DISCOVEh'IKS WITH THE MICROSCOPE 215 



plate-culture method for bacteria. To the discoverer of a 

 new method, an unknown region is opened up. Finally, 

 we have the random method : trying in all directions, trust- 

 ing to industry to find something of sufficient importance. 



Two Great Discoveries. — It is the writer's opinion that 

 there have been two discoveries, wholly due to the micro- 

 scope, of such fundamental nature that they may be placed 

 among the few greatest discoveries. Both these discoveries 

 were due to the achromatic microscope. The long period 

 of about two hundred years which passed while the uncor- 

 rected magnifying lens and the compound microscope with 

 the uncorrected objective were in use, seems to have been 

 mainly devoted to detail. 



The Cell Theory of Organisms. — The first great micro- 

 scopical discovery was probably the cell theory of living 

 organisms. ''Every cell from a cell" was, and still is, the 

 foundation of ontogeny and even phylogeny. It gives the 

 writer, as a botanist, pleasure to note that this discovery 

 was made at least as much from microscopical observations 

 of plants as from studies of animal tissues. The important 

 deductions are: that each plant and animal normally 

 comes from a single cell; that all the different tissues are 

 formed by (usually) permanent changes occurring in certain 

 of the cell progeny of the original cell; and that the faculty 

 of reproduction, and of that partial reproduction called 

 ''regeneration," shows, in the tissues of plants and animals, 

 all grades — from complete reproduction by any cell, to 

 the absence of any cell division at all. These more or less 

 permanent changes in the cells as they multiply to build 

 up the tissues are almost without any explanation today, 

 and remain a challenge to investigators. Even tissue 

 cultures do not seem to have given a clue to the remarkable 

 stability of the tissues when in situ. 



It was found also that two reproductive cells, usually of 

 different ancestry and appearance, combined to form the 

 original cell of each new being; but why this happened was 

 not clear. 



