34 GENERAL CONCEPTS 



fellow countryman and a zoologist, formulated the generalization which 

 has since developed into the cell theory: The bodies of all plants and 

 animals are composed of cells, the fundamental units of life. The cell 

 IS both the structural and functional unit in all organisms, the funda- 

 mental unit possessmg all the characteristics of living things. A further 

 generalization, first clearly stated by Virchow in 1855, is tnat new cells 

 can come into existence only by the division of previously existing cells. 

 The corollary of this, that all cells living today can trace their ancestry 

 back to the earliest living things, was stated by August VVeismann about 



1880. 



The bodies of higher animals are made of many cells, which are not 

 all alike, but differ in size, shape and functions. A group of cells which 

 are similar in form, and specialized to perform one or more particular 

 functions, is called a tissue. A tissue may contain nonliving cell products 

 in addition to the cells themselves. A group of tissues may be associated 

 into an organ, and organs into organ systems. For example, in a verte- 

 brate, the digestive system is composed of a number of organs: esophagus, 

 stomach, intestine, liver, pancreas, and so on. Each organ, such as the 

 stomach, contains several kinds of tissue-epithelium, muscle, connective 

 tissue, nerves-and each tissue is made of many, perhaps millions, of cells. 

 If a single cell is placed in the proper environment it will survive, 

 grow, and eventually divide. For most single-celled animals, a drop of 

 sea water or pond water will provide the environment required. It is 

 more difficult to culture cells removed from a multicellular animal— a 

 man, chick or frog. This was first accomplished in 1907 by Ross Harrison 

 of Yale, who was able to grow cells from a salamander in a drop of 

 nutrient medium containing blood plasma. Since then, many different 

 kinds of cells from animals and plants have been cultured in vitro,* and 

 many important facts about cell physiology have been revealed in this 

 way. 



The cells of diflferent organs and different animals present a be- 

 wildering variety of sizes, shapes, colors and internal structures, but all 

 have certain features in common. Each cell is surrounded by a plasma 

 membrane, contains a nucleus, and has in its cytoplasm mitochondria, 

 microsomes, Golgi bodies and a centriole. 



Each cell is completely enclosed by a thin sheet of protoplasm, the 

 plasma membrane. This is a living, functional part of the cell, which 

 controls the entrance and exit of nutrients, secretions and waste prod- 

 ucts. The plasma membrane is permeable to certain substances and not 

 to others; in addition it is capable of doing work to "pump" substances 

 into and out of the cell. Very few substances are found at the same 

 concentration within the cell and in the surrounding fluid; some con- 

 centrations are much higher, others are lower, than in the environment. 

 The activities of the plasma membrane are responsible for maintaining 

 these difterences. When it fails to do this, the cell dies. Nearly all plant 

 cells have, in addition to the plasma membrane, a thick cell wall made 

 of cellulose. This nonliving wall, lying outside the plasma membrane, is 



* In vitro, Latin, "in glass." The cells are removed from the animal body and incu- 

 bated in glass vessels. 



