CELLS AND TISSUES ,.,,, 



Living Things Composed of Cells 



A very small proportion of living plants on the earth :uc uniccliui-ir 

 but accordnig to Hegncr, tiio number of species of protozoa or single- 

 celled ammals must be nearly, if not quite, as great as all the other 

 species of animals put together. He bases his estimate upon the fa<-t 

 that practically every kind of animal has its own species of parasitic 

 protozoa living upon or within it. Nevertheless the mctazoa, as the 

 many-celled animals are called, make up most of the living animals 

 that we know about on earth today, just as the many-celled plants 

 make up the visible and familiar plant life. 



Just how the many-celled forms of life evolved from the unicellular 

 forms is a matter of conjecture. Two theories of origin in animals 

 have arisen, one of which, the colonial theory, postulates many-celled 

 organisms evolving as colonies of cells, which hold together after fi.ssion 

 to form plants or animals, instead of separating into individual isf>- 

 lated cells. As these cell masses evohcd, they became more and 

 more complex, different systems of organs appearing in more highly 

 organized forms. In the animal series shown on ])age 146, this 

 theory seems to be pretty well substantiated. But another theory, 

 the organismal theory, considers the living thing as a whole, being 

 divided into units of structure in the many-celled organism. Accord- 

 ing to such a theory unicellular organisms would become first much 

 chfferentiated within their own bodies, as is .seen in many of the 

 protozoa. These theories need not concern us further at present. 

 Both have many facts to support them, substantiatetl by the devel- 

 opment and structure of various types of organisms. 



Plant and Animal Cells Differ in Size, Shape, and Structure 



An examination of the figure on page 140, will sliow that cells 

 are far from uniform in size and shape. They differ in size from the 

 smallest bacteria which can just be distinguished with an ultra-micro- 

 scope that magnifies 3000 diameters, to cells that can be seen with the 

 naked eye. The egg-cell of the chick, for example, includes the con- 

 spicuous yolk, while certain cells in the human spinal cord, altluiu^h 

 microscopic in size, may have prolongations reaching down irito the 

 muscles of the fingers or toes. Cells are not of n(>cessity lar-jcr in 

 large animals or plants, some of our largest cells being found li\ing 

 isolated and alone. But under normal conditions a cell of a given 

 size and shape always reproduces the same kind of cell as itself. 



