ORGANIZATION. 31 



the division of labor emerges. First comes a distinction 

 between the governing and governed classes ; then follow 

 and multiply the various civil, military, ecclesiastical, and 

 industrial occupations. 



In like manner, as we advance in the animal series, we 

 find the body more and more heterogeneous and complex 

 by a process of differentiation, i. e., setting apart certain 

 portions of the body for special duty. In the lowest 

 forms, the work of life is carried on by very simple appara- 

 tus." But in the higher organisms every function is per- 

 formed by a special organ. For example, contractility, 

 at first the property of the entire animal, becomes centred 

 in muscular tissue; respiration, which in simple beings 

 is effected by the whole surface, is specialized in lungs 

 or gills; sensibility, from being common to the whole or- 

 ganism, is handed over to the nerves. An animal, then, 

 whose body, instead of being uniform throughout, is made 

 up of different parts for the performance of particular 

 functions, is said to be organized. And the term is as ap- 

 plicable to the slightly differentiated cell as to complex 

 Man. Organization is expressed by single cells, or by 

 their combination into tissues and organs. 



1. Cells. A cell is the simplest form of organized life. 

 In general, it is a microscopic globule, consisting of a del- 

 icate membrane enclosing a minute por- 

 tion of protoplasm. The very simplest 

 kinds are without granules or signs of 

 circulation ; but usually the protoplasm 

 is granular, and contains a defined sep- 

 arate mass called the nucleus, within 

 which are sometimes seen one or two. 



n FIG. 1. Parts of a Cell 



rarely more, dark, round specks, named a,i>,i/,ceii-waii; 



nucleoli. The enveloping membrane is cleQ8; "' uacleolQ8 - 

 extremely thin and transparent, and structureless : it is 

 only an excretion of dead matter acting as a boundary to 



