HISTOLOGICAL DEMONSTRATIONS. 87 



of bone may be seen growing inwards from the perichon- 

 drium to meet its fellow from the opposite side. Between 

 the two there are irregular medullary spaces filled with cells. 

 Blood-vessels extend into them from the perichondrium 

 through canals in the bone. Rows of cells may be seen in 

 the cartilage near the medullary spaces. 



(H.) The proliferation of the cartilage cells close to the 

 medullary spaces may be readily seen. A single cavity 

 contains a number of cells, and these cavities finally be- 

 come continuous with the medullary spaces into which 

 blood-vessels have grown from the perichondrium. The 

 cartilage matrix is entirely absorbed', as has long been 

 known, but whether the cartilage cells become marrow 

 cells or osteoblasts, or what becomes of them, it would be 

 difficult to say, for one may detect something very like the 

 diapedesis of white blood corpuscles through the walls of 

 the capillaries, and the emigrated blood corpuscles may play 

 some part in the process of forming osteoblasts, as has been 

 suggested by Gegenbaur and others. 



128. (H.) A V. S. of a somewhat older bone of a 

 kitten, prepared as above, shows the rows of cartilage cells, 

 the medullary spaces with their capillaries, and cells around 

 them. The fate of the plates of cartilage matrix between 

 the medullary spaces may be clearly traced. 



The cartilage matrix becomes encrusted with the matrix 

 of the future bone, and slowly disappears. Osteoblasts are 

 gradually enclosed in the bony matrix, and thus become 

 bone cells. It would be difficult to say what is the precise 

 genetic relation between the osteoblasts and the bony matrix, 

 for a thin layer of the matrix may be seen extending up 

 towards the cartilaginous ends of the medullary spaces, 

 without any appearance of osteoblasts in it, in the first in- 

 stance. The exact mode of origin of the osteoblasts in in- 

 tra-cartilaginous formation of bone, must therefore still be 

 held as doubtful. 



129. Marrow (H.) As yellow marrow consists chiefly 

 of fat cells, it need not here be alluded to. Red marrow, 

 however, is worthy of careful study. Take a rib from a 

 rabbit, cat, or dog, not fully grown ; divide it transversely, 



