the Department of Natural Science, Michigan 

 State University, has demonstrated (personal 

 communication) sections of early embryos that 

 clearly revealed the primordial germ cells. 

 Even in sections they were considerably larger 

 than any of the lilood cells seen in the embryo, 

 including the primary generation of erythrocytes. 

 Very probably we have never seen primordial 

 germ cells in our smears. 



A final bit of data is the macrophage shown in 

 figure 318. It was not obtained by cardiac punc- 

 ture but in a drop of blood from the Ijasilic vein 

 of a chick hatched only a few hours before the 

 sample was taken. In some ways the high degree 

 of vacuolization and the rather delicate nuclear 



pattern resemble the yolk macrophage of a 2-day- 

 old embryo more than they do the macrophages 

 of a 10-day-old embryo. 



An impression smear from the spleen of a late 

 embryo that, after killing, was left at room tem- 

 perature for about 2 hours produced still another 

 type of phagocytic cell (fig. 317). The strong 

 acidophilic reaction of the cytoplasm is charac- 

 teristic of post mortem degeneration and this type 

 of vacuole formation can be seen in tissue-culture 

 cells when the culture is held under unfavorable 

 conditions. The nuclear structure is entirely dif- 

 ferent from that found in the 10-day-old embryo; 

 in fact, tlie entire cell is morphologically different 

 from any macrophage described thus far. 



ADDENDUM 



Throughout this book and in a recent publica- 

 tion (Lucas, 1959), it has been repeatedly men- 

 tioned that in smears or touch preparations the 

 nuclei of erythrojjlasts and thromboblasts usu- 

 ally reveal their nucleoli, whereas the nuclei oi 

 granuloblasts and lymphoblasts rarely do. Dur- 

 ing the period following the completion of the 

 manuscript, an effort has been directed (1) to- 

 ward the identification of the tissue components 

 in different hematopoietic organs from which the 

 various blood cell lines take their origin, and (2) 

 toward the identification of equivalent cells in 

 sectioned material with the named stages of de- 

 velopment found in touch preparations. 



From our studies thus far it appears that the 

 presence or absence of a nucleolus is a variable 

 that is dependent more upon the size of the cell 

 and its level of metabolic activity than upon its 

 being a fixed morphologic structure. We are 

 not including here tlie pseudo-nucleoli that are 

 chromocenters in some animal cells. From sec- 



tions showing stages in development of the granu- 

 locyte series the nucleolus appears first in the 

 metagraiudoblast, reaches its maximum size in 

 the promyelocyte and then disappears during the 

 subsequent stages. The areas on the riglit of the 

 nuclei in figures 382 and 383 that stain a faint 

 bluish color are probal)ly nucleoli of the baso- 

 phil promyelocytes. 



Ackei-man and Knouff (1959) picture the 

 lymphoblast in the bursa of Fabricius with a 

 nucleolus. Our own studies indicate that this is 

 probably true also in the thymus. l>ut it is still 

 imcertain that this is the case in spleen and bone 

 marrow. 



Regardless of how universally a imcleolus 

 may prove to be present, based on sections of 

 avian tissues, the usefulness of its variable visi- 

 bility as a tool in the study of smear prepara- 

 tions is not nullified; by this tool, erythroblasts 

 and thromboblasts can be distinguished from 

 granuloblasts and lymphoblasts. 



140 



