228 MAMMALIAN DESCENT. [Lkct. IX. 



("osteoblasts") by tlio deposit in them of jiliospliati' of lime. Ami 

 this takes place l)y a vegetative; process, anterior to the time -when 

 proper animal functions are exercised. This little ring made; of 

 a paste, half eartli and lialf protoplasm, grows larger, and lic- 

 comes a sheath; then another, larger than it, is formed over it, and 

 so on, sheath over sheath. Thus the hone grows e.axjcnduxhiy 

 like the trunk of the oak, the bark of fibrous tissiie yielding 

 fresh and fresh mat(!rials ; this fibrous layer now takes the name of 

 " periosteum," or th(> covering of the bone. When the bone is 

 sciircel}- one fourth the length it attains in the adult, the cartilage on 

 which the l)one was formed degenerates into a lower kind of tissue,, 

 and is partly absorbed ; the cells tliat remain become red, and after- 

 wards they luidergo a further degradation into yellow marrow-cells. 

 This degradation of the inner part of the rod is followed, after a time, 

 Ijy the al)Sorption of the earlier rings of bony deposits, so that the- 

 ca\'it3' for the marrow grows larger and larger. All this is doni; 

 without detriment to the sti'ong pillar of bone; it is a living pro- 

 cess, and although there is degeneration of tissue, it is not like the 

 hollowing out of an old tree trunk. After a time the concentric 

 growth of bony rings or tidies is supplemented by bony deposits in tlu- 

 top, the "trochanter," and the base of the bone; in the uiiper and lower 

 parts, the cartilage itself becomes transformed into a new and mori> 

 solid tissue, and the new bony substance, after a further lapse of tinu', 

 becomes excavated into small galleries, so as to make the ends of tlu^ 

 bone spongy. All the cartilage is not so transformed ; at the end of 

 the bone its remains are seen as a \)ni\ or buffer ; so important in the- 

 pressure of the body during the motion of the joints. This account^ 

 meagre as it is, must serve ns here as typical of Avliat takes place 

 when cartilage is transformed into bone ; it will do duty equally Avell 

 for the "femur" of all the other INIammals — with the exceptiim of 

 the Ornitliorhynchus and Echidna. 



The bones that form the roof and most of the side walls of the- 

 skull, and the outermost bones of the ui)per face, are formed by trans- 

 formation of a mere web of librons tissue without any pre-existing 

 cartilage. But the whole skeleton, with its various cartilages, bones, 

 joints, ligamentous bands, and the like, is l)ut the rougher and coarser 

 part of this organic building. 



The furniture of the iippt-r chand)er, its crypts, and cells, and 



