39 CLASSIFICATION OF TISSUES. 
medullary spaces, or secondary areole. These spaces are occupied 
by embryonic marrow and osteoblasts. Around these spaces bony 
material is finally deposited to form the true bone. 
What becomes of the cartilage cells in the process of ossification ? 
There are two theories, one being that they are destroyed and dis- 
appear, the other that they are changed to form the osteoblasts. 
What does the absorption and the final precipitation of the bony 
material in the form of lamella seem to depend upon ? 
The osteoclusts; these are large irregular cells, having. two to ten 
nuclei. These cells are thought to break up and form osteoblasts. In 
cell reproduction the nucleus divides into two, each nucleus appro- 
priating a portion of the cell-protoplasm. Here the nucleus may 
divide into many parts, and each nucleus take a portion of the cell- 
protoplasm. In this manner it is supposed that the osteoclast breaks 
_up and forms osteoblasts. 
What are the physical characteristics of blood ? 
It is a fluid of a reddish color, a little heavier than water, salty to 
the taste, alkaline in reaction, and having a faint peculiar odor. 
Of what elements is it composed ? 
Of a colorless fluid called Liquor sanguinis or plasma. This con- 
tains quantities of little cells of two kinds, the white and red bloodcells. 
wit ie. the estimated, number of cells in a cubic millimeter of 
ood ? 
5,000,000 of red bloodcells. 
10,000 of white bloodcells. 
Describe the microscopical appearance of the red bloodcell. 
In human blood they look like round biconcave disks having no 
nuclei. In the blood of cold-blooded animals like the frog the cells 
are nucleated. All red bloodcells are elastic. 
What is the structure of the red bloodcell ? 
It is composed of a colorless membrane, enclosing a watery solution 
of hemoglobin, which gives the characteristic color to blood. 
Describe the appearance and structure of the white bloodcell. 
They are colorless and finely granular. They have no limiting mem- 
brane, and have one or more nuclei. 
What peculiar property is possessed by them ? 
The power of ameboid motion, and the faculty of incorporating 
little masses of foreign material within their substance. : 
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