THE HISTORY OF BIOLOGY 3S1 



Cesalpino (1519-1603), Jung (1587-1657), and van Helmont (1577-1644), 

 occupy high places. 



Ingen-Housz (1730-1799) was the first to show that carbon dioxide 

 from the air is broken down in the leaf when the plant receives sunlight, 

 and that the carbon is retained which thus assists materially in nutrition 

 and growth. 



De Saussure (1740-1799) showed further that water and various salts 

 from the soil produced the remaining factors in this process, while Bous- 

 singault (1802-1887) gave us our knowledge of chlorophyl. 



Haller and van Leeuwenhoek were what is called pre-formationists. 

 They supposed that each sperm or egg-cell already contained an embryo 

 somewhat fully formed, and that all that occurred during the growth 

 period was an enlarging of the parts which were already present. Such 

 an idea meant that every human germ-cell must have every other com- 

 plete human being that could ever descend from it, within itself, fully 

 formed, but very small. We know now that both those who held this 

 theory and those who opposed it were wrong. There must, of course, 

 be a potentiality present in each germ-cell which .can develop into what 

 it is to become, but this by no means signifies that the embryo possesses 

 a definitely formed embryo within it in turn. The new embryo is always 

 organized little by little until it becomes the completed individual adult 

 organism. 



However, it is natural to see how and why observers thought they 

 saw the complete embryo in the egg. In our study of Embryology we 

 shall see that when the hen lays an egg, it is already from twenty-four 

 to thirty-six hours old, and consequently, even when w r e have a freshly 

 laid egg (provided it is fertile) there is already an embryo which can be 

 seen. It was with material of this kind that these men had to work. 



Wolff (1733-1794) had proved that the performationists were in 

 error, but Haller, who held the intellectual reins of workers in zoology at 

 the time, refused to accept it and so the lesser lights also refused. 



It was but natural that after Hooke had observed that plants were 

 composed of cells that something should be done with such a finding. 

 Brown (1773-1858), working on the cell, discovered the cell nucleus in 

 1831, and the botanist Schleiden (1804-1881), and the zoologist Schwann 

 (1810-1882) published their works in 1838 and 1839, respectively, show- 

 ing that plants are developed from cells and that plants and animals are 

 alike in being composed of cells. 



An important point was made in suggesting that each cell has two 

 functions : one to perform the work of itself and the other to perform a 

 task which makes it an integral part of a larger organism. 



Schultze (1825-1874) in the early sixties established the idea of pro- 

 toplasm as the living substance of all cells. This protoplasm was called 

 by Huxley the "physical basis of Life." 



In Embryology Fabricius (1537-1619) published a paper describing 



