446 GENERAL ZOOLOGY 



made up of very small parts called cells, and the next year 

 Schwann made a similar discovery with respect to the animal 

 body, thus laying the foundations for the " cell-theory." The 

 main propositions involved in the cell-theory, as stated by 

 Professor Thomson, are, first, that all organisms are either 

 built up of single cells or combinations of such cells ; second, 

 that all organisms begin life as a single cell, which, in the 

 case of the many-cell organisms, gives rise to a more or less 

 complex body ; and third, that the function of a multicellu- 

 lar organism can be expressed in terms of the activities of 

 its component cells. Though the third proposition may now 

 require some modification, the cell-theory proved an invalu- 

 able unifying conception in biology. In 1846 the word "pro- 

 toplasm," originally used in a different sense, was applied by 

 von Mohl to the substance inclosed by the cell-wall in plants ; 

 and in 1861 Max Schultze established the essential identity of 

 protoplasm with the life-substanoe in the animal cell, which 

 had been called sarcode. Embryology was broadened through 

 the influence of von Baer (1792-1876), who, in 1827, de- 

 scribed the primary germ-layers in the vertebrate embryo. We 

 owe the appreciation of the importance of the embryological 

 basis of classification to von Baer. 



The nineteenth century was prolific in experiments to test 

 whether there could be spontaneous generation of organisms, 

 and a war of discussion was waged between those who sup- 

 ported the idea and those whose experiments seemed to show 

 that all the living organisms experimented upon came from 

 preexisting life. The experiments of Francesco Redi (1626- 

 1697) in the seventeenth century had shown that maggots 

 did not appear in decaying meat if flies were prevented from 

 having access to it, and the discussion took a new turn 

 about the possibility of the spontaneous generation of the 

 animalcules (such minute animals as the infusorians and roti- 

 fers). Some of the earlier experimenters did not succeed in 



