these the chromatin is clumped 

 into heavily stained blocks. As mentioned 

 earlier, at low magnification there are no visible 

 significant changes in cytoplasmic sti-ucture dur- 

 ing the differentiation of the lymphocyte, but 

 under high magnification (figs. 334—338) mito- 

 chondrial spaces may be seen in some of the 

 young cells. The spaces disappear as the cell 

 grows older. 



The framework of the thymus is composed of 

 reticular cells and their fibers. These cells are 

 often called epithelial or epithelioid cells. In 

 section they are readily recognized by the acido- 

 philic cytoplasm, which is large compared with 

 the small nucleus. Cells in section show very 

 little structural detail in the cytosome but in 



smears considerable structure is visible, the most 

 conspicuous being the mitochondrial spaces. A 

 well-defined plasmosome nucleolus lies near the 

 center of the nucleus in cell 1 of figure 333. The 

 color taken by the cytoplasm is similar to that of 

 the plasmacyte of the spleen (fig. 331). Cell 2 

 is a slightly crushed reticular cell. 



Not as many reticular cells are found in smears 

 as might be expected from the large number seen 

 in sections because in making them the loose cells 

 in the meshes of the reticulum are given up more 

 readily than the framework itself. The reticular 

 cells seen here are very similar to the pri- 

 mordial osteogenic cells of the embryo bone mar- 

 row, and if the cells of figure 332 A could be laid 

 out, separate from each other, it is probable that 

 both the reticular cells and the primordial osteo- 

 genic cells would have an appearance quite sim- 

 ilar to the mesenchyme of the embryo. The 

 reticular tissue of the adult organism is said to 

 be more like the embryonic mesenchyme es- 

 pecially in potentiality for producing other cell 

 types than any other connective-tissue cell of the 

 body. 



Danchakoff (1916b) in her study of thymus 

 development in the chick embryo concluded that 

 the cells of the thymus cortex are derived from 

 small lymphocytes that originally came from 

 mesenchyme cells. The small lymphocytes are 

 said to produce fibroblasts, macrophages, plasma 

 cells, and myelocytes. 



Granulocytes are common in the thymus, and 

 in sectioned material the heterophils often lie in 

 a mass of reticular cells and debris that form a 

 Hassall's corpuscle. Other cells found in the 

 avian thymus but not in the mammalian gland 

 are the isolated striated muscle cells. The eosin- 

 staining affinities of these cells cause them to 

 stand out conspicuously from the blue nuclei of 

 the lymphocytes (Wassjutotschkin, 1913 and 

 1914). It was expected that these cells would 

 be seen in smears but none were found. 



FEATHER SHEATH CELL 



This is not a blood cell and is not directly 

 related to one. It is a contaminant that was 

 found on many slides made from late embryos. 

 Before its significance was known, it caused a 

 great deal of confusion and concern. It was 

 called an x-cell when discovered, and that name 



169 



