272 



BONE OR OSSEOUS TISSUE. 



OSSIFICATION IN CARTILAGE. 



It has already been stated that, in by far the greater number of bones the 

 mesodermic tissue with closely packed cells, of which they originally consist, is 

 very quickly succeeded by cartilage, in which the ossification begins. One of the 

 long bones taken from a very small embryo, just before ossification has commenced 



Fig. 310. HUMERUS OF A 



FOJTUS, NATURAL SIZE. 



(Sharpey. ) 



The upper part is divided 

 longitudinally, a, cartilage, 

 6, line of junction of bone 

 and cartilage. The perios- 

 teal bone looks lighter than 

 the endochondral bone pro- 

 per. 



Fig. 311. SECTION OF PHALANGEAL BONE OP HUMAN FCETUS, AT THE TIME OF COMMENCING OSSIFICA- 

 TION. Photographed from a preparation by Mr. F. A. Dixey. Magnified about 75 diameters. 

 (E. A. S.) 



The cartilage cells in the centre are enlarged and separated from one another by dark-looking calcified 

 matrix ; im, layer of bone deposited underneath the periosteum ; o, layer of osteobJasts by which this 

 layer has been formed. Some of the osteoblasts are already embedded in the new bone as lacunae. The 

 cartilage-cells are becoming enlarged and flattened and arranged in rows above and below the calcified 

 centre. At the ends of the cartilage the cells are small and the groups are irregularly arranged ; the 

 fibrous periosteum is not sharply marked off from the cartilage. 



in it, is observed to be distinctly cartilaginous. In the tibia of a sheep, for example, 

 at a time when the whole embryo is not more than an inch and a quarter in length, 

 we can plainly see that the substance consists of cartilage-cells imbedded in a 

 pellucid matrix. These cells can scarcely be said to be collected into groups, and 

 are very irregular in size and shape. They become enlarged in the middle part of 



