The Life of the Individual 



95 



ated portion of the protoplasm which is especially active 

 during the division of cells and carries the special organs 

 (chromosomes), instrumental as the bearers of hereditary 

 characters. The nucleus and surrounding protoplasm are 

 closely united in their functions and are incapable of sepa- 

 rate existence. The nucleus is, in its resting condition, 

 usually rounded in form, while the remaining protoplasm 

 is of various shapes according to the special functions of the 

 cell. Protoplasm "is characterized by ability to take in 

 nourishment, to grow, 

 to give off waste, to 

 divide and to move in 

 response to stimuli, but 

 in each organ the cells 

 become specialized to 

 do some one thing .es- 

 pecially well and they 

 often lose some of the 

 functions of primitive 

 protoplasm. For exam- 

 ple, a nerve cell loses its 

 power of contractility 

 but becomes specialized 

 for transmitting nervous 

 impulses, while a muscle 

 cell has a marked power of contractility. A detailed discus- 

 sion of the structure and function of the various parts of 

 the cell in different tissues is, of course, impossible here, 1 

 but these few suggestions are sufficient to indicate the ex- 

 treme complexity of the organization of each tissue that 

 goes to make up any organism, such as the bee. 



The egg. 



The egg, as it leaves the ovaries of the queen where it is 

 formed, is essentially a single cell. The eggs of most ani- 



FIG. 50. Group of tissue cells from skin 

 of young salamander. 



1 The interested reader is referred to Wilson, E. B., The cell in development 

 and inheritance. New York : Macmillan, and to other works on cytology. 



