50 



GENERAL ANATOMY. 



somewhat granular. Lying below this cartilaginous layer (i. e., nearer to the 

 centre of ossification), is a layer, b, consisting of large round clear cells (the 



Fig. 20. 



Longitudinal section through the ossifying portion of a long bone in the human embryo, a. Cartilaginous 

 region, b. Region of the round clear cells, g. Region of the dark granular cells. 



" osteoblasts" of some anatomists), with granular contents, also arranged in 

 somewhat parallel rows, each row and each pair of superimposed cells being 

 separated by a transparent cartilaginous matrix the arrangement being com- 

 pared by Rollett to a ladder. In the lower part of this region the matrix is 

 encroached upon by calcareous matter, so that if a transverse section be made 

 here, rings of dark granular calcareous deposit are seen inclosing the large 

 round clear cells. As the section is taken deeper and deeper into the ossifying 

 part, the calcified rings or areolse are seen to inclose numerous smaller granular 

 masses ("primitive marrow" of some authors) which have replaced the single 

 clear cells, and may be formed by the proliferation of those cells. This, how- 

 ever, is doubted by Rollett and others, who believe that these masses are fur- 

 nished by the underlying periosteal vessels. In the longitudinal section (Fig. 

 20) these masses are seen at g to succeed very suddenly to the separate clear 

 cells. If they are detached from the surface, they are found to have one or 

 more processes. Deeper down in the ossifying or ossified portion, bloodvessels 

 are met with, which proceed from the periosteum. 



The next step in the process is that the above-described areolaa ("primary 

 areolas") break into each other, so as to give rise to the "secondary areoloe," or 

 medullary spaces of H. Miiller. These spaces are filled with the red or foetal 

 marrow above described. The cells of this marrow appeared to be furnished 

 directly from the bloodvessels which are abundantly supplied to these spaces 



