THE BIOLOGY OF THE CELL SURFACE 



digestion, excretion, reproduction. Thus we" speak of the 

 various systems — nervous, muscular, respiratory, etc. 

 Each system is made up of organs; each organ is a combi- 

 nation of different tissues; each tissue is a collection of cells 

 of the same origin and usually of the same form. 



Out of a single cell, the egg, emerges all this complexity 

 of organization found in the adult human being or in most 

 other multicellular organisms, animal or plant. The human 

 eg^ during its development forms tissues, these aggregate 

 to form organs, and the organs are united into systems. 

 In an animal's origin, as well as in its definite adult struc- 

 ture, therefore, the cell is the unit of organization. This 

 fact, together with that of the existence of unicellular 

 animals and plants, shows that a large part of the world 

 of living things is organized on the basis of cell-structure. 



The history of the multicellular organism as it develops 

 from the egg, a single cell, to the adult is ver}' much like a 

 synopsis of the history of the whole world of multicellular 

 organisms; this has most probably evolved from a single- 

 cell ancestor. Thus biologists classify animals and plants 

 beginning with the single-cell individuals and ascend- 

 ing through grades of increasing complexity. Among 

 animals, for example, the Protozoa stand first; next are the 

 sponges, animals possessed of tissues only; above the 

 sponges stand animals whose tissues are organized into 

 organs and finally come animals with systems of organs. 

 From the point of view of the history, both of the individual 

 multicellular organism and of the world of multicellular 

 organisms, the cell is the unit of the state of being alive. 



That this cell must be regarded as a unit becomes further 

 evident if we consider the fact that nucleus without cyto- 

 plasm or cytoplasm without nucleus is incapable of living. 

 It is true that pieces of cytoplasm devoid of nuclei, as the 

 red blood corpuscles of mammals, occur normally and live 

 for a time; but these are specialized structures incapable of 



36 



