CONNECTIVE TISSUE IN AMPHIBIA 449 



according to Merkel, are really strands of the amorphous ground 

 substance and they increase in size through an assimilation of 

 the surrounding ground substance. The cells play no direct 

 part in the formation of the fibers but serve only to produce the 

 ground substance by secretion. Merkel also believes that the 

 formation of the connective fibers is due in every case to the 

 influence of mechanical factors acting on the ground substance; 

 e.g., where there is a decided stretching, as in a tendon, parallel 

 fibers are formed from the ground substance. 



According to the intracellular theory, the ground substance 

 which is early formed is regarded as living material, although 

 it is intercellular in position. The fibers which form in this 

 ground substance are considered by some investigators to be 

 strands of cell cytoplasm w^hich have been given off from the 

 periphery of the connective tissue cells. Others beUeve that a 

 transformation of the entire cell into fibers takes place. In 

 either case, in the first stages of fiber development, all the fibers 

 are regarded as. being merely modified strands of cell cytoplasm. 

 The fibers when once formed by the cells may, and often do, 

 become separated from the cells but nevertheless it is held that 

 they are able to grow both in thickness and in length by intus- 

 susception due to the assimilation of food material. The living 

 intercellular material is also able to grow in the same manner 

 and it increases greatly in amount in proportion to the cells 

 present. 



The intracellular view has been modified by the Hansen-Mall 

 theory, which is widely held at present. In accordance with 

 this theory, the ground substance arises as a result of a sjm- 

 cytium of the mesenchyme cells and is therefore regarded as 

 being a common living exoplasm of the mesenchyme cells. In 

 this exoplasm, each of the nuclei of the mesenchyme cells are 

 surrounded by a minute mass of cytoplasm which constitutes 

 the endoplasm of the cells. According to this theor}^ the fibers 

 arise in the common exoplasm either in the regions in immediate 

 contact with the cells or in the regions of the exoplasm which 

 may be far removed from cells. 



