all vertebrates and, if possible, all theories of 

 hematopoiesis. 



Table 2 shows the lineages of cell lines as seen 

 in the birds. For eacli line, there is listed at the 

 top of the column a primordial stem cell or blast 

 cell. In human hematology, blast cells are pic- 

 tured and described as containing one or more 

 nucleoli that appear as pale blue homogeneous 

 bodies following the commonly used blood stains. 

 In avian blood, no more than one nucleolus has 

 been seen in any cell line except in the primary 

 generation of embryonic erythrocytes, and nu- 

 cleoli are absent from granuloblasts, lympho- 

 blasts, and probably monoblasts, at least by the 

 same technics that have revealed them in human 

 blood. 



Erythroblasts, thromboblasts, lymphoblasts, 

 granuloblasts, and primordial osteogenic cells of 

 birds are somewhat more easily distinguished 

 than in the human species. Each represents the 

 earliest recognizable member of its respective 

 line, and although some do not show nucleoli, 

 each is believed to be equivalent functionally to 

 the same blast cell by the same name in the hu- 

 man series. The important point is that in many 

 cases they can be separated on the basis of their 

 structure, even when isolated from the other cells 

 on the slide. This does not alway hold true in 

 the human field, and in the atlas by Osgood and 

 Ashworth (1937) it is stated (p. 36), "In the 

 authors opinion [Osgood], the individual 

 granuloblast (myeloblast) is morphologically in- 

 distinguishaljle from the most immature lympho- 

 blast, monoblast, plasmoblast, or karyoblast 

 (megaloblast). The differentiation of the type 

 of cell has to be made by identification of the 

 cells found associated with the stem cell under 

 consideration. A stem cell found in association 

 with a progranulocyte (promyelocyte) is classed 

 as a granuloblast." (References to figures have 

 been omitted in the quotation just given.) It is 

 this type of dependence upon the presence of 

 more highly differentiated cells in the same field 

 for exact placement of the different blast cells 

 that has led the unitarian hematologist to sug- 

 gest that if blast cells all look alike and are all 

 characterized by a narrow rim of ]:)lue-staining 

 cytoplasm around the nucleus, and the nucleus 

 contains a nucleolus, it is just as logical to say 

 tliat there is a single common stem cell capable 

 of differentiation into any blood cell type. As 

 already mentioned, the criticism leveled at this 



view of blast cells in mammals cannot be applied 

 as readily to avian blood because, as seen by 

 the smear method, cytologic differences do exist 

 among the blast cells. 



All schools of hematology recognize the fact 

 that if the genealogy of cell types is carried far 

 enough back into the embryology of the organism, 

 there will be a common cell for all blood-cell 

 types. The point of controversy is not on this 

 matter but on whether a blast cell of a particular 

 cell line is a fixed type, incapable of differentia- 

 tion under stimulus into other cell types. An- 

 swers to such questions must come from experi- 

 mentation and, as far as the present study is 

 concerned, there is no evidence that blood cells 

 have or have not any potentiality beyond the par- 

 ticular line that they represent. 



On the basis of what has been observed in 

 the chicken, its embryology, and its hematopoietic 

 organs, a blast cell may be defined as the earliest 

 recognizable cell belonging to a particular cell 

 type, and all following stages observed consist of 

 progressive steps toward the mature cell. To this 

 should be added the observation that if, antece- 

 dent to the blast cell, there does exist a totipotent 

 cell type other than mesenchyme, reticular, and 

 possibly connective and endothelial tissues, it has 

 not been found in this study. It should be added, 

 also, that the terminology presented in table 2 is 

 based on the assumption that the usual medium 

 and small lymphocyte found in circulating blood 

 is a mature cell, on a par with all other fully 

 differentiated mature cells found in the circulat- 

 ing blood, and that it is not a totipotent primor- 

 dial cell capable, at least in the course of normal 

 hematopoiesis, of producing all other blood-cell 

 types. In normal blood only two types of mature 

 lymphocytes exist, medium and small; the large 

 lymphocyte is not a mature cell but is an imma- 

 ture cell usually standing early in the lineage of 

 the cell line to which it belongs, which may not 

 necessarily be the lymphocyte line.^ 



More than one erythrocyte series exists in the 

 life of the embryo up to hatching. There is a pri- 

 mary series representing the first generation of 

 red cells in the embryo; then follow several 

 generations, each less precocious in its hemo- 

 globin formation than the preceding one and, 



"For further clarification of viewpoint see: 



Lucas, A. M. 1959. A discussion of synonymy in avian and 

 mammalian hematologic nomenclature. Amer. Jour. Vet. Res. 

 78:887-897. 



