Cell and Protoplasm. 103 



(Metazoa and Metaphyta). In short, all plants and 

 animals have a cellular structure. 



(2) Embryological. Every organism, reproduced in 

 the ordinary way, begins its life as a single cell. The 

 simplest organisms rarely get beyond this stage ; almost 

 all remain strictly unicellular. But in all other cases 

 the original single cell in which the individual life begins 

 the fertilized ovum divides and redivides into a co- 

 herent mass of cells, and gradually gives rise to a more 

 or less complex body. 



(3) Physiological. The functions of a multicellular 

 organism are expressible in terms of the activities of its 

 component cells. "Each cell", Schleiden said, "leads 

 a double life : an independent one, pertaining to its own 

 development alone ; and another incidental, in so far as 

 it has become an integral part of a plant" (1838). 

 "The whole organism", Schwann said, "subsists only 

 by means of the reciprocal action of the single elemen- 

 tary parts" (1839). "Every animal", Virchow said, 

 "appears as a sum of vital units, each one of which 

 bears with it the characteristics of life" (1858). 



These three conclusions combine in impressing us 

 with the unity of organic nature, for although a plant- 

 cell is often very different from an animal-cell, and one 

 animal- or vegetable-cell may be very different from 

 another in the same or in another body, yet the points 

 of agreement in structure, in development, and in func- 

 tion are at least as striking as the observable differ- 

 ences, and often more striking. 



Before the cell-theory could attain to the dominant 

 influence which it has exerted for half a corrobora- 

 century on biology, it required to be cor- tion of the 

 roborated in various directions. 



It had been recognized that the ovum was a single 

 cell, and the spermatozoon likewise; as early as 1824 

 Prevost and Dumas had studied the cleavage of the 

 fertilized ovum ; it remained to follow the segmentation- 

 cells on to their final differentiation into tissues. At 

 early dates strong steps on this line of research were 

 taken by Reichert (1840), Henle (1841), Remak (1841- 

 1852), and Kolliker (1843-1846). 



