I2O THE TISSUES. 



In the upper portion of figure 87 is observed a zone composed 

 of groups of cartilage-cells, adjoining this a zone composed of 

 columns of vesicular and shrunken cartilage-cells, the nuclei of which 

 are indistinctly seen. These columns are separated by septa and 

 trabeculae of calcified matrix. This zone is followed by one in 

 which the cartilage-cells have disappeared, leaving spaces into 

 which the osteoblasts and small blood-vessels have grown. In cer- 

 tain parts of the figure, the osteoblasts are arranged in a layer on 

 the trabeculae of calcified cartilage, some of which are enveloped 

 in a layer of osseous matrix, less deeply shaded than the darker car- 

 tilage remnants. 



As the development of endochondral bone proceeds from the 

 center of ossification toward the extremities of the diaphysis in the 

 manner described, the primary marrow spaces at the center of ossi- 

 fication are enlarged, a result of an absorption of many of the smaller 

 osseous trabeculae and the remnants of calcified cartilage matrix 

 enclosed by them. In this process are concerned certain large 

 and, for the most part, polynuclear cells, which are differentiated 

 from the embryonic marrow. These are the osteoclasts (bone break- 

 ers) of Kolliker (73). They are 43 // to 91 p. long and 30 jn to 40 // 

 broad, and have the function of absorbing the bone. The spaces 

 which they hollow out during the beginning of the process appear 

 as small cavities or indentations, containing osteoclasts either single 

 or in groups, and are known as Howship's lacuna. All bone 

 absorption goes hand in hand with their appearance. At the same 

 time, the osseous trabeculae not absorbed become thickened by a 

 deposition of new layers of osseous tissue (by osteoblasts), during 

 which process some of the osteoblasts are enclosed in the newly 

 formed bone and are thus converted into bone-cells. In this way 

 there is formed at the center of ossification a primary or embryonic 

 spongy or cancellous bone, surrounding secondary marrow spaces or 

 Havcrsian spaces, filled with embryonic marrow. This process of 

 the formation of embryonic cancellous bone follows the primary 

 ossification from the center of ossification toward the extremities of 

 the diaphysis. It should be further stated, that long before the 

 developing bone has attained its full size indeed, before the end of 

 embryonic life the embryonic cancellous bone is also absorbed 

 through the agency of osteoclasts. The Haversian spaces are thus 

 converted into one large cavity, which forms a portion of the future 

 marrow cavity of the shaft of the fully developed bone. The 

 absorption of the embryonic cancellous bone begins at the center 

 of ossification and extends toward the ends of the diaphysis. 



Some time after the beginning of the process of bone develop- 

 ment at the center of ossification of the diaphysis, centers of 

 ossification appear in the epiphyses, the manner of the develop- 

 ment of bone being here the same as in the diaphysis. Several 

 periosteal buds grow into each center of ossification, filling the 

 irregular spaces formed by the breaking down of the degener- 



