THE FROG 49 



which agree in that the intercellular substance is fibrous and 

 large in amount. Examples of connective tissue have been 

 encountered in the study of cross-sections of various organs. 

 For more careful study, connective tissue may be prepared 

 as follows: From a preserved frog cut out a small piece of 

 the white fibrous connective tissue that fastens the skin to 

 the body, spread it on a slide in a drop of water, and cover 

 it with a cover glass. Under high power observe the nu- 

 merous fibers, which are usually unbranched and of a char- 

 acteristic wavy appearance. Irregularly distributed among 

 the fibers are the connective tissue corpuscles or cells; 

 these have conspicuous granular nuclei and vary considerably 

 in form and in the appearance of the cytoplasm. Sketch a 

 small portion of connective tissue. 



XL THE SKELETON. 



The skeleton, which forms the hard internal parts of the 

 frog, is composed partly of cartilage and partly of bone. In 

 the early stages of development the skeleton consists entirely 

 of cartilage; in the adult this primary cartilaginous skeleton 

 is replaced to a considerable extent by bone. Bones formed 

 thus by replacement of cartilage are called cartilage bones. 

 By far the greater portion of the skeleton of the frog is 

 made up of cartilage bones. Cartilage may also be calcified 

 (i. e., become hardened by a deposit of calcareous material) 

 without taking on the histological character of true bone. 



Bones may also be developed in places where there is no 

 preexisting cartilage and are then, in distinction from car- 

 tilage bones, called membrane bones. Membrane bones 

 are formed as ossifications in the cutis or deeper layer of the 

 skin; in many fishes they retain this primitive position, but 



