120 REMINISCENCES OF 



saying anything of the results of these investigations, 

 it will be well to give some preliminary explanation, 

 and this explanation is the more desirable since 

 biological science has just undergone a most 

 remarkable development, which has placed most 

 students of life and organisation in a wholly new 

 position. 



In the very youngest state of all the mammalia, 

 including man, almost every bone is represented by 

 a cartilage, or what is familiarly known as gristle. 

 This cartilage consists of a more or less flexible and 

 structureless substance, in which are lodged numerous 

 clusters of minute hollow spheres, known as cells, 

 and each of these undeveloped bones is invested by 

 a closely adherent thin layer of membrane known, 

 in the earliest stages of growth when the future 

 bone is still cartilaginous, as " perichondrium," but 

 later on, when hard bony matter becomes formed, as 

 " periosteum." The first earthy matter deposited in 

 the bone accumulates in the structureless parts of 

 the cartilage between the clusters of cells. This 

 first process of ossification is soon followed by 

 another, in which the thin membrane referred to as 

 " perichondrium " also becomes hardened by phos- 

 phate of lime being deposited in its substance. 



This perichondrium is now replaced by a new layer 

 of membrane like its predecessor, but which, having 

 now a layer of true bone beneath it, is designated 

 " periosteum." I have called these two forms of 

 bony matter by names which I shall directly have to 



