THE FROG AND VERTEBRATES IN GENERAL 173 



the larger part of the tissue. It begins to develop very early in 

 the Frog embryo as a homogeneous substance which later becomes 

 modified in various ways to form the matrix of the different types 

 of connective tissue and bone. For example, it may remain largely 

 unchanged, as in some of the simpler types of connective tissue ; 

 it may become a dense fibrillar material, as in a tendon ; or it 

 may become largely infiltrated with certain inorganic salts, chiefly 

 calcium carbonate, and form the basis of the hard bone tissue. 

 (W. f. 32, B, G, D.) 



White Fibrous Tissue. There are several different kinds of 

 connective tissues recognized. One type, which has a very wide 

 distribution, is known as the white fibrous tissue. This may be 

 obtained from almost any region of the body ; for example, in the 

 skin, or surrounding the muscles and nerves, or forming the sup- 

 porting framework in various organs, or as tendons attaching the 

 muscles to the bones. When examined microscopically, the white 

 fibrous tissue will be found to consist of a considerable proportion 

 of ground substance in which are bundles of collagenous fibers 

 having a characteristic wavy appearance. The white fibrous 

 tissue also contains a number of elastic fibers, mentioned below. 

 Scattered among the fibers and embedded in the ground sub- 

 stance are numerous connective tissue cells, known as fibroblasts. 

 These are elongated, spindle-shaped cells which have a tendency 

 to stretch out along the fibers. (W. f. 32, B.) 



Elastic Tissue. Another type of connective tissue, which is 

 known as elastic tissue, consists of heavier, straight fibers 

 which frequently branch. The elastic fibers are found in abun- 

 dance in the walls of the blood vessels, and they also form certain 

 ligaments. The nature of elastic tissue is such that when tension 

 is applied it will stretch and then return to the original condition 

 when the tension is released. This condition is very different 

 from that present in the tendons of the body, which consist almost 

 entirely of white fibers. A tendon lacks elasticity and will break 

 under tension rather than stretch. Generally the white fibers and 

 the elastic tissue are found to be more or less intermingled in the 

 various connective tissues. For example, the ligaments contain a 

 great deal of the elastic tissue and very few of the white fibers, 

 while the reverse is true of the tendons. (W. f. 32, B.) 



Fatty or Adipose Tissue. This type of tissue is found more 

 or less scattered throughout the body, and is generally regarded 



