ORGANIZATIONAL UNITS 23 



Such being the case we reach another great generalization : 

 all organisms have the same elementary structure, just as 

 we have seen that all organisms are composed of a similar 

 fundamental life-stuff, protoplasm. Therefore a cell may 

 be described as a small mass of protoplasm, either living 

 apart as a microscopic plant or animal, or forming a building 

 block, as it were, in the architecture of one of the higher 

 organisms. Indeed, organisms are organisms because of 

 specific local differentiations in the living material this 

 differentiation being possible largely because the protoplasm 

 is disposed in microscopic unit masses, or cells. (Figs. 5, 6.) 



The appreciation of this dual similarity protoplasmic 

 basis and cellular organization of all living things, which 

 was finally attained about the middle of the last century, 

 firmly established the fact that all living nature is one, 

 the corner stone of modern biology. 



A. THE CELL 



Having taken a general survey of the building materials 

 of living nature, microscopic unit masses of protoplasm, 

 termed cells, we are now in a position to consider in some 

 detail the structure of a typical cell. With the diversity 

 of gross structure of animals and plants in mind, one is not 

 surprised that there are considerable, even great, variations 

 in their component elements. In fact the characteristics of 

 an organism or part of an organism are determined by those 

 of its cells. But there are certain generalized cell characters 

 which are common to all cells by virtue of which they 

 are cells and it is important to emphasize these. 



In its simplest form a cell is a small spherical mass of 

 protoplasm. Such are the eggs of various animals and the 

 complete body of some of the lowest plants and animals. 



