CELLULAR STRUCTURE OF ORGANISMS 95 



biological principle that sharp lines dividing groups can hardly 

 ever be drawn, and it is almost always possible to find inter- 

 mediate forms connecting widely separate types. 



True Multicellular Organisms. Multi cellular organisms are 

 always made up o'f more than one cell; but the fact that they 

 consist of many cells is not enough to define them accurately. 

 A brief account of the manner in which multicellular organisms 

 develop will explain the meaning of the term. In all cases 

 they begin as a single cell, which may be either an egg or a 

 spore. This cell divides into two parts, these into four, and 

 so on, the number of cells increasing indefinitely; but after 

 dividing, the cells remain attached instead of separating. 

 After a while some of the cells assume a variety of types, i.e., 

 they become differentiated in form and function, and play 

 different parts in the life of the organism. Such a differentia- 

 tion of cells occurs in all true multicellular organisms. Hence 

 we may define a multicellular organism as one composed of many 

 cells which show a differentiation in structure and function. 



With this differentiation of cells, tissues appear for the first 

 time. Cells with similar structure and function are commonly 

 grouped together, to form a tissue. The cells with special 

 contractile power, for example, form muscle tissue; cells with 

 power to secrete bone form bony tissue; and those in which 

 conductility and irritability are particularly developed are 

 grouped together to form nervous tissue; and so on. Tissues are, 

 of course, impossible among unicellular organisms, but univer- 

 sal among multicellular organisms. 



With the multiplication of cells and their differentiation, 

 there also appears the formation of true organs. Among the 

 unicellular animals and plants there may be certain parts 

 of the cell, like the mouth and nucleus, set apart for certain 

 functions, and these are, to be sure, cell organs. But they are 

 not organs in the sense in which the term has been used among 

 the multicellular animals, where groups of cells, usually of 

 various kinds, are aggregated to form distinct parts with 



