THE SKELETON 409 
day in the femur of the chick. The process extends towards the 
ends, and faster at the periphery of the cartilage (7.e., next to 
the perichondral bone) than in the center. In this way there 
remain two terminal, cone-shaped cartilages, and the ends of the 
cones project into the marrow cavity (Fig. 231 A). 
(3) Calcification of Cartilage. Salts of lime are deposited in 
the matrix of the cartilage at 
the ends of the marrow cavity; 
such cartilage is then removed VEN 
by osteoclasts, large multinu- poze 
cleated cells, of vascular en- 
dothelial origin, according to 
Brachet (seventeenth or eigh- 
teenth day of incubation). 
(4) Endochondral Ossifica- 
tion. Osteoblasts within the 
marrow cavity deposit bone on 
the surface of the rays of cal- 
cified cartilage that remain 
between the places eaten out 
by osteoclasts, and on the 
inner surface of the perichon- 
erie 0852 oats = oft Cort. 
dral bone. Fic. 231 A. — Longitudinal section of 
f le 96 ae) Shae 
These processes gradually the femur of a chick of 196 hours’ in 
cubation; semi-diagrammatic. (After 
extend towards the ends of 
Peat 1 tl Hi Brachet.) 
the bone, and there 1s never art. Cart., Articular cartilage. C.C., 
any independent epiphysial Calcified cartilage. end. B., Endochon- 
Pentortor Gscification anslonc dral bone. M., Marrow cavity. P’ch., 
Saabs Y“o  Perichondrium. P’os., Periosteum. 
bones of birds, as there is in p’os. B., Periosteal bone. Z. Gr., Zone 
of growth. Z. Pr., Zone of proliferation. 
mammals. The ends of the 7 R., Zone of resorption 
ane s : 
bones remain cartilaginous 
and provide for growth in length. Growth in diameter of the 
bones takes place from the periosteum, and is accompanied by 
enlargement of the marrow cavity, owing to simultaneous ab- 
sorption of the bone from within. It is thus obvious that all of 
the endochondral bone is removed from the shaft in course of 
time; some remains in the spongy ends. 
The details of the process of ossification will not be described 
here, and it only remains to emphasize a few points. At a stage 
shortly after the beginning of absorption of the cartilage in the 
