ORGANS, TISSUES AND CELLS. 



THE term organism, as used in biology, designates in 

 general an individual animal or plant, and implies that they 

 are composed of organs. For example, the ears, eyes, legs 

 feet, of a dog are familiarly known as organs; and the same 

 is true of such internal structures as stomach and liver. It 

 is perhaps not so well known that organs are likewise 

 complex structures made up of simpler components, and 

 as in general these are composed of a network of similar 

 elements they are usually spoken of as tissues. To demon- 

 strate the organic structure of a frog it is only necessary to 

 critically observe its external features, or perhaps dissect 

 and lay open its interior. To demonstrate that organs are 

 composed of tissues will require the use of the microscope, 

 and in most cases some means of dissecting and preparing 

 them for study. This phase of biology is known as micro- 

 scopic anatomy, or histology. Finally, such a study will 

 reveal the fact that a given tissue is composed of still more 

 elemental .structures which are called cells. The following 

 outline of laboratory study will afford a direct introduction 

 to these phases of our subject. 



1. Mount in water a small fragment of frog epidermis, 

 and examine with the high power. Make a careful drawing 

 of a group of cells, showing cell walls, and nuclei. 



2. Strip the epidermis from the upper surface of the leaf 

 of Tradescantia, mount in water and examine. Compare 

 the shape, size, and general appearance of the cells with 

 those of the animal epidermis. Make a drawing of a group 

 of cells. 



3. With a razor cut very thin transverse and longitudinal 



