208 



BLOOD AND BLOOD-FORMING ORGANS. 



The typical cellular elements of red bone-marrow are : 

 I. The Marrow-cells, or Myelocytes. These are cells, slightly 

 larger than the leucocytes, possessing a relatively large nucleus of 

 round or oval shape, rarely lobular, containing a relatively small 

 amount of chromatin. In the protoplasm of these cells are found 

 (in man) neutrophile granules and now and again small vacuoles. 

 They are said to contain various pigment granules. These cells 

 are not found in normal blood, but are found in circulating blood in 

 certain forms of leukemia, where they may be distinguished from 

 the mononuclear leucocytes partly by their structure, more particu- 



Fig. 167. Cover-glass preparation from the bone-marrow of dog; X I2O (from 

 preparation of H. F. Miiller) : a, Mast-cell ; l>, lymphocyte ; c, eosinophile cell ; of, red 

 blood-cell ; <?, erythroblast in process of division ; f, /, normoblast ; g, erythroblast. 

 Myelocyte not shown in this figure. , 



larly by the presence of neutrophile granules not found in the 

 mononuclear leucocytes. 



2. Nucleated Red Blood-cells containing Hemoglobin. Two 

 varieties of these cells are recognized structurally, with interme- 

 diary stages, as one variety is developed from the other. The 

 cry throb lasts, being genetically the earlier cells, possess relatively 

 large nuclei with distinct chromatin network, surrounded by a 

 protoplasm tinged with hemoglobin, and are often found in a stage 

 of mitosis. The other variety of nucleated red blood-cells, the 

 normoblasts, are developed from the erythroblasts. They contain 

 globular nuclei, staining deeply, in which no chromatin network 

 is recognizable, and surrounded by a layer of protoplasm containing 

 hemoglobin. The normoblasts are changed into the nonnucleated 

 red blood-discs by the extrusion of the nucleus. This process 

 occurs normally in the red bone-marrow, or in the venous spaces 



