382 General Biology 



Hall (1790-1857). The greatest of the physiologists is undoubtedly 

 Johannes Muller. 



In botanical physiology, Hale (1677-1761), is the greatest, while 

 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 and 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 

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



Haller and van Leeuwenhoek were "pre-formationists." They sup- 

 posed that each sperm or egg-cell already contained an embryo some- 

 what 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 complete 

 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 

 present in each germ-cell a potentiality 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-two 

 to thirty-six hours old, and consequently, even when we 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 pre-formationists 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, 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 itself and the other to perform a 

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



