228 MAMMALIAN DESCENT. [Lect. IX. 



(" osteoblasts ") by the deposit in them of phosphate of lime. And 

 this takes place by a vegetative process, anterior to the time when 

 proper animal functions are exercised. This little ring made of 

 a paste, half earth and half protop)lasm, grows larger, and be- 

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

 so on, sheath over sheath. Thus the bone grows exogenoiidii, 

 like the trunk of the oak, the bark of fibrous tissue yielding- 

 fresh and fresh materials ; this fibrous layer now takes the name of 

 " periosteum,'' or the covering of the bone. When the bone is 

 scarcely one fourth the length it attains in the adult, the cartilage on 

 which the bone was formed degenerates into a lower kind of tissue, 

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

 wards they undergo a further degradation into yellow marrow-cells- 

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

 by the absorption of the earlier rings of bony deposits, so that the 

 cavity for the marrow grows larger and larger. All this is done 

 without detriment to the strong 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 tnmk. After a time the concentric, 

 growth of bony rings or tubes is supplemented by bony deposits in the 

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

 parts, the cartilage itself becomes transformed into a new and more 

 solid tissue, and the new bony substance, after a fm-ther lapse of time, 

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

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

 the bone its remains are seen as a pad or buffer ; so important in the- 

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

 meagre as it is, must serve us here as typical of what takes place 

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

 for the " femur " of all the other Mammals — with the exception of 

 the Ornithorhynchus and Echidna. 



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

 skull, and the outermost bones of the upper face, are formed by trans- 

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

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

 joints, ligamentous bands, and the like, is but the rougher and coarser 

 part of this organic building. 



The furniture of the upper chamber, its crypts, and cells, and 



