itoX MICROSCOPIC ANATOMY OF THE ORGANS. 



by a layer of flat epithelium. This is to be considered as a 

 serous membrane. Often there extend from the sy no vial mem- 

 brane into the joint cavity the so-called syuovial villi These 

 are found abundantly on the borders of the joint-surfaces, and 

 consist of a connective-tissue axis often containing blood capil- 

 laries and an epithelial covering. The synovial fluid (synovia) 

 contains a few fat droplets and fragments of epithelium broken 

 off from the joint-surfaces. 



(6*) Development of Bones. 



Bony tissue develops later than any other tissue, and arises 

 from some preformed tissue, such as hyaline cartilage or con- 

 nective tissue. In young embryos the future skeleton exists 

 as cartilage or connective tissue. 



(1) Development of Bone from Cartilage. 



In bones which are developed from cartilage the bony 

 tissue is laid down in two different places, either in the interior 

 of the cartilaginous forerunner of the skeleton (endochondral 

 ossification), or on the surface of the cartilage (perichondral 

 ossification, wrongly called periosteal ossification). 



The endochondral bone formation begins with the increase 

 in size and number of cartilage cells through karyokinesis, so 

 that many cells come to lie in each cartilage lacuna (Figs. 193 

 and 194). Certain changes then begin in the homogeneous 

 ground substance of the cartilage. Calcium salts are laid 

 down, so that the ground substance becomes opaque. The 

 cartilage lacunae become large and the cells shrink. Places 

 where such changes have taken place may be quite numerous 

 in a bone, and are known as areas of ossification or calcifica- 

 tion. In the long bones such centres usually appear first in 

 the diaphysis. 



While this process is going on inside the cartilage certain 

 changes take place on its outer surface. In the deeper cel- 

 lular layers of the perichondrium an ossification (perichondral 

 o i(i<-;ition) begins. These layers of perichondral cells, richly 

 supplied with blood-vessels, are known as osteogenous tissue. 



