Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 121 



is a good example of a membrane bone. The first sign of ossifi- 

 cation here is the formation of a network of minute bars or 

 spicules. The bars are fibrous, and composed of osteogenetic fibres, 

 which are at first soft and pliant, but become hardened by the 

 deposition within them of calcareous salts. Between the fibrous 

 bundles are large fusiform cells, the corpuscles. These cells 

 are instrumental in forming the bone, whence they have been 

 termed osteoblasts. As ossification proceeds some of these cells 

 become completely surrounded by the osseous tissue, and thus 

 give rise to the bone corpuscles, each lying in a hollow space or 

 lacuna. 



Meanwhile the bone has become invested by a definite con- 

 nective tissue membrane, the periosteum. The inner layer of 

 this (osteogenetic layer) is largely composed of osteoblasts, which 

 deposit successive lamellae of bone on the surface of the growing 

 parietal. At the same time lamellae are being deposited in the 

 meshes of the original network of minute fibrous bars. Thus 

 the interstices of the network are converted into narrow channels 

 which contain blood-vessels. And thus the whole bone is con- 

 verted into a more or less compact structure. Finally, as the 

 bone increases in thickness, its middle layer becomes hollowed 

 out by absorption of the bony matter, so as to become spongy 

 in structure. This internal spongy bone is called the diploe. 



We may take the femur of the rabbit as our example of a 

 cartilage bone. In a still young rabbit this bone consists of a 

 shaft (diaphysis) containing a central space in which is lodged 

 the marrow. At each end, separated from the shaft by a carti- 

 laginous interspace, is a terminal ossification (or epiphysis). In 

 the adult rabbit the epiphyses coalesce with the shaft by ossifi- 

 cation of the intermediate cartilage. 



In the embryo the minute femur consists of a rod of foetal 

 cartilage invested by a sheath of vascular perichondrium. Within 

 the substance of the cartilage the cells arrange themselves in 

 columns near the growing ends, and in groups in the more 

 central portions. In the groups, and at the deeper ends of the 

 columns, the cell cavities are seen to be enlarged by the swelling 



