PROTOPLASM AND CELLULAR ORGANIZATION 



29 



distinct throughout the interphase stage of 

 the nucleus. Observations indicate that 

 chromosomes do not move at random dur- 

 ing the interphase stage, but form a sort of 

 mosaic with respect to one another in a 

 definite order. That chromosomes retain 

 their individuahty and genetic continuity 

 from generation to generation is indicated 

 by breeding experiments. 



Discovery of cells 

 and protoplasm 



As noted previously, we owe the term cell 

 to Hooke, who in 1665 described the struc- 

 ture of cork and pith. Many other early 

 investigators who used the compound micro- 

 scope, which was then being developed, re- 

 ported the presence of cells in all sorts of 

 plants and animals. In 1674, a Dutch micro- 

 scopist, Leeuwenhoek, discovered unicellular 

 animals, the Protozoa. For many years the 

 cell wall was considered the important part 

 of the cell, but later the protoplasm within 

 the wall was recognized as the essential cel- 

 lular substance. A nucleus had been seen in 

 cells, but was not recognized as a regular 

 constituent until 1833, when an English 

 botanist, Robert Brown, made this general- 

 ization and called it by that name. Two 

 years later, in 1835, Dujardin, a French pro- 

 tozoologist, described the semi-fluid sub- 

 stance in unicellular animals and coined the 

 term sarcode. Not until 1840 were the cell 

 contents called protoplasm by Purkinje; and 

 in 1846 the term was also used by the Ger- 

 man von Mohl for the "slime" that is pres- 

 ent in plant cells. In the meantime, the 

 German botanist Schleiden in 1838 and the 

 zoologist Schwann in 1839 concluded that 

 all plants and animals are made up of simi- 



lar cellular units. Another German zoologist, 

 Max Schultze, in 1861, furnished the final 

 proof that protoplasm is the essential living 

 substance. 



Cell theory 



The modern cell theory may be expressed 

 thus: organisms are made up of cells and 

 cell products, or are free single cells. Ex- 

 amples of the products of cells are the inter- 

 cellular substances of plants and animals. 

 The cell is not only the unit of structure but 

 also of function. A human being begins life 

 as a cell (the fertilized egg) which by multi- 

 plication and differentiation develops into a 

 complex, multicellular organism. The cell 

 principle has exerted an important influence 

 on the development of all biology. 



SELECTED COLLATERAL 

 READINGS 



DeRobcrtis, E.D.P., Nowinski, W.W., and 

 Saez, F.A. General Cytology. Saunders, 

 Philadelphia, 1954. 



Gerard, R.W. Unresting Cells. Harper, New 

 York, 1949. 



Heilbrunn, L.V. An Outline of General Phy- 

 siology. Saunders, Philadelphia, 1952. 



Hughes, A. The Mitotic Cycle. Academic Press, 

 New York, 1952. 



Schrader, F. Mitosis. Columbia Univ. Press, 

 New York, 1953. 



Sharp, L.W. Fundamentals of Cytology. Mc- 

 Graw-Hill, New York, 1943. 



Symposium. Fine Structure of Cells. Intersci- 

 ence Publishers, New York, 1955. 



Wyckoff, W.G. The World of the Electron 

 Microscope. Yale Univ. Press, New Haven, 

 1958. 



