VI 



HISTOLOGY OF THE FROG 123 



and the effect of this common movement is to create a 

 current which carries small objects away from that region. 

 The action of cilia may easily be demonstrated by sprinkling 

 some powdered carmine on the roof of a frog's mouth. Soon 

 one may observe that the substance is slowly carried back- 

 ward down the esophagus into the stomach. 



The connective tissues embrace a large number of tissues 

 whose general function it is to ^upport and hold together 

 the various other parts of the body. While in the other 

 kinds of tissue the intercellular substance is relatively very 

 small in amount, in the connective tissues it is usually very 

 abundant. Nearly all of the connective tissue is derived 

 from the middle germ layer, or mesoderm. It arises chiefly 

 from scattered cells, or mesenchyme, and in the early stages 

 of its differentiation the amount of intercellular substance is 

 very small, and of a jelly-like consistency. The intercellular 

 substance becomes modified in various ways in the different 

 varieties of connective tissue. In some cases it remains 

 soft, in others it becomes fibrous, in bone it becomes hard- 

 ened through deposits of carbonate and phosphate of lime. 

 The principal kinds of connective tissue found in the frog 

 are the following : — 



White fibrous connective tissue is the variety which has 

 the widest distribution. A good example of this may be 

 obtained from the membranes which connect the skin with 

 the body wall. If a portion is spread out on the slide and 

 examined with the microscope, it will be seen to be made up 

 of a clear homogeneous portion, or matrix, of a gelatinous 

 substance in which are imbedded numerous fibers ; the 

 fibers are usually unbranched and have a characteristic wavy 

 appearance. They are frequently united in bundles which 

 run in all directions. When treated with acetic acid, they 

 swell up and disappear, and when boiled, become converted 



