CHAPTER VII. 



HISTOLOGY OF THE FROG 



Every living individual, plant or animal, being- able to live an inde- 

 pendent existence and possessing the ; four characteristics of irritability 

 ( ), ability to take and digest food, to grow by in- 



tussusception ( ), 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, is 

 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 quite readily that the cell is intensely impor- 

 tant 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 devel- 

 ops 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 procured 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 



