208 THE ORGANS 



down layer within layer of bone until the Haversian space is reduced to a mere 

 channel, the Haversian canal. In this way are formed the Haversian canals 

 and the Haversian systems of lamella. Some of the interstitial lamellae are the 

 remains of the spongy bone which was not quite removed in the enlargement of 

 the primary marrow spaces to form the Haversian spaces; other interstitial 

 lamcllic appear to be early formed Haversian lamellae which have been more or 

 less replaced by Haversian lamellae formed later. 



While these varieties of ossification have been described, we would emphasize 

 the essential unity of the process. The likeness between intramembranous and 

 subperiosteal ossification has been already noted. The differences observed in 

 intracartilaginous ossification are more apparent than real. In intracartilagi- 

 nous ossification the bone is developed in cartilage but not from cartilage. As 

 in intramembranous and in subperiosteal ossification, intracartilaginous bone 

 is developed /row osteogenetic tissue. This osteogenetic tissue is a differentiation 

 of embryonal connective tissue, in this case carried into the cartilage from the 

 periosteum in the periosteal buds. In intramembranous ossification the bone 

 is developed within and directly from the embryonal connective tissue of which 

 the membrane is composed. In intracartilaginous ossification there is the same 

 embryonal connective-tissue membrane, but within this membrane the form of the 

 bone is first laid down in embryonal cartilage. Surrounding the cartilage there 

 remains the embryonal connective tissue of the membrane, now perichondrium. 

 It is from tissue which grows into the cartilage from this m.emhra,ne— embryonal 

 connective tissue — that the bone, although developed in cartilage, is formed. 



Marrow develops from the mesenchymal tissue which enters the cartilage 

 anlage in the periosteal buds, at the beginning of ossification. 



Growth of Bone 



The growth of intramembranous bone by the formation of successive layers 

 beneath the periosteum has been already described (page 204). 



Intracartilaginous bones grow both in diameter and in length. 



Growth in diameter is accomplished by the constant deposition of new layers 

 of bone beneath the periosteum. During this process, absorption of bone from 

 within by means of osteoclasts leads to the formation of the marrow cavity. The 

 hard bone of the shaft of a long bone is entirely of subperiosteal origin, the intra- 

 cartilaginous bone being completely absorbed. 



Growth in length takes place in the following manner: Some time after the 

 beginning of ossification in the shaft or diaphysis, independent ossification 

 centres appear in the ends of the bone (epiphyses). So long as bone is growing, 

 the epiphyses and diaphysis remain distinct. Between them lies a zone of grow- 

 ing cartilage, the epiphyseal or intertnediate cartilage. Increase in length of the 

 bone takes place by a constant extension of ossification into this cartilage from 

 the ossification centres of the epiphyses and diaphysis. After the bone ceases to 

 grow in length, the epiphyses and diaphysis become firmly united. 1'! 



TECHNIC 



(i) Developing Bone — Intramembranous. — Small pieces are removed from 

 near the edge of the parietal bone of a new-born child or animal. These pieces 



