410 THE DEVELOPMENT OF THE CHICK 
center of the shaft, the perichondral bone is invaded by capillary 
vessels and connective tissue that break through into the cavity 
formed by absorption; it 1s supposed by many that osteoblasts 
from the periosteum penetrate at the same time. The marrow 
of birds is derived, according to the best accounts, from the 
original cartilage cells, which form the fundamental substance, 
together with the intrusive blood-vessels and mesenchyme. The 
endochondral osteoblasts are believed by some to be of endo- 
chondral origin (7.e., derived from cartilage cells), by others of 
periosteal origin. For birds, the former view seems to be the 
best supported. | 
In birds. calcification does not precede absorption of the 
‘artilage, as it does in mammals, until the greater part of the 
marrow cavity is formed. The cones of cartilage, referred to 
above, that are continuous with the articular cartilages, are 
absorbed about ten days after hatching. 
On the whole, perichondral ossification plays a more extensive 
role in birds than in mammals. The endochondral bone forma- 
tion begins relatively much later and is less extensive. The 
bodies of the vertebrae, which ossify almost exclusively in an 
endochondral fashion, form the main exception to this rule. 
Ossification in membrane proceeds from bony spicules de- 
posited between the cells in the formative center of any given 
membrane bone. It spreads out from the center, the bony 
spicules forming a network of extreme delicacy and beauty. 
After a certain stage, the membrane bounding the surface becomes 
a periosteum which deposits bone in dense layers. Thus a mem- 
brane bone consists of superficial layers of dense bone, enclosing 
a spongy plate that represents the primitive bone before the 
establishment of the periosteum. 
The formation of bones proceeds from definite centers in all 
three stages of their formation; thus we have centers of mem- 
brane formation, centers of chondrification and centers of ossifi- 
cation. Membranous centers expand by peripheral growth, 
cartilage centers expand by the extension of cartilage formation 
in the membrane from the original center of chondrification, and 
bony centers expand in the original cartilage or membrane. 
Several centers of chondrification may arise in a single primitive 
membranous center; for instance, in the membranous stage, the 
skeleton of the fore-limb and pectoral girdle is absolutely con- 
