6 
The Bulletin 
some time remains cartilaginous. Subsequently, a similar process com¬ 
mences in one or more places in these extremities (epiphyseal ossifica¬ 
tion), and gradually extend from them. Growth of long bones in length 
in the fowl takes place from the cartilaginous portion of the ends. 
Ossification of Cartilage 
The first step in the ossification of cartilage is the enlargement of the 
cartilage cells, which arrange themselves in rows at a point termed the 
center of ossification. The matrix in which they are imbedded increases 
in quantity, so that the cells become further separated from each other. 
A deposit of calcareous material now takes place in this matrix, pre¬ 
senting a granular and opaque appearance. Here and there the matrix 
between two cells of the same row also become calcified, and the trans¬ 
verse bars of calcified substance stretch across from one calcareous 
column to the other. Thus there are longitudinal growths of cartilaginous 
cells inclosed in oblong cavities, the walls of which are formed of a 
calcified matrix which cut off all connection between other groups of 
cells, and thus primary areolar spaces are formed. 
During the same time this process is going on in the center of the solid 
bar of cartilage of which the fetal bone consists, certain changes also 
are taking place on its surface. This surface is covered by a vascular 
membrane, the perichondrium, the inner surface of which is in contact 
with the cartilage, and on which are gathered the formative or genetic 
cells, called osteoblasts. These cells form a thin layer of bone tissue 
between the perichondrium and the cartilage. In this first stage of 
ossification there are two processes going on simultaneously: (1) In the 
center of the cartilage the formation of a number of oblong processes, 
and (2) on the surface of the cartilage the formation of a layer of true 
bone. 
Second Stage of Bone Formation 
The second stage in the formation of bone consists of prolongation 
into the cartilage of processes of the osteogenic layer of the perichon¬ 
drium, which has now become periosteum. The processes consist of 
blood vessels and cells, osteoblasts or bone formers and osteoclasts or 
bone destroyers. The latter are similar to the giant cells found in the 
marrow. They excavate passages through the newly formed bony layers 
and grow through into the calcified matrix. 
Wherever these processes come in contact with the calcified walls or 
the primary areolas they absorb them, and thus cause assimilation of 
the original cavity and the formation of large spaces known as the 
medullary spaces. These spaces become filled with embryonic marrow, 
consisting of vessels carrying osteoblasts on their walls, and are derived 
in the manner described above from the osteogenic layer of the priosteum. 
