finitive specific granule is present, but in immature 

 eosinophils, all degrees of development from the pre- 

 cursor substance to the definitive granule may be 

 found. Therefore, this stage overlaps the next, and 

 the term "mesomyelocyte" has been used to cover both 

 stages. 



Mesomyelocyte. — Definitive granules arise at this 

 stage, and all cells are included under this term that 

 have less than half the usual number found in the adult 

 cell. The nucleus is more definite and chromatin more 

 condensed than in the heterophil. 



Metamyelocyte. — This is an immature myelocyte with 

 more than half the number of specific granules. The 

 nucleus may be round, indented, or constricted, and its 

 chromatin clumped irregularly. 



Mature eosinophil. — Contains a full complement of 

 specific granules, and the number of lobes in the nu- 

 cleus may vary from one to five. The chromatin 

 clumps are close together. 



BASOPHIL SERIES 



Granuloblast. — Morphologically this cell is the same 

 as described for the heterophil series. 



M eta granuloblast. — This stage is present but. concur- 

 rently with the characteristic vacuolization of the cyto- 

 some, the magenta granules appear also, and thus the 

 term "promyelocyte" has been used for the two stages 

 combined. 



Promyelocyte. — Scattered magenta bodies are pres- 

 ent. They are closely similar to those of the heterophil, 

 except that there is less tendency to form rings. If 

 rings are present, they are usually small. Vacuoles are 

 present in the cytoplasm but they are less uniform in 

 size than for the other two preceding granulocytes. 

 The nucleus is eccentric and has a coarse chromatin 

 pattern. 



Mesomyelocyte. — This cell is smaller than the pre- 

 ceding one. The nucleus usually is not in the center 

 but is not completely eccentric. The cytosome con- 

 tains less than half the number of granules found in 

 the adult cell. The water solubility of the granules 

 is a confusing factor in estimating number. 



Metamyelocyte. — This cell contains more than half 

 the total number of basophil granules. The nucleus is 

 near the center and is composed of a dense chromatin 

 network. 



Mature basophil. — This cell contains a full comple- 

 ment of granules. The nucleus is a single body in 

 most cases, but occasionally it is divided into two lobes. 



OSTEOGENIC CELLS 



Primordial osteogenic cell. — A large amoebid cell 

 with lightly stained cytoplasm containing clear vacu- 



oles. There are relatively few mitochondrial spaces. 

 The cytosome is large relative to the nucleus. The 

 nuclear chromatin is finely punctate and a nucleolus 

 is present that stains light blue. 



Osteocyte Series 



Immature osteoblast. — The nucleus is placed eccen- 

 trically. Large and definite mitochondrial spaces are 

 present. Usually there is a clear area on the side 

 of the nucleus toward the center of the cell. Clear 

 spherical vacuoles are present in the cytosome. The 

 nucleus stains darkly with uniform granulation, and 

 usually one blue-stained nucleolus is present. 



Mature osteoblast. — A darkly stained cell with well 

 developed, clear areas adjacent to the nucleus. All 

 parts of the cytosome are filled with mitochondrial 

 spaces. The nucleus has a pattern of granular chrom- 

 atin, and a nucleolus is present. 



Osteocyte. — This cell has not been seen in smear 

 preparations of bone marrow since it is embedded 

 within the substance of the bone. 



Osteoclast Series 



Mononuclear osteoclast. — This cell closely resembles 

 the primordial osteogenic cell. The nucleus may be in 

 the center of a round cell or at one end of an elongated 

 cell. It contains a mixture of delicate vacuoles com- 

 posed of mitochondrial spaces and eosinophilic accum- 

 ulations. Also present are some clear vacuoles with a 

 definite spherical shape. The nucleus is composed of 

 particulate granules on a linin network and beneath 

 this a nucleolus is visible. 



Muhinuclear osteoclast. — This is the only giant cell 

 of the avian bone marrow. The cytosome contains 

 basophilic and sometimes eosino])hilic substances, and 

 the latter usually are concentrated in the central part 

 of the cell. Many vacuoles and granules and some- 

 times irregularly shaped bodies are present in the 

 cytosome. The borders of the cell are poorly defined. 

 The nucleus is large and round; it contains relatively 

 fine, punctate granules; a single nucleolus is present. 



PLASMACYTE SERIES 



Plasmablast. — This cell may be a primordial osteo- 

 genic cell or a reticular type cell. It has not been 

 identified thus far. 



Early immature plasmacyte. — The ratio of cytosomal 

 to nuclear size is about the same as in the monocyte. 

 The cytosome contains vacuoles and mitochondrial 

 spaces, and the ground substance stains a clear blue 

 that is more transparent than the basophilic cytoplasm 

 of most other young cells. 



Late immature plasmacyte. — The cytoplasm in this 

 cell is colored the same shade of blue as in the pre- 

 ceding stage, but there are fewer mitochondrial spaces 



13 



