CHAPTER VII 



HISTOLOGY OF THE FROG 



EVERY living individual, plant or animal, which is able to live an 

 independent existence and which possesses the four characteristics 

 of irritability, ability to take and digest food, to grow by intus- 

 susception, and to reproduce its own kind, is called an organism. 



The higher organisms are made up of separate specialized organs, 

 each organ consisting of a series of tissues, and each tissue, in turn, 

 made up of a sheet of similar functioning cells. 



The cell is the biological unit, and the modern world attempts to 

 explain all living things in terms of cellular construction. 



It can be appreciated readily that the cell is intensely important 

 in the study of all living organisms when it is realized that every living 

 thing, plant or animal, originally grows from a solitary cell, and any 

 tiny structure capable of producing so wondrous an animal as the frog 

 or still more wondrous an animal as the human being, is certainly of 

 importance. 



In fact, if one could find all the possibilities of any given cell, and 

 then find why it has these possibilities, and just how and why it 

 develops into the particular structure that it does and no other, the 

 riddle of life would be solved. 



It must be remembered that every living thing starts life as a single 

 cell, and then, if it is to become a multicellular animal, it passes through 

 a cell-dividing stage. Some plants and animals remain in the one-celled 

 stage, while others, as soon as they begin to divide, adhere together and 

 form tissues, which in turn develop into organs. This means that a 

 study of the origin, development, and content of the unit cell gives us 

 a sort of bird's-eye view of how living things work and grow. A study 

 such as this presents a more complete view than could be obtained in 

 any other way. 



First, therefore, it is necessary to know the different kinds of tissues 

 that may be encountered. These are grouped under four distinct heads : 



1. Epithelial. 



2. Connective. 



3. Muscular. 



4. Nervous. 



1. Epithelial tissues (Fig. 39) are always surface tissues. They 

 lie in layers with a small amount of intercellular substance. The sur- 

 faces of organs, the linings of cavities of organs, and the lining of glands, 

 blood vessels, and ducts of all kinds, possess this tissue. In fact, it is 



