BONE 295 



constitute the perichondrium. When cartilage is replaced by bone, the perichondrium becomes 

 the periosteum. 



In hyaline cartilage the matrix remains hyaline. In fibro-cartilage the fibrillations of the 

 primitive ectoplasm are converted into white fibers. In elastic cartilage yellow elastic fibers 

 are formed in the hyaline matrix, according to Mall; before the hyaline matrix is differentiated, 

 according to Spalteholz. Most of the bones of the skeleton are laid down first in the form of 

 cartilage. Later, this is gradually replaced by the development of bone tissue. 



BONE 



Bone is a tissue appearing relatively late in the embryo. There are de- 

 veloped two types, the membrane bones of the face and cranium and the cartilage 

 bones which replace the cartilaginous skeleton. Cartilage bones are not simply 

 cartilage transformed into bone by the deposition of calcium salts, but represent 

 a new tissue which is developed as the cartilage is destroyed. 



Membrane Bone. The bones of the face, the parietals, frontals and parts 

 of the occipital, temporals and sphenoid are not preformed as cartilage; the man- 

 dible is developed around a pair of cartilages (of Meckel). The form of a mem- 

 brane bone is determined by the development of a periosteal membrane from 

 the mesenchyma. The bone matrix is differentiated within the periosteum from 

 enlarged cells, the osteoblasts (bone-formers) . Osteoblasts appear in clusters and 

 from their cytoplasm is differentiated a fibrillated ectoplasmic matrix like that 

 which precedes the formation of connective tissue and cartilage (Fig. 286 A). 

 This fibrillated matrix, by a chemical change apparently, is converted into a 

 homogeneous bone matrix, which first takes the form of spicules. The spicules 

 coalesce, form a network of bony plates and constitute the bone matrix upon the 

 surfaces of which osteoblasts are arranged in a single layer like the cells of an 

 epithelium (Fig. 286 B). These cells may be cuboidal, columnar or flatten out as 

 bone formation ceases. As the matrix of the bone is laid down, osteoblasts 

 become enclosed and form bone cells. The bone cells are lodged in spaces termed 

 lacuna. These are connected by microscopic canals, the canaliculi, in which 

 course delicate cell processes and anastomose with those of neighboring cells. 



The plates of the spongy membrane bone are formed about blood-vessels 

 as centers. As the bone grows at the periphery, the bone matrix is resorbed 

 centrally. At this time large multinucleated cells (43 to 91 /x long) appear upon 

 the surfaces of the bone matrix. These cells are known as osteoclasts (bone- 

 destroyers). There is, however, no positive evidence that the osteoclasts are 

 active in dissolving the bone. They may be interpreted also as degenerating 

 osteoblasts. The cavities in which they are frequently lodged are known as 



