GENERAL BIOLOGY. 5 



very much as to whether they belong to the realm of animals or 

 plants. The relation of the sciences which may be regarded 

 as subdivisions of general biology is well shown in the accom- 

 panying table. * 



THE CELL.f 



All living things, great and small, are composed of cells. 

 Animals may be divided into those consisting of a single cell 

 (Protozoa), and those made up of a multitude of cells (Metazoa) ; 

 but in every case the animal begins as a single cell or ovum 

 from which all the other cells, however different finally from 

 one another either in form or function, are derived by processes 

 of growth and division ; and, as wUl be seen later, the whole 

 organism is at one period made up of cells practically alike in 

 structure and behavior. The history of each individual animal 

 or plant is the resultant of the conjoint histories of each of its 

 cells, as that of a nation is, when complete, the story of the total 

 outcome of the lives of the individuals composing it. 



It becomes, therefore, highly important that a clear notion 

 of the characters of the cell be obtained at the outset ; and 

 this chapter will be devoted to presenting a general account of 

 the cell. 



The cell, whether animal or vegetable, in its most complete 

 form consists of a mass of viscid, semifluid, transparent sub- 

 stance (protoplasm), a cell wall, and a more or less circular 

 body (nucleus) situated generally centrally within ; in which, 

 again, is found a similar structure (nucleolus). 



This description applies to both the vegetable and the ani- 

 mal cell ; but the student will find that the greater proportion 

 of animal cells have no cell wall, and that very few vegetable 

 cells are without it. But there is this great difference between 

 the animal and vegetable cell : the former never has a cellulose 

 wall, while the latter rarely lacks such a covering. In every 

 case the cell wall, whether in animal or vegetable cells, is of 

 greater consistence than the rest of the cell. This is especially 

 true of the vegetable cell. 



It is doubtful whether there are any cells without a nucleus, 

 while not a few, especially when young and most active, pos- 



* Taken from the General Biology of Sedgwick and Wilson, 

 f Tlie Illustrations of the sections following will enable the student to 

 form a generalized mental picture of the cell in all its parts. 



