LAD DR W. B. DRUMMOND. 
various cells present in the marrow in their natural relations 
to one another. The result of these investigations has been 
to establish the supreme position of the bone-marrow as a 
blood-forming organ, and to displace the older theories of 
blood formation. The bone-marrow, then, is the birth-place 
of the red corpuscles of the blood. Whether all the red 
corpuscles are born there, we need not discuss at present. 
We do not know of any other site for their formation in 
Man in normal conditions. 
The Structure of the Bone-marrow.—When a thin section 
of marrow is examined under the microscope, it is found 
that the marrow tissue proper is arranged in a network of 
thin strands, in the meshes of which are the large fat cells 
of which the marrow is so largely composed. The marrow 
tissue is formed almost entirely of cells, there being scarcely 
any supporting stroma. These cells are of several kinds. 
First of all, there are the marrow cells proper, which make 
up a considerable proportion of the substance of the 
marrow. These are colourless cells, each possessing a single 
large rounded or oval nucleus and finely granular protoplasm. 
The nucleus is relatively poor in chromatin. It has a 
distinct nuclear membrane, and contains a fine nuclear net- 
work, with some scattered fragments of chromatin, and one 
or more nucleoli. The protoplasm contains exceedingly 
minute granules, which stain with eosin like those of the 
finely granular leucocytes of the blood. 
In addition to these ordinary marrow cells, a few coarsely 
granular cells may be noticed, which differ from the first 
only in the character of their granules. 
The cells of most striking appearance in the marrow are 
the giant cells—very large cells which measure in diameter 
four or five times as much as the marrow cells. 
Now we come to the cells in which we are specially 
interested at present, the nucleated predecessors of the red 
corpuscles. These cells are sometimes divided into coloured 
and colourless varieties, according as they contain hemo- 
globin in their protoplasm or not; but it is not possible to 
make any such absolute distinction, as the hemoglobin may 
be present in such small amount that one cannot be quite 
certain whether it is there or not. The typical coloured variety 
