THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



173 



sents a modification of the "ground substance" of the original embryonic 

 tissue. The fibers in the matrix are probably derived from the cells in the 

 same manner as the fibers in the fibrillar forms of connective tissue (Fig. 150). 

 Osseous Tissue. Here again the basis for development is embryonic 

 connective tissue, although in one type of development cartilage precedes the 

 bone. Two types of ossification are recognized intramembranous and intra- 

 cartilaginous or endochondral. In intramembranous ossification calcium salts 

 are deposited in ordinary embryonic connective tissue. In intracartilagi- 



Capillary 



Embryonic 

 connective tissue 



Arteriole 



FIG. 149. Developing fat from subcutaneous tissue of pig embryo 5 inches long. Fat (stained 

 black) developing in embryonic connective tissue cells. At the right are five individual cells 

 showing stages of development from an embryonic cell to an adult fat cell. 



nous ossification hyalin cartilage first develops in the same general shape as 

 the future bone and the calcium salts are afterward deposited within the mass of 

 cartilage. It is customary to speak also of another type of ossification sub- 

 periosteal in which the calcium salts are deposited under the periosteum. 



INTRAMEMBRANOUS OSSIFICATION. 



This is the type of ossification by which many of the flat bones of the skull 

 and face are formed. The region in which these bones are to develop consists 

 of embryonic connective tissue. At certain points in this region bundles of 

 connective tissue fibers become impregnated with calcium salts. Such areas are 

 known as calcification centers. In each of these areas the cells increase in num- 

 ber, the tissue becomes very vascular and some of the cells, becoming more or 

 less round or oval, with distinct nuclei and a considerable amount of cytoplasm, 



