APPENDICES 1443 



Lavoisier (1743-94), the main founder of modern chemistry, 

 tirst made it clear, with the help of Priestley's discovery (or, rather, 

 re-discovery) of oxygen, that living always involves a process of 

 combustion or oxidation, and thus contributed one of the most 

 fundamental ideas and initiatives of physiology. 



After Stephen Hales's (1677-1761) great initiative of vegetable 

 physiology and Priestley's (1733-1804) discovery of oxygen, and the 

 hardly less important steps taken by Ingenhousz (1730-99) and 

 De Saussure (i 767-1845), there began to be some understanding of 

 photosynthesis, the most important vital process of the world, in 

 which green plants, using the energy of red-orange rays, break up 

 carbon dioxide, liberating oxygen, and build up carbon compounds, 

 such as sugars. 



Liebig (1803-73) was pre-eminent in demonstrating the circu- 

 lation of matter, a fact of the deepest importance in biology. He 

 showed how various elements, such as carbon and nitrogen, iron 

 and phosphorus, are continually passing through a cycle of diverse 

 combinations, entering successively into the composition of very 

 varied molecules, and taking part in very different organic embodi- 

 ments or incarnations. 



In 1838-39 Schwann and Schleiden formulated the Cell-Theory, 

 Goodsir and Virchow being contemporary in apprehension, and 

 Lamarck a notable anticipator. The Cell-Theory (or better Cell- 

 Doctrine) includes three propositions: (i) All organisms have a 

 cellular structure, being either single cells (Protophytes and Proto- 

 zoa), or collocations of cells and modifications of cells (multicellular 

 plants and animals) ; (2) every multicellular organism, reproduced 

 in the ordinary way, begins its life as a fertilised egg-cell, and 

 develops by its continued division, whence multiplication and dif- 

 ferentiation of cells ; (3) the life of the multicellular organism is the 

 sum of the activities of the component cells— and yet something 

 more, since there is an integration which makes the behaviour of 

 the whole more than the additive sum of the activities of all the 

 component parts. 



The Russo-German investigator, Karl Ernst von Baer (1792- 

 1876), must be ranked as the founder of Comparative Embryology. 

 He elucidated the cleavage of the egg (including the mammal's), 

 the formation of germinal layers, and the gradualness of embryonic 

 development. He was also one of the first to show that in a general 

 way, in organogenesis in particular, the individual development 

 tends to be a condensed recapitulation of the racial evolution. 



To be linked with the Cell-Theory was a step associated with the 

 names of Dujardin, Von Mohl, Max Schultze, and Cohn, and dating 

 from about the middle of the nineteenth century— the extension of 

 scrutiny beyond the cell to its living matter or protoplasm. This 

 implied what Sir Michael Foster called "a change of front" in biology. 



