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 is 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 (i.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 

 cartilage, 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- 



