.AN INTRODUCTION TO ZOOLOGY. 



out the aid of the microscope; and hence the student 

 of zpology requires to learn the use of the microscope. 

 Cells of different kinds vary much in size, some being 

 very much larger than others, and yet of microscopic 

 size. 



When not furnished with a wall of cellulose, the 

 cell is simply a little mass of jelly, which has in its 

 centre a dot, of varying size and shape, which is 

 slightly darker than the rest, especially when the 

 cell has been treated with staining fluid. This dot 

 is called the nucleus : from the fact of its staining 

 more deeply, and from its resisting the action of 

 certain solvents, which by removing some of . the 

 surrounding jelly cause the dot to be seen more 

 clearly, it is concluded that the nucleus consists of 

 a firmer variety of the same jelly as the rest. Under 

 ^ very high power of the microscope it is seen to 

 be traversed by a network of darker (therefore no 

 doubt firmer) material ; and under certain circum- 

 stances, presently to be explained, this network de- 

 velops itself into a set of definite shapes. The 

 nucleus is to be considered the most essential part of 

 a cell, for it is seen to initiate all changes connected 

 with growth. In old cells it sometimes disappears, 

 where the cell assumes some particular structure- 

 associated with some particular function, i.e. some 

 particular make adapted for some particular use. An 

 example of this is to be found in the fibres of voluntary 

 muscle; these, when very young, are seen to consist 

 of a number of cells with nuclei : but afterwards 



