DIFFERENTIATION AND REJUVENATION 149 



cells in the salivary gland, are found to have dimin- 

 ished in size and become very much smaller indeed 

 than they were in their earlier state when charged 

 with the zymogen destined to be given out. In this 

 case also we have an illustration of a functional 

 variation in the size of the cells. This ends the series 

 of pictures which I wanted especially to show to you 

 as illustrating the changes of the cells as their differ- 

 entiation progresses. We can see in the bodies of the 

 cells the changes which have occurred. 



Here is a picture (Fig. 56) which teaches us one 

 thing more about these cells. Notice the scattered 

 nuclei, each surrounded by protoplasm, completing 

 the cell. The protoplasm of each of these cells is 

 connected across with the protoplasm coming from 

 another, so that the whole set of cells forms an ir- 

 regular protoplasmic network. Now in the spaces 

 between these cells are fine lines. These represent 

 delicate structures which we call connective tissue 

 fibrils, which have a mechanical function. By their 

 tensile strength, their power to resist a pull, they give 

 a certain supporting power to the tissues. Our pic- 

 ture represents one of the tissues which support and 

 connect other portions of the body. Now the fibrils 

 apparently lie entirely disconnected from the cells, 

 but a more careful study of the history of the con- 

 nective tissue has revealed the very interesting and 

 instructive fact that the fibrils, now separate from the 

 cells, arose by a metamorphosis of the protoplasm of 

 the cells that they are first formed out of some of 



