ii8 THE FROG chap. 



osteolilasts (o), by which new lamellae of bone are formed on 

 the outside of that already existing : thus the outer layer of 

 bone (d) grows from within outwards. Between the marrow 

 and the inner surface of the bone is another layer of osteo- 

 blasts {o') which forms new lamellae on the inner side of 

 the existing bone, so that the inner layer (//) grows from 

 without inwards. 



Summary. — The various sitnple tissues studied in the 

 present chapter consist either entirely of cells, or of cells 

 separated by an intercellular substance. Formed entirely 

 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 tissues consisting of cells with 

 intercellular 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 intercellular 

 substance. 



Cells, wherever they occur, have the same essential struc- 

 ture, 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 — covering the outer surface of the body or lining 

 the inner surface of the enteric canal. Striped muscle forms 

 the " flesh," unstriped muscle the outer layer of the enteric 

 canal (p. 70). Bone and cartilage form the framework of 

 the body, while connective tissue is the packing betweeii 

 the other tissues, 



