254 CONNECTIVE TISSUE. 



DEVELOPMENT OF BONE FROM FIBROUS CONNECTIVE TISSUE. 



A great part of the skeleton grows from fibrous connective 

 tissue, independently of preformed cartilage. The strict distinc- 

 tion which in former times was made between "cartilaginous" 

 and "covering 7 ' bones (see page 237) can be upheld only to a 

 certain extent. It is true that in the production of covering 

 bones no preformed cartilage participates, yet all the other 

 bones of the skeleton, including the flat, the short, and the shaft 

 bones, obtain only a part viz., their cancellous portion, from 

 preformed cartilage. Their cortical or compact portion is formed 

 entirely from periosteum. Many observers have noticed, in dif- 

 ferent places beneath the periosteum, cartilaginous layers ; but 

 no importance need be attached to the presence of such a layer, 

 as the formation of bone-tissue is materially similar, whether it 

 is developed, from cartilage or from fibrous connective tissue. 



In the shaft-bones of human embryos ten to twelve weeks old, 

 when no trace of calcification is yet noticeable, the cartilage is 

 found to be surrounded by a delicate fibrous investment, the 

 .perichondrium, between which and the surface of the cartilage 

 there is a broad layer of medullary tissue. From the latter 

 tissue arises the future cortex, simultaneously with the calcifica- 

 tion and ossification of the cartilage in the center. At the four- 

 teenth week there is already a distinct cortex around the 

 diaphysis, at the same time when the transformation of the 

 cartilage into medulla and cancellous bone has also commenced 

 in the central portion of the diaphysis. The cortex and the 

 calcified cartilage occupy about the same height, the middle por- 

 tion always being the starting-point for their formation. Both 

 portions, however, at first exhibit the cancellous structure, while 

 the formation of regular lamellae begins to appear much later. 



The first-formed cancellous bone of the cortex has a striated 

 structure agreeing with the spindle-shaped territories of the 

 mother-tissue the periosteum. The first-formed cancellous 

 bone of the center, on the contrary, is composed of globular ter- 

 ritories corresponding to the globular territories of the mother- 

 tissue the cartilage. It is only on this ground that we are able 

 to understand the difference in the structure of the earliest- 

 formed bones, the formation of "perforating fibers/' and the 

 vascular system of the cortex. The law, however, according to 

 which the peripheral lamellae and the Haversian lamellae are 

 formed is not known. Probably there are three main layers in 



