I 1 6 THE TISSUES. 



(b) Development of Bone. Nearly all the bones of the adult 

 body are, in the earlier stages of embryonic life, preformed in embry- 

 onic cartilage. As development proceeds, this embryonic cartilage 

 assumes the character of hyaline cartilage, its cells becoming vesic- 

 ular, and probably disappearing. In the matrix, however, there 

 are formed spaces that are soon occupied by cells and vessels which 

 grow in from a fibrous-tissue membrane (the future periosteum) sur- 

 rounding the cartilage fundaments of the bones. These cells deposit 

 a bone matrix in the cartilage spaces. Bone developed in this man- 

 ner is known as endochondral or intracartilaginous bone. In certain 

 bones namely, those of the vault of the skull and nearly all the 

 bones of the face there is no preformation in cartilage, these bones 

 being developed from a connective -tissue foundation. They are 

 known as intramembranous bones. As will become evident upon 

 further discussion of the subject, the formation of fibrous-tissue 

 bone (intramembranous) is not confined to bones not preformed in 

 cartilage. In bones preformed in cartilage, fibrous-tissue bone de- 

 velops from the connective-tissue membrane surrounding the carti- 

 lage fundaments, the two types of bone-development going on simul- 

 taneously in such bones. Attention may further be drawn to the 

 fact that nearly all endochondral bone is absorbed, so that the 

 greater portion of all adult bone, even that preformed in cartilage, 

 is developed from a foundation of fibrous tissue. The two modes 

 of ossification endochondral or intracartilaginous and intramem- 

 branous even though appearing simultaneously in the majority of 

 bones, will, for the sake of clearness, be discussed separately. 



I . Endochondral Bone-development. The cartilage that forms 

 the fundaments of the bones preformed in cartilage has at first the 

 appearance of embryonic cartilage, consisting largely of cells with 

 a small amount of intercellular matrix. These fundaments are sur- 

 rounded by a fibrocellular membrane the perichondrium. Ossifi- 

 cation is initiated by certain structural changes in the embryonic 

 cartilage, in one or several circumscribed areas, known as centers of 

 ossification. In the long bones a center of ossification appears in the 

 middle of the future diaphysis. In this region the intercellular 

 matrix increases in amount and the cells in size ; thus the embry- 

 onic cartilage assumes the character of hyaline cartilage. This is 

 followed by a further increase in the size of the cartilage-cells, at 

 the expense of the thinner partitions of matrix separating neighbor- 

 ing cells, while at the same time lime granules are deposited in the 

 matrix remaining. During this stage the cells appear first vesicu- 

 lar, distending their capsules, then shrunken, only partly filling the 

 enlarged lacunae. They stain less deeply, and their nuclei show 

 degenerative changes. The center of ossification, in the middle of 

 which these changes are most pronounced, is surrounded by a zone 

 in which these structural changes are not so far advanced and which 

 has the appearance at its periphery of hyaline cartilage. 



Simultaneously with these changes in the cartilage, a thin layei 



