PROTOZOA 289 



The fundamental elements of organisms are called 

 cells.^ With a few exceptions the cells are too small 

 to be seen with the naked eye ; they can be seen with 

 the microscope as corpuscles of various shapes. But all 

 of them contain a frothy, fluid substance, protoplasm. 

 Inside this is a tiny vesicle, the nucleus. This in turn 

 has the same texture as the protoplasm of the sur- 

 rounding cell-body. Only the nucleus is enclosed by 

 a delicate membrane, while there is no membrane 

 enclosing the entire cell, at least not in most animal 

 specimens. Plant -cells have always a membrane, 

 sometimes a comparatively thick one. 



Thus the principal characteristic of a cell is its 

 nucleus. But this is not the whole of its contents ; it 

 has other invariable elements besides the nucleus. The 

 nucleus itself is not a simple structure, but always 

 contains certain compact particles which are comprised 

 together under the name of " chromatin." It is believed 

 that these contain the matter that is the vehicle of 

 heredity. On this view they are the most important 

 parts of the cell. 



Thus the protoplasm of which the cell consists 

 contains a number of deposits, and this shows that 

 it is not a formless mass. It is especially the frothy 

 character of the protoplasm that gives it a definite 

 structure.^ We know from the seventh chapter that 



^ The cell was discovered in 1667 by Robert Hooke, who examined 

 a cork, and found that it was composed of cells like a honey-comb. 

 The term " cell " is still retained, though we now know that the cell is 

 a solid particle, and often has no membrane at all, so that it has no 

 resemblance to the honey-comb cell. 



" The theory of the frothy structure of protoplasm was advanced by 

 O. Biitschli, and has generally displaced other theories. 



