THE CELL 17 



structure, but it is now a matter of common knowledge 

 that this is not the case. There is such a dissimilarity 

 'that it is practically impossible to give a definition that 

 will hold for all cells, but in this chapter by cell is meant 

 a structure which is capable of displaying all the vital 

 manifestations, but not capable of being divided into 

 simpler vital units. 



In regard to size, cells vary enormously. Many of 

 the bacteria are so small as to be visible only with the 

 highest powers of the microscope, while other cells are 

 quite visible to the naked eye, and may even reach, in 

 the case of yolk-laden eggs, some inches in size. Size 

 therefore is unimportant. 



In regard to structure, cells vary as much as they do 

 in size. Some cells may be a mere microscopic speck 

 in which no structure can be made out, others may have 

 a definite structure and contain numerous clearly-defined 

 *parts. But as there are many cells in which no one, 

 <up to the present time, has been able to identify those 

 Iwell-known parts, we may take it that the essential 

 /living substance is the protoplasm or cytoplasm. 

 Numerous attempts have been made to find a definite 

 structure in the cytoplasm ; it has been said by some 

 to be reticular, others say it is fibrillar, while others 

 maintain that it is granular. The truth probably lies 

 in all three of these views, as it is quite possible that at 

 one time it may be reticular, at another fibrillar, and 

 at yet another granular. 



We may therefore say that the cytoplasm is a sub- 

 stance with no definite structure, in which we find a 

 large number of granules. These granules are probably 

 "of different nature and function, as they react differently 



B 



