266 Cell as Basis oj Organic Activity 



Among those individuals who looked at nearly everything that was 

 within reach, the name of Leeuwenhoek is an outstanding one. He was 

 a Dutch merchant who in his spare time ground lenses and made ob- 

 servations. He lived for 91 years (1632-1723), made at least 400 mi- 

 croscopes, and sent numerous communications to the Royal Society 

 of London. 



Robert Hooke (1635-1703) may be considered to be one of the 

 best of the early microscopists. His fame in biology is assured by the 

 fact that he was the first man to use the word cell to describe the struc- 

 tural unit of the plants he examined. 



The name cell, meaning hollow space, was in this way given to 

 these building blocks of tissue. This name is perhaps not the best 

 possible, for it tends to emphasize the wall rather than the important 

 contents. 



Due to the poor quality of the microscopes of this early period, 

 more detailed discoveries were delayed until the early part of the nine- 

 teenth century. At the beginning of the nineteenth century, a French- 

 man, Bichat, introduced the word tissue to describe the groups of simi- 

 lar cells massed together. Although his work cannot be considered to 

 be of the greatest importance, it does indicate the renewal of interest 

 in the microscopic structure of living materials. 



Robert Brown, whose name is familiar in Brownian movement, 

 in the year 1831 studied cells in orchids and first noted that nuclei were 

 a regular feature of plant cells. Another man, Purkinje, in 1839, named 

 the contents of the cell, protoplasm. 



The final formulation of the cell theory is credited to two men, 

 a botanist, Schleiden, and a zoologist, Schwann. Schleiden in 1838 

 conceived the idea that the cell was the essential unit of the living 

 organism after his studies on many different plants ; Schwann in 1839 

 showed that the same principle applied to animals. 



Since this period, interest has centered more on the contents of 

 the cells and their functions. A vast number of investigators has dem- 

 onstrated that the cell's contents is not a homogeneous mass, but rather 

 that it is made up of a number of specialized structures with particu- 

 lar functions. This work has, of course, been made possible by techni- 

 cal advances in the manufacture of microscopes. The electron micro- 

 scope represents a new advance by which great magnifications up to 

 20,000 times are possible. In this microscope, a beam of electrons 

 is utilized and the image is recorded on photographic film. Further 

 knowledge has also been made possible by the development of the phase 



