INTRODUCTORY 



very lowest organisms where the whole animal is a single 

 cell, solid particles may be taken into the cell through 

 definite openings or "mouths." In others, the cell may 

 change its shape so as to wrap itself about the particle 

 to be taken in. But even in these instances, the particles 

 must be dissolved or digested before they can be built up 

 into the protoplasm of the cell. 



"At the beginning of its life, the animal consists of a 

 single cell only, an egg, but as it grows the number of 

 cells increase. When a child grows to manhood his 

 increase in size is not due to growth of the individual 

 cells making up his body but to the increase in their 

 number. The cells of the adult are not larger than those 

 of the child but more numerous. This multiplication is 

 the result of repeated division of the original cell and in 



this process, every part of the cell divides 



Each of the resulting cells immediately commences to 

 grow and continues until it becomes as large as the one 

 from which it started, then it divides and the story is 

 repeated. 



"We have thus seen that the body is made up of organs, 

 that the organs are made of tissues and that the tissues 

 are made of cells. Is it possible to carry this division 

 further or is the cell the final unit? To this question 

 we must reply that the cell is the final unit. It is true 

 that the cell has parts, cell wall, nucleus, cell substances, 

 etc., but no one of them can live by itself, while a com- 

 plete cell may be an independent body and live an inde- 

 pendent life. Although our own bodies are composed of 

 many millions of these cells, there are some organisms 

 made up of one cell only these are microscopic and are 

 called unicellular animals and plants. Although very 



tiny, each lives an independent life 



They vary in shape and differ in structure. Some of 



