60 THE CELL. 



at intervals throughout the matrix. In some such cases the cells 

 possess distinct individual membranes in addition to the matrix 

 or rather the cell-membranes consist of two layers, the inner being 

 distinct, and the outer being fused with the corresponding layer 

 of other cell-membranes to form the matrix ; and filially, after or 

 during the formation of the matrix, various deposits, often 

 mineral, may take place within it (p. 18). 



The facts just described are illustrated by the development of 

 cartilage and bone. Young cartilage (as well as certain forms of 



O O C7 N 



the adult tissue) consists of rounded cells, surrounded by thin 

 membranes, called the capsules, composed of a soft translucent 

 substance (Fig. 30, A). As the tissue develops the cells continu- 

 ally multiply by successive division into two, four, eight, etc., and 

 at each division the daughter-cells surround themselves with new 

 capsules within the original one. The older capsules increase in 

 thickness and fuse more or less completely together to form the 

 matrix, the last-formed capsules alone remaining distinct. In 

 many kinds of cartilage the lines of fusion between the capsules 

 disappear, though in some cases they can be brought into view by 

 the use of suitable reagents. The process just described is illus- 

 trated by Fig. 30, which represents the origin of four cartilage- 

 cells from a single mother-cell, and shows the primary, secondary, 

 and tertiary capsules. 



Many bones consist at first of cartilage like that shown in 

 Figs. 6 and 30. Solid calcareous salts are afterwards deposited 

 at various points in the matrix (simultaneously with other changes 

 which need not be mentioned here). These deposits increase in 

 number, and ultimately fuse into a hard continuous mass. The 

 cells at the same time undergo various modifications, and finally 

 assume the branching form which is characteristic of them in the 

 adult bony tissue (Fig. 8). 



Summary. The ultimate units or cells of the body are con- 

 structed upon the same plan, sometimes appearing almost without 

 modification in those known as typical, unmodified, or undiffer- 

 entiated cells. Such cells consist of three parts, viz., cell-sub- 

 stance, nucleus, and membrane. Modifications or differentiations 

 of the cell affect either its form or structure, or both, and may 

 involve extensive chemical as well as physical changes. Modifica- 

 tion of the cell-substance consists in the deposit within it of 

 various lifeless substances; or of chemical or physical alteration-- 



