THE GROWTH OF BIOLOGICAL IDEAS 157 



of plant and animal tissue are heated in an oven until all the 

 organic matter is burned away and only inorganic ash is left. 

 The process is so carefully carried out, however, that the ash 

 remains exactly where it was, and the microscope reveals the 

 exact location of the inorganic constituents within individual 

 cells. 



America has led the world in originating and developing 

 novel methods for investigating the substances of which the 

 cell is composed. Professor R. R. Bensley of Chicago, youth- 

 ful despite his years, has been and still is a pioneer in this 

 work. It was he who first showed how the minute com- 

 ponents of cells can be separated from one another by pass- 

 ing: tissues throuo^h fine sieves and then centrifusrino^ the ma- 

 terial at carefully regulated speeds. In this way some of the 

 most elusive cell constituents, previously only peered at under 

 the highest powers of the microscope, have been obtained in 

 masses that one can hold in one's hand. Instead of having 

 to rely on conjecture as to their composition, one can now 

 subject the material to direct chemical analysis. 



But we must return to the outburst of discovery in various 

 fields that followed the formulation of the cell theory. The 

 phenomena of reproduction began to be put upon a cellular 

 basis. In 1855, for the first time, the German botanist 

 Nathaniel Pringsheim saw the essential feature in the act of 

 fertilization. As early as 1823 the microscopist Giovanni 

 Amici had observed the tube formed by the pollen grain and 

 seen it enter the ovule. Pringsheim now saw the cellular 

 nature of fertilization. He was working with Vaiicheria, 

 one of the lowly plants that form masses of branching green 

 threads in our ponds and ditches. He found that two cells, 

 the active male spermatozoid and the female ovum or egg, 

 fuse together to form a single cell and that the single cell 

 grows and differentiates until it becomes a new plant indi- 

 vidual. Spermatozoa had been known since the seventeenth 

 century and the corresponding spermatozoids of ferns since 

 the forties, and it seems rather surprising that an understand- 

 ing of the general principles of fertilization came so slowly. 



