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tinguishes three kinds of osseous medulla, the red, the yellow, 
and the gelatinous. The red occupies the most important 
position with respect to the formation of blood. It consists 
of three varieties of cells. 
1. Cells analogous to the white corpuscles of the blood. 
These are from ‘005 mm. to ‘010 mm. in diameter, are some- 
times without a nucleus, sometimes contain a divided nucleus, 
or even two. Their contractility is very remarkable, and 
was observed by Bizzozero so long ago as 1865. He has 
also directly observed in four frogs multiplication of these 
cells by division; the actively moving cell drew itself out, 
became constricted in the middle, and finally separated into 
two parts. ‘The obvious objection that such cells might be 
migrated blood cells, was met by the experiment of carefully 
washing out the vessels of rabbits recently killed by bleeding 
with solution of common salt before examination of the me- 
dulla. The number of bodies resembling leucocytes was not 
in any degree diminished. It was also observed that the 
number of such cells contained in the medulla was very far 
out of proportion to any that could be contained within the 
vessels. 
2. Red nucleated cells, discovered by Neumann. These 
vary from ‘008 mm. to ‘(012 mm., or more, in diameter. They 
show every transitional form, from the colourless nucleated 
cells to the red blood discs; some showing a large nucleus 
and colourless protoplasm, others one or more small nuclei, 
and a protoplasmic mass of the same colour as the red blood 
discs. The vanishing of the nucleus takes place by a kind 
of atrophy, the nucleus breaking up into granules. Elon- 
gated cells with two nuclei, one at each end, were also ob- 
served; they are either spindle-shaped or narrow in the 
middle, and show the process of division of red cells. 
3. “ Gigantic ” (or myeloid) cells, with proliferating central 
nucleus, were observed; their size is from ‘025 mm. to 
045 mm. They have an irregular round, oval, or kidney 
shape. They differ from the myéloplaxes of Robin in shape, 
size, and consistence, as well as in their locality. 
4. White cells containing red globules were first dis- 
covered by Bizzozero himself in 1868, and are commonly, 
though not constantly, present. The shape of these is ex- 
tremely various, in animals most round or oval; in man 
more often angular or spindle shaped. Their size is from 
distinguished three kinds of medulla—the red, the yellow, and the gela- 
tinous; and also pointed out the analogy of the cells of the red medulla 
with granulation cells, as well as the occurrence of pigmented cells in that 
structure. 
