THE FINE STRUCTURE OF THE FROG 



In the previous laboratory studies of the frog the gross struc- 

 ture only has been emphasized. The body was found to be 

 composed of organs associated as systems of organs each with a 

 particular service to perform. When a small portion of any 

 organ is examined microscopically it is found to bo composed of 

 great numbers of units called celh. Kach cell is a minute mass 

 of protoplasm. When more extensive microscopic studies of the 

 body are made two configurations are readily recognized: 



1. The protoplasm of the body rather than existing as a mass 

 is everywhere divisioned as cells. The body is therefore to be 

 regarded as formed of cells and is said to be niiilti<clli(1or in its 

 structural nature. 



2. The cells are not all alike. Different groups of cells while 

 agreeing among themselves differ from those of other groups and 

 each different group is concerned with different categories of 

 functions. Each of these groups constitutes a tissue which may 

 be defined as an association of cells differentiated in relation to a 

 particular category of functions. There are four primary tissues 

 as enumerated below. 



a. P^pithelium or the covering, protecting and secreting tissue. 

 Normally nowhere in the body is any other kind of tissue exposed. 

 Not only the external surface but also the lining of every cavity 

 is composed of this kind of tissue. I'^pithelial tissues constitute 

 the active elements of glands. 



b. Connective tissue, as the name suggests, functions as the 

 connecting and supporting substance of the body. It fills spaces 

 in the body, unites the various parts of the body, and binds 

 organs together. 



c. Muscular tissue is the tissue of movement. 



d. Nervous tissue comprises elements adapted to the conduc- 

 tion of impulses and serving as the communicating lines in the 

 body. 



When a cell is studied microscopically it is fouml to ho com- 

 posed fiot of a homogeneous substance but of organized 

 protoplasm. 



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