THE OSSEOUS SYSTEM. 359 



soft, reddish substance — -fatal medulla. This substance at first 

 consists of nothing but a small quantity of fluid and many 

 rounded cells, with one or two nuclei and faintly granular 

 contents, of which I am unable to say how they originate, but 

 only this much, that they are altogether new formations. In 

 process of time these cells, which are in all respects identical 

 with thoscwhich occur, in the adult, in certain bones (vid. supra), 

 are developed in the usual way into connective tissue, blood- 

 vessels, fat-cells, and nerves. The formation of blood-vessels 

 proceeds with great rapidity, so that the bones, very shortly 

 after the development of the medullary spaces, exhibit blood- 

 vessels in them; that of the fat and nerves takes place more 

 slowly, although the latter, at the period of birth, of course 

 with fewer filaments than subsequently, may be very readily 

 perceived in the large cylindrical bones, even more readily than 

 in the adult, because at this time the medulla may be more 

 easily washed away from them and the great vessels. The fat- 

 cells at this period are but few in number; the medulla, in man 

 at least, being coloured entirely red by the blood and the light 

 reddish medulla-cells. After birth they gradually multiply, 

 till at last, the marrow, in consequence of their great increase 

 and the disappearance of the medulla- cells, which are ultimately 

 all transformed into the elementary tissues of the permanent 

 medulla, acquires its subsequent colour and consistence. 



In many of the primarily cartilaginous bones of Birds and 

 Amphibia, the ossification of the cartilage commences, according 

 to Rathke and Reichert (1. c), on the outer aspect of the 

 cartilage, so that at first a cylinder of bone is formed with 

 cartilage internallv and at the extremities. The remainder 

 of the internal cartilage then affords space to the medulla, 

 whilst the epiphyses are formed out of that of the ex- 

 tremities. 



[If the contents of the cartilage-cells, the " cartilage-cor- 

 puscles" of authors, be really surrounded by a membrane, as 

 Virchow supposes, it may be assumed that a similar tunic, 

 analogous to the primordial utricle of the plant-cell (vid. sup. 

 § 8), exists also around the contents of the bone-cells, and that 

 it takes an essential part, by its throwing out stellate processes, 

 in the first formation of the canaliculi, their further elouga- 



