A review of the differences in the histology of 

 the avian and mammalian spleens has been given 

 by Lucas et al. (1954). 



Whereas the embryo bone marrow produced 

 both granulocytes and erythrocytes in about equal 

 numbers, in the embryo spleen it is the granulo- 

 cyte that is the dominant cell. After hatching, 

 the spleen becomes predominately a lymphocyto- 

 genic and monocytogenic organ. The shift in 

 the cell picture is shown graphically by a com- 

 parison of figure 329 (8 days of incubation), 

 figure 330 (12^ days of incubation), and figure 

 331 (35 days after hatching). In the first of 

 this series of three plates are two typical meta- 

 granuloblasts (cells 1 and 2) and in the same 

 field are numerous others (cells 3-6) that are 

 covered too heavily with stained serum to show 

 the details of their structures. 



The next stage in development of the granulo- 

 cyte is the promyelocyte, in which vacuoles and 

 magenta granules and rings are present in the 

 cytoplasm adjacent to an eccentric nucleus. 

 Most of these characteristics are to be found in 

 cell 7, and lightly stained orange spheres are 

 present also. This cell has been classified as a 

 promyelocyte. Had some of the spheres taken 

 on the dense coloration that occurs antecedent to 

 the transformation into definitive rods, the cell 

 would have been classed as a mesomyelocyte. 



Cell 8 clearly fulfills the characteristic of the 

 mesomyelocyte. This particular cell has no 

 definitive rods but now there are present numer- 

 ous darkly stained orange spheres that represent 

 the precursor substance. In this cell the nucleus 

 has become smaller and the chromatin more con- 

 densed. Other developmental stages are not 

 shown in the field; cell 9 and the cell below 8 

 have already differentiated into the mature form. 



Cell ii of figure 329 is an erythroblast ; the 

 chromatin pattern of the nucleus and the faint 

 nucleolus are the features which most readily 

 identify it. The unnumbered blast cell at the 

 bottom of the plate is probably of the same type 

 but the details of its structure are masked in part 

 by the serum. There are at least five small em- 

 bryo thrombocytes in the field; two of them are 

 indicated at 12. One cell {10) , in the prophase 

 of mitosis, cannot be identified because the cyto- 

 plasmic structure is not sufficiently distinctive 

 and the nuclear pattern has been lost in the proc- 

 ess of cell division. 



The interval between 193 and 299 hours of 

 incubation has wrought developmental changes 

 that advance the architecture of the spleen to a 

 level that will be maintained up through the 

 hatching process. Since a low-power drawing 

 had been made of the bone marrow both at 12 

 days and at 20 days of incubation, it was orig- 

 inally planned that a low power drawing of the 

 spleen would also be made at these ages, but a 

 study of spleen impression smears made after 12 

 days of incubation showed tliat no significant 

 changes had taken place in cell types; therefore 

 a drawing at 20 days of incubation has been 

 omitted. The spleen at 8 days of incubation is 

 at about the same level of development as bone 

 marrow 4 days later, as far as the general ap- 

 pearance of the cells in the smears is concerned. 



When the spleen has reached its 12-day level of 

 development, blast forms of granulocytes become 

 relatively rare. Only two examples (cells 1 and 

 2) are shown in figure 330. One is shown only 

 in part, Init the narrow, dense blue-stained ring 

 of cytoplasm in cell 2 is typical of the structure 

 of such cells in the hatched chick. Most of the 

 cells of this line in figure 330 have reached the 

 metagranuloblast and promyelocyte stages of de- 

 velopment. However, figure 330 cannot be 

 taken as representing an exact replica of what 

 every slide examined at 299 hours of incubation 

 will show because among smears from a dozen 

 embryos there will be definite shifts in the dom- 

 inant cell type of a particular series. This might 

 be accounted for on the basis of slight differences 

 in developmental rates that always exist among 

 embryos or on the basis of cycles in cell produc- 

 tion. The latter would agree with the sugges- 

 tions given in the literature for blood-cell de- 

 velopment in the yolk sac and bone marrow. 



Cells 3-6 (fig. 330) are identified as meta- 

 granuloblasts of the heterophil line but they are 

 not so clearly typical of this stage as is the eosino- 

 phil metagranuloblast (cell 11). A question 

 might be raised regarding cells 3-6, in which the 

 nuclear structure, instead of retaining the deli- 

 cate, lightly stained pattern of the granuloblast, 

 shows an increased density of staining and clump- 

 ing of chromatin, which is characteristic of the 

 early erythrocyte line; had nucleoli been visible 

 in these cells as in cell 12, these 4 cells would 

 have been called erythroblasts. One expects 

 to find in the metagranuloblast stage an eccentric, 

 faintly stained nucleus that has an indefinite 



156 



