there was so much distortion of the cells and 

 because the serum precipitate was so dense, it 

 was difficult to distinguish between mesenchymal 

 cells and young lymphocytes. In 332 A no at- 

 tempt has been made to place a label of lympho- 

 cyte on any of the cells, although some of them 

 appear to be undergoing a transition from pri- 

 mordial (mesenchyme) tissue to lymphoblasts. 



Cell 1, which is fairly well presei-ved, shows 

 a faint nucleolus in the lower part of the nucleus, 

 and other cells in the same field show nucleoli 

 vaguely. It is this type of cell in the older 

 chicken that would be called the reticular cell. 

 Somewhere in the transition to the lymphoblast 

 the nucleolus is lost, or at least it becomes hidden 

 beneath the delicate reticular pattern of the 

 chromatin at the surface of the nucleus. In 

 figure 334 this is true even when the chromatin 

 is lightly stained. As stated before, whether a 

 nucleolus is actually present must be determined 

 from sections but, if present, it definitely disap- 

 pears before the stage of the small lymphocyte 

 has been reached. This has been confirmed in 

 sections by Dantschakoff (1908b and 1909a). 



In Sundberg's study of lymphocytogenesis 

 (1947) in man and other mammals, obsei^va- 

 tions were made that were almost identical to 

 those reported here for birds. She found that 

 a reticidar cell was the precursor for the lympho- 

 cyte line. The nucleus of the reticular cell 

 showed a nucleolus, whereas this organelle if 

 present in the lymphoblast was not visible. The 

 reticular cell described by Sundberg closely re- 

 sembles in appearance the primordial osteogenic 

 cell of the embryo chick bone marrow (fig. 320, 

 cells i-4). 



Broken nuclei produce long strands of stained 

 basichromatin across the slide and sometimes it 

 looks as if a particular strand could be traced 

 back to the granule in the nucleus out of which 

 it had formed a streamer (fig. 332 A, 8) . Some 

 cells (like the two to the right of cell 1) stain 

 intensely and the chromatin is clumped, but the 

 cells are crowded and thus fail to show clearly 

 the type to which they belong. They resemble in 

 a general way the amoeboid wandering cell de- 

 scribed by Dantschakoff. 



Cells from an embryo that has been incubated 

 11 days (fig. 332 B) were somewhat better pre- 

 served in smears than were cells taken from em- 

 bryos of younger ages; hence the cells that 

 are going to produce granulocytes can be identi- 



fied. The thymus has a lesser granulopoietic 

 function than the spleen. Cell 11 has many 

 structural characters of the promyelocyte but the 

 magenta masses are not the sharply defined 

 rings and granules usually found, which might 

 be due to the unfavorable environment in which 

 it was fixed. Cell 12 is definitely atypical. Some 

 might identify it as a macrophage, and cell ii as 

 its early stage. It also is reminiscent of the pe- 

 culiar defect noted in bone marrow where large 

 spheres and masses of magenta material were 

 deposited in the cytosome of primordial osteo- 

 genic cells (figs. 320 and 328). Cell 12 may 

 well be a naked nucleus surrounded by cyto- 

 plasmic residue and serum artifacts. 



Cell 1 in figure 332 B closely simulates the 

 primordial cells of 332 A but, also, it is identical 

 in appearance with the type of cell called the 

 lymphoblast (fig. 334). The lymphoblast de- 

 creases in size during differentiation and, al- 

 though the chromatin is not clumped, it does be- 

 come coarser (cells 2 and 3) and forms what is 

 called an early phase of the immature lympho- 

 cyte; soon the forming of blocklike clumps of 

 chromatin begins, usually at one side of the 

 nucleus, which indicates that the differentiation 

 process has reached the late phase of the im- 

 mature stage (cells 4 and 5). Cell 5 in the cir- 

 culating blood would probably be called a ma- 

 ture lymphocyte of medium size. Small mature 

 lymphocytes are present at this age but none are 

 shown in the field that has been illustrated. 



The thymus at this age is not a suitable source 

 of material for making smears and evidence for 

 this is shown by the distortion exliibited by di- 

 viding cells 6 and 7; these may be compared 

 with dividing cells in the spleen at 12^/4 days, 

 where the mitotic figures are well presei-ved. 



The cells shown in figures 334^338 represent 

 a developmental series taken from the thymus 

 when the embryo had been incubated 14 days 11 

 hours. Search was made for well-preserved 

 cells. All cells as large as figure 334 had nuclei 

 that took the stain poorly. This poor reaction 

 has happened before with May-Griinwald Giemsa 

 but usually there were similar cells on the same 

 slide or equivalent slides that showed the nuclear 

 surface well stained. With Wright's stain, cells 

 this large and immature rarely show the nucleus 

 properly stained. 



By the methods used here a nucleolus is not 

 visible at any stage of lymphocytogenesis al- 



168 



