1O STRUCTURE. AND FORMATION OF BONE. 
fills the cavities of tubular bones; the latter occurs at 
the ends of tubular bones, and in the spongy bone 
substance. There are also present in the marrow a 
series of various sized. miulti-nucleated cells, the 
myeloplaxes of Robin; which, according to Kolliker, 
are concerned with the process of bone absorption, 
while other authors regard them as entering into the 
formation of blood corpuscles. 
Formation and Growth of Bone.— Bone is de- 
veloped in two ways, viz. in the cartilage of the 
embryo, or directly from the osteogenetic layer of the 
periosteum ; these two methods are known respectively 
as intracartilaginous or endochondronal and intra- 
membranous or periosteal. 
(he futurey skeleton; of the embryo. ariseseas) a 
series of mesoblastic cells; these undergoing certain 
modifications, and becoming cartilaginous, and later 
impregnated with calcareous matter. It has been 
pointed out, however, that bone arises by another 
mode, distinct from the skeleton of the embryo, and 
atua later period.o Ite will, theretore;-benwell? to girst: 
consider that first formed in the embryo. 
1. Intracartilaginous or endochondronal ossifi- 
cation. The ossification of a cartilage,such for instance 
as that: which in the adult will form the humerus; 
commences in the middle, the ossification gradually 
proceeding towards the ends. 
The cells in this central portion of cartilage first 
