94 STUDIES IN ADVANCED PHYSIOLOGY. 



developed directly from a membrane. A typical example 

 of such a membrane bone is the parietal bone of the 

 skull. 



Growth of Cartilage Bones. Cartilage bones are those 

 bones which are preceded by cartilage which has been 

 removed and, later, bone deposited in its place, or as com- 

 monly stated in our text-books, bones which ossify from 

 cartilage. If the limb of a young animal be examined in 

 embryonic life even before the cartilages have made their 

 appearance, the following series of changes may easily be 

 noticed: 



In earliest life such a limb budding out from the body 

 would consist throughout of perfectly similar cells, the 

 original descendants of the primitive egg cell. Soon after, 

 among many other changes, there might be noticed, in the 

 place where the cartilage is to appear, to form, say the 

 humerus, a number of cells similar to the others in appear- 

 ance, which, however, begin to secrete between themselves 

 a substance familiar to us as cartilage. In this cartilaginous 

 matrix these cells, which we may now call cartilage cells or 

 chondrioblasts, multiply and by a continued secretion from 

 these the matrix of the cartilage family results. As this 

 cartilage is plastic, and may extend by interstitial growth, 

 it soon comes to possess the form intended for the future 

 bone. A membrane soon invests this cartilage except at the 

 ends, which membrane will become the future periosteum, 

 which now, as it surrounds cartilage, is called the peri- 

 chondrium. 



This membrane becomes filled with blood-vessels, and 

 underneath it, that is between it and the cartilage, there 

 come to lie numerous little corpuscles, the osteoblasts of 

 the future bone. At this stage of the process no bone is yet 

 present, there being but a solid cartilage rod of the form of 

 the intended humerus, surrounded by the membrane. Soon 

 after this the process commonly called the process of ossi- 

 fication begins. 



