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place in the arrangement and in the size and shape of the 
cartilage cells, previous to ossification. When this alteration 
is sufficiently advanced, a deposit of calcareous matter takes 
place around the cells, and a general opacity of the cartilage 
is produced. It is then necessary to separate this matter 
from the tissue by the application of an acid, in order to 
observe the process of ossification in its more advanced 
stages. Certain spaces are then found below the plane of 
ossification, in the parts more advanced in the process. 
These are called medullary spaces, and are due to the ab- 
sorption of the calcified tissue. These spaces are filled with 
spherical cells and blood vessels. The cells are arranged in 
concentric layers within the spaces, which latter eventually 
intercommunicate; and blood vessels, which run through 
the spaces, can be traced as continuations of the vessels in 
the perichondrium, or membrane investing the cartilage. 
Kecent observations appear to show that the cells of the 
medulla are the offspring of similar cells found beneath the 
perichondrium, and that they follow the course of the blood 
vessels as the latter extend themselves into the newly 
formed medullary spaces. These cells give origin to the 
cells of marrow, to certain stellate cells, and to osteoblasts 
or bone-forming cells. The osteoblasts, again, give origin 
to larger, many-nucleated cells, called osteoclasts, which, 
according to Kolliker, are the agents by which the calcified 
cartilage is absorbed in the formation of medullary spaces. 
This absorption of calcified cartilage precedes the production 
of true bony tissue. When the formation of true bone is 
about to occur, an external layer of osteoblastic cells lining 
a medullary space assumes a granular appearance ; a finely 
reticulated matrix of osteogenic substance, secreted by the 
osteoblasts, surrounds the latter, and a deposit of osseous 
particles takes place at the periphery of each cell, and in 
the intervening osteogenic matrix. This deposit unites 
