30 ANATOMY OF THE RABBIT. 
The way in which bones are formed on a cartilage basis serves to 
explain many peculiarities of the adult skeleton. In the embryonic 
condition the cartilage rudiments are associated to form a com- 
plete but primitive skeleton. In many cases the replacement of 
these elements by bone is not direct, certain readjustments being 
necessary, both for purposes of growth and on account of the much 
more special functional requirements of the adult skeleton. 
In the embryonic condition the cartilage rudiments are en- 
closed by a connective tissue sheath, equivalent to the periosteum 
of a bone, but described as the perichondrium. The osteoblasts 
of this layer are concerned*with the formation of bone material, 
both in the interior (endochondral bone) and on the surface 
(intramembranous bone). The formation of endochrondral 
bone is preceded by certain changes 
tr. ma. which take place in the interior of the 
cartilage. In the latter, in certaifi 
J areas, known as the centres of ossifi- 
cation, the matrix becomes partly 
trnmi. dissolved, the cells enlarged and 
ultimately broken down. These 
changes are associated with a deposi- 
tion of calcareous material, or calci- 
Fic. 11. Outline sketch of the prox- > : 3 
imal end of the femur of a young fication, by which the portion of the 
animal: c.f., principal epiphysis for the a : 6 
head of the femur. The accessory Cartilage undergoing transformation 
fers (erat ond chia. (er ty cee is temporarily strengthened. Into 
at this, “area? the -active ‘cells, .onaerne 
perichondrium are carried through the agency of vascular in- 
growths, the periosteal buds, and the result of their presence is 
the deposition of bone material in association with the remaining 
portions of the matrix. This condition is partly illustrated in the 
distal epiphysis of the humerus shown in Fig. 13, A, the figure being 
from a vertical section of the elbow-joint of a four-day-old rabbit. 
In the long bones the formation of the first or main centres of 
ossification takes place in the shaft, and there are formed afterwards 
accessory or epiphysial centres for the extremities. A divided 
extremity, such as the proximal end of the femur (Fig. 11), may 
possess several such centres—a principal one for the chief epiphysis 
or actual extremity of the bone and several subsidiary centres for 

