THE WORLD OF LIFE 5 



proportions capable of infinite variations or whether in varying 

 proportions we are unable to say, but the bit of protoplasm 

 floating in a pond which develops into a frog must be very 

 different from the bit beside it which develops into a fish, 

 even though under the highest powers of the microscope and in 

 the laboratory of the chemist we may be unable to distinguish 

 between them. 



The simplest forms of life consist of microscopic bits of pro- 

 toplasm, within which we can in most cases distinguish a more 

 highly refractive spot, often spherical, which is called the 

 nucleus. This is merely differentiated protoplasm, which be- 

 cause of its special structure and functions is sometimes called 

 nucleoplasm, while the surrounding protoplasm is called cyto- 

 plasm. Such a nucleated mass of protoplasm is termed a cell, 

 and is frequently surrounded by a more or less firm membrane, 

 called the cell membrane or wall, a product of the cytoplasm; 

 but this membrane is not considered an essential part of the 

 cell as usually defined, since it is often absent. 



Among the simplest animals and plants each individual con- 

 sists of a single cell, which is capable of performing all the 

 functions necessary for its own existence and for reproducing 

 itself ; but most organisms are much more complicated in 

 structure, for they consist, not of a single cell, but of many cells, 

 some of which perform one function and some another, so that 

 there is a division of labor amongst them. These special 

 groups of cells are called tissues, and necessarily undergo con- 

 siderable modification of structure to fit them for their special 

 use, so that we may briefly define a tissue as a group or com- 

 plex of similarly differentiated cells, which have a common 

 function to perform. When we consider the functions of a 

 plant or animal, however, we see that they are not performed 

 by one tissue exclusively, but that several tissues are brought 

 into play, even for a very simple form of activity ; thus the 

 tissues of the body are more or less united into organs, such as 

 the root of a plant or the foot of an animal, each of which per- 

 forms some one or more of the functions necessary for the 

 existence of the individual or the race. It is for this reason 

 that we speak of plants and animals as organisms, — composed 

 of organs, which in turn are composed of tissues, and these of 



