CELLS AND THE CELL THEORY 



39 



crystals form in a supersaturated solution of sugar, the cytoblas- 

 tema being described as a complex, supersaturated solution 

 formed by the living body. This theory did not last many years, 

 however, because ^ it was shown that 

 cells arise only from other cells. Even 

 as early as 1846, Schultze and others 

 proved that cells have no other origin 

 except from previously existing cells. 

 Starting with an egg, which is easily 

 demonstrated to be a single cell (Fig. 

 15 A), and then carefully studying its 

 development, it can be shown that 

 its growth is by the method of re- 

 peated division and sub-division (Fig. 

 15 B, C, D, E, F) until the single- 

 celled egg gradually becomes the 

 many-celled adult. Although the 

 cells become very numerous, they all 

 arise by the process of division from 

 the original egg cell. For many years, 

 however, it was considered possible 

 for a cell to arise in some other way 

 than by division of the original egg 

 cell; and even as late as 1880 discus- 

 sions took place as to whether "free 

 cell origin" was possible. By this 

 term was meant the origin of cells 

 from any source except from a previ- 

 ously existing cell. In time this ques- 



,., j . ,, ,. i F and G show side folding inward 



tlOn Was Settled in the negative, and. to form what becomes the digestive 



we are now certain that cells never 



arise except from the division of earlier cells, and that all the 

 cells of an adult animal body, though there may be millions, 

 have arisen by the process of division from the original egg, 

 which was in itself the single cell from which the life of the 



en 



FIG. 15. THE DEVELOP- 

 MENT OF THE EGG OF A 

 SEA-URCHIN 



Showing how a singli 

 (A), by division (B toG) 

 a many-celled animal. 



en, endoderm; 

 ec, ectoderm. 



celled egg 

 grows into 



