THE NEW CELL DOCTRINE 5 



land and America, and also on the -European Continent. 

 Everybody discussed at that time the question whether proto- 

 plasm was really the physical basis of life or not. The solution 

 of this problem we have not fully reached even yet. The de- 

 scription of the cell which we owe to Max Schulze dominates 

 everywhere and yet with the progress of science it has be- 

 come insufficient. 



The size of the cell is of the greatest significance to biologists. 

 Cells for the most part are rather small, and the size is ex- 

 tremely variable. The cells of the human body, according to 

 an estimate I have made, have an average diameter of perhaps 

 X^ I 0.014 mm. Variations, however, are considerable; some cells, 

 like the blood-corpuscles, are very small; certain nerve cells, 

 on the other hand, attain a considerable size. The largest 

 cells of all, known to us at present, are eggs. Those of certain 

 animals appear as true giants in comparison with other cells. 

 The largest eggs occur in birds. The entire yolk of the bird's 

 egg corresponds to but a single cell. The albumen which 

 surrounds the yolk and the shell do not belong to this cell, but 

 are simply layers which are added by the oviduct to the egg 

 proper, and which are secretions of the glands of the oviduct. 

 Of all the animals now living the ostrich has the largest egg 

 and the yolk of the ostrich egg is certainly the largest living 

 cell known to us. These enorrhously enlarged eggs might be 

 described as the monsters of the cellular world. They are ex- 

 ceptions. By far the majority of cells are of such dimensions 

 that they are visible with the microscope alone. The smallest 

 organisms which we know are the vegetable germs, which may 

 have a diameter of not more than one-tenth of a milUmeter. 

 As is well known to all, certain of these smallest organisms 

 cause diseases which may be extremely dangerous to man. 

 The investigators of infectious diseases have made the inter- 



