THE CONNECTIVE TISSUES. 121 



ated cartilage-cells. Osteoblasts are arranged in rows on the 

 trabeculae of cartilage thus formed, which they envelop in osseous 

 tissue. As development proceeds, the primary osseous tissue is 

 converted into embryonic cancellous bone as above described. 



In the development of the epiphyses, as in the development of 

 the smaller irregular bones, the formation of bone proceeds from 

 the center or centers of ossification in all directions, and not only 

 in a direction parallel to the long axis of the bone as described for 

 the diaphysis. The epiphyses grow, therefore, in thickness as well 

 as in length, by endochondral bone-development. 



There remains between the osseous tissue developed in the dia- 

 physis and that in the epiphyses, at each end of the diaphysis, a zone 

 of hyaline cartilage in which ossification is for a long time delayed ; 

 this is to permit the longitudinal growth of the bone. These layers of 

 cartilage constitute the epiphyseal cartilages. Here the periosteum 

 (perichondrium) is thickened and forms a raised ring around the 

 cartilage. As it penetrates some distance into the substance of the 

 cartilage, the latter is correspondingly indented. '(Fig. 86.) The im- 

 pression thus formed appears in a longitudinal section of the bone 

 as an indentation, the ossification groove (encoche d 'ossification, 

 Ranvier, 89). That portion of the perichondrium filling the latter 

 is called the ossification ridge. The relation of the elements of the 

 perichondrium to the cartilage in the region of the groove just 

 described is an extremely intimate one, both tissues, perichondrium 

 and cartilage, merging into each other almost imperceptibly. It is 

 a generally accepted theory that so long as the longitudinal growth 

 of the bone persists, new cartilage is constantly formed at these 

 points by the perichondrium. In the further production of bone 

 this newly developed cartilage passes through the preliminary 

 changes necessary before the actual commencement of ossification 

 /'. e., it goes through the stages of vesicular cartilage and the 

 formation of columns of cartilage-cells, in place of which, later, the 

 osteoblasts and primary marrow cavities develop. 



By the development of new cartilage elements from the encoche 

 the longitudinal growth of the bone is made possible ; at the same 

 time, those portions of the cartilage thus used up in the process of 

 ossification are immediately replaced. (Fig. 88.) 



The following brief summary of the several stages of endochon- 

 dral bone-development may be of service to the student : 



1. The embryonic cartilage develops into hyaline cartilage, 

 beginning at the centers of ossification. 



2. The cartilage-cells enlarge and become vesicular. In the 

 diaphysis of long bones such cells are arranged in quite regular 

 columns, while in the epiphyses and irregular bones this arrange- 

 ment is not so apparent. 



3. Calcification of the matrix ensues ; the cartilage-cells disap- 

 pear (degenerate) ; primary marrow spaces develop. 



4. Ingrowth of periosteal buds. The osteoblasts are arranged 



