228 MAMMALIAN DESCENT. [Lixr. IX. 



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

 this takes place by a vegetative process, anterior to the time \vlu-n 

 proper iinimnl functions are exercised. This little ring made of 

 a paste, half earth and half protoplasm, grows larger, and !>- 

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

 so on, sheath over sheath. Thus the bone grows c.,-/,,/, //>>*/</, 

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

 fresh and fresh materials; this fibrous layer now takes the name <>t 

 " periosteum," or the covering of the bone. When the bone i> 

 srarc.oly 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 ] artly 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 thin-, 

 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 trunk. After a time the concent in- 

 growth of bony rings or tubes is supplemented by bony deposits in tin- 

 top, the "trochanter," and the base of the bone; in the upper and lowi-r 

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

 solid tissue, and the new bony substance, after a further 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 >f 

 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 Ormthorhynchus 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 tr; in- 

 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 coarse i 

 part of this organic building. 



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



