DEVELOPMENT OF BONE. 135 



the statements made by Henke,* respecting the origin of the 

 cartilage elements at the plane of ossification, according to 

 which even the vertical rows of cells contained in the medul- 

 lary spaces are formed at the expense of blood corpuscles that 

 have escaped from the vessels. 



We have already pointed out that coincidently with the 

 change that occurs in the nature of the material occupying 

 the interspaces of the calcified trabeculse a differentiation of its 

 constituent cells may be observed to take place into an outer 

 and an inner layer. 



The granular cells of the outer layer were first described by 

 Gegenbaur,*f~ who applied the term Osteoblasts to them, whilst 

 he considered the more transparent internal tissue to be the 

 proper medulla in an early stage of development. The osteo- 

 blastic layers are found in the cavities already described 

 (primary medullary spaces), sometimes forming thin and some- 

 times thicker layers, interposed between the remains of the 

 original cartilage on the one hand, and the vascular medulla 

 on the other. 



The formation of the osteoblastic layer constitutes the imme- 

 diate precursor of true osseous tissue. The latter occurs in 

 the walls of the primary medullary cavities, at some distance 

 from the margin of the cartilage, and when first seen con- 

 stitutes a thin, lustrous, highly refractive lamella, in which the 

 peculiar stellate form of the bone corpuscles is already visible. 

 Wherever this tissue is formed, osteoblasts have previously 

 been deposited in layers, and just as in the first instance the 

 remains of the trabeculse of the calcified cartilage, so at a later 

 period the young bony tissue deposited on the surface of these 

 is again covered by a layer of osteoblasts separating it from 

 the medulla. 



All the peculiarities above described, with the above-men- 

 tioned exception of the relation of the cartilage cells to the 

 medulla, may be traced with perfect accuracy. 



It is more difficult to ascertain the relation of osteoblasts to 

 newly formed bone. 



* Zeitschrift fur rationale Medicin, 3 R., Band xviii., p. 61. 

 t Loc. cit., p. 360. 



