122 ELEMENTARY ZOOLOGY. 



132. The Changes that Come to Cells with Age. The student 

 should try to realize the great changes that have come to the cells 

 of the body in passing from the fertilized ovum to such a condition 

 as we find in one of the higher animals. All the living cells of the 

 body, numerous, different, specialized, have come from the proto- 

 plasm of this first cell. (See Fig. 8.) Some of them, as muscle 

 and nerve cells, are so changed that they have lost the power of 

 division; some, as the red blood corpuscles, have degenerated to 

 such an extent that they cannot repair the wastes and are contin- 

 ually dying out and being replaced by new ones; others, as bone 

 cells, have secreted materials about themselves so that they are 

 practically buried alive and depend on the nourishment that can 

 percolate through minute passages in the bony matter ; some, as 

 the epithelial cells, have retained much of their power of division, 

 as is shown by the way the skin is restored if cut. Yet all have 

 a common origin in the ovum. On the whole, we are compelled to 

 say that with all the division, growth, waste, and repair, and differen- 

 tiation they have undergone, they show signs of loss of powers, and 

 of old age. 



133. Summary. 



1. Divisions of the fertilized ovum produce first a mass of undif- 

 ferentiated cells; the cells gradually become differentiated through 

 inherited tendencies and from the result of the different positions 

 in which they find themselves. 



2. Groups of similar cells with similar powers are called tissues. 

 In addition to the cells, there may be intercellular substance se- 

 creted among the cells. 



3. Tissues may be classed either with respect to their structure 

 or their activities. 



A. Passive tissues. 



I. Epithelial (cells in layers, little intercellular substance). 

 Kinds: pavement, columnar, ciliate, glandular, sensory, 

 reproductive. 



Functions: protection, absorption, secretion, sensation, 

 reproduction. 



II. Supportive or connective (usually much intercellular 

 substance). 



Kinds: gelatinous, fibrous, cartilaginous, bony. 

 Functions: binding, support, protection. 



