cell uuall 



nucleus 



nucleolus 

 chromatin 



nuclear membrane 



UNITS OF LIFE-CELLS 

 '^ centre sphere - » 



57 



eel 

 inclusions 



vacuole 

 plastid 



centrosome 



cyToplosm 

 mitochondria 



plasnoa membrane 



Golgi apparatus 



Fig. 3-1. A generalized cell. 



of these tiny units, and they paid little or 

 no attention to the internal parts of the cells 

 for over one hundred years. By 1824, Dutro- 

 chet in France had noted that plants grew 

 by an increase in number and size of the 

 cells of which they were composed. By 1831, 

 Brown had seen and described the cen- 

 trally located body within the cell which he 

 called the nucleus. This, together with the 

 increased use of the microscope, centered 

 the attention of investigators on the internal 

 parts of the cell. Purkinje saw the universal 

 occurrence of protoplasm within cells and 

 gave it the name which it bears today. 

 About this time (1839), two Germans, 

 Schleiden (a botanist) and Schwann (a 

 zoologist ) advanced the Cell Theory, which 

 was merely a concise statement of what had 

 been learned by a great many men up to 

 this point, namely, that all living things 

 were composed of cells. 



The Cell Theory gave biologists for the 

 first time a tangible theory upon which to 

 base further studies. It meant to them what 

 the molecular theory did to the chemist and 

 physicist, and as a result, biology became 



more and more a study of cells rather than 

 a study of the organism as a whole. Be- 

 cause cells occupy the central core of stud- 

 ies in embryology, reproduction, growth, 

 heredity, and behavior, they have been 

 given steadily increasing attention through 

 the years by investigators of fundamental 

 biology. Today there is more activity in this 

 field than ever before. Once a thorough un- 

 derstanding of the cell is acquired many of 

 our most perplexing problems in biology 

 will be closer to solution. 



THE STRUCTURE OF CELLS 



While all plant and animal cells possess 

 specific morphological features that identify 

 them as particular kinds of cells, they are 

 all fundamentally alike in certain respects 

 (Fig. 3-1). All are composed of protoplasm 

 bounded by a semi-permeable plasma mem- 

 brane, and somewhere within the proto- 

 plasm lies a nucleus, which is likewise 

 bounded by a nuclear membrane. The pro- 

 toplasm in which the nucleus floats is the 

 cytoplasm. In addition to the plasma mem- 



