CONNECTIVE TISSUES. 29 
myéloplaxes or giant cells. In the marrow of developing bone are 
seen oval or polyhedral granular cells of good size with a single large 
nucleus, these are the osteoblasts. 
In what two general ways is bone developed ? 
By intra-cartilaginous and intra-membranous ossification. 
What bone may be taken as an example of intra-membranous ossi- 
fication ? 
The parietal bone. 
What are the relations of the parietal bone with adjacent parts in 
the embryo ? 
Without, integument ; within, dura mater ; while between these two 
layers there is found an intermediate layer in which ossification begins. 
What is the appearance of this commencing ossification ? 
There is a network of fibres similar to white fibres. At the centre 
of this mass of fibres is seen a deposit of lime salts in the form of 
spiculae or bars. These spiculee interlace, and the spaces between 
them gradually become filled up with the lime salts. This process 
continues to spread until the parietal meets the other cranial bones. 
How are long bones produced ? 
By cartilaginous ossification. 
What is the first step toward bony formation ? 
There is first what might be termed a pattern of the bone to be 
formed made up of hyaline cartilage. 
How does the ossification begin ? 
At the centre of the bone the cartilage cells begin to swell. The 
matrix is infiltrated with a granular precipitate of lime salt. The 
cartilage cells above and below the point of beginning ossification are 
arranged in rows in tke calcified matrix. The spaces occupied by 
them are called primary areole. 
At this time what change is going on outside the cartilage under- 
neath the periosteum ? 
A layer of bony tissue is being formed by the osteoblasts, which are 
large nucleated cells contained in the periosteum. 
What is the next step in the process ? 
There is an irruption of sub-periosteal tissue, and of osteoblasts 
into the middle of the cartilage. Here the osteoblastic cells absorb the 
calcareous deposit between the primary areole forming larger, or 
