THE CONNECTIVE TISSUES. 



I2 3 



Intrarnembranous bone first appears in the form of a thin lamella 

 of bone, which increases in size and thickness by the formation of 

 trabeculse about the edges and surfaces of that previously formed 

 and in the manner above described. A layer of intramembranous 

 bone thus surrounds the endochondral bone in bones preformed in 

 hyaline cartilage. The two modes of ossification may, therefore, 

 be observed in either a cross or a longitudinal section of a develop- 

 ing bone preformed in hyaline cartilage. In such preparations 

 the endochondral bone can be readily distinguished from the intra- 



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Fig. 89. Section through the lower jaw of an embryo sheep (decalcified with picric 

 acid) ; X 3' At # and immediately below are seen the fibers of a primitive marrow 

 cavity lying close together and engaged in the formation of the ground- substance of the 

 bone, while the cells of the marrow cavity, with their processes, arrange themselves on 

 either side of the newly formed lamella and functionate as osteoblasts. 



membranous bone by reason of the fact that remnants of calcified 

 cartilage matrix may be observed in the osseous trabeculae of the 

 former. It will be remembered that these osseous trabeculae de- 

 velop about the calcified cartilage matrix remaining after the dis- 

 appearance of the cartilage -cells. In figure 90, which shows a 

 cross-section of a bone from the leg of a human embryo, these facts 

 are clearly shown. A study of this figure shows the endochondral 

 bone, with the remnants of the cartilage matrix (shaded more 



