118 THE FROG CHAP. 



formed on the outside of those already existing : thus 

 the outer layer of bone (b) grows from within outwards. 

 Between the marrow and the inner surface of the bone 

 is another layer of osteoblasts (o r ) which forms new 

 limillae on the inner side of the existing bone, so that 

 the inner layer (b r ) grows from without inwards. 



Summary. The various simple tissues studied in the 

 present chapter consist either entirely of cells, or of cells 

 separated by an intercellular substance. Formed en- 

 tirely of cells are the various kinds of epithelium 

 columnar, ciliated, and squamous, and unstriped muscle. 

 In striped muscle the cells have elongated into fibres 

 and their nuclei have multiplied. Of the supporting 

 tissues, consisting characteristically of cells with inter- 

 cellular substance, connective-tissue has the matrix soft 

 and homogeneous, with fibres imbedded in it ; in hyaline 

 cartilage it is structureless and tough, though elastic ; 

 and in bone laminated and calcified. In the blood, the 

 plasma may be looked upon as a kind of liquid inter- 

 cellular substance. 



Cells, wherever they occur, have the same essential 

 structure, being formed of protoplasm with a nucleus. 

 In nearly all cases they increase by binary fission, first 

 the nucleus and then the protoplasm dividing into two. 



The distribution of the various tissues throughout the 

 body is worth noting. Epithelium always bounds a free 

 surface e.g., that covering the outer surface ot the body 

 or lining the inner surface of the enteric canal. Striped 

 muscle forms the " flesh," unstriped muscle, e.g., the 

 outer layer of the enteric canal (p. 70) . Bone and carti- 

 lage form the framework of the body, while connective- 

 tissue is the packing between the other tissues. 



