AN INTRODUCTION TO ANIMAL BIOLOGY 



41 





called nuclear material. Careful experiments have proved 

 that a cell cannot continue 

 to live without a nucleus. 

 Organisms grow by multi- 

 plication of cells, and the 

 nucleus plays an important 

 part in that process of cell- <? "_ Sf ' g 

 division. The boundary of 

 a cell is called cell-wall or 

 cell-membrane; it is some- 

 times exceedingly delicate, 

 and sometimes very thick 

 and hard, as in certain plant 

 cells. The substance be- 

 tween the cell-wall and the 

 nucleus is called the cell-body. 



' ^^~ 



c.s 



*:. V'. 



c' 



FIG. 12. Cartilage from end of femur 

 of frog, c, cartilage cells; m, matrix 

 of inter-cellular substance formed by 

 the cells; c.s, empty cell spaces. (From 

 Parker and Parker.) 



Im, 



*'< 



Cells are composed of cell-sub- 

 stance, part of which 

 is living substance and 

 part stored food and 

 other lifeless materials 

 to be considered later. 

 40. Inter-cellular 

 Substance. In some 

 animal tissues all the 

 substance does not lie 

 within cell-walls ; some 

 of it is between cells or 

 inter-cellular. In the 

 frog's skin and muscles 

 there is a cement sub- 

 of bone from leg of stance between cells 

 which holds them to- 

 gether. This is called 



FIG. 13. Cross section 



frog. Ic, lacunae or spaces in which lie 



bone cells; m, marrow cavity in center of 



leg bone; c, branching tubes from bone 



spaces and containing branches of the cells; i nter . ce U u l ar substance, 



Im, concentric layers of bone deposited by . 



the bone cells. (After Parker and Parker.) and IS lormed Or SC- 



