566 POPULAR SCIENCE MONTHLY. 



when microscopes of the best construction are employed, the determina- 

 tion of the intimate nature of the tissue, and the precise relation which 

 one element of an organ bears to the other constituent elements, is, in 

 many instances, a matter of difficulty. Hence additional methods have 

 had to be devised in order to facilitate study and to give precision and 

 accuracy to our observations. It is difficult for one of the younger 

 generation of biologists, with all the appliances of a well-equipped 

 laboratory at his command, with experienced teachers to direct him in 

 his work, and with excellent text-books, in which the modern methods 

 are described, to realize the conditions under which his predecessors 

 worked half a century ago. Laboratories for minute biological research 

 had not been constructed, the practical teaching of histology and em- 

 bryology had not been organized, experience in methods of work had 

 not accumulated; each man was left to his individual efforts, and had 

 to puzzle his way through the complications of structure to the best of 

 his power. Staining and hardening reagents were unknown. The 

 double-bladed knife invented by Valentin, held in the hand, was the 

 only improvement on the scalpel or razor for cutting thin, more or less 

 translucent slices suitable for microscopic examination; mechanical 

 section-cutters and freezing arrangements had not been devised. The 

 tools at the disposal of the microscopist were little more than knife, 

 forceps, scissors, needles; with acetic acid, glycerine and Canada balsam 

 as reagents. But in the employment of the newer methods of research 

 care has to be taken, more especially when hardening and staining 

 reagents are used, to discriminate between appearances which are to be 

 interpreted as indicating natural characters, and those which are only 

 artificial productions. 



Notwithstanding the difficulties attendant on the study of the more 

 delicate tissues, the compound achromatic microscope provided an- 

 atomists with an instrument of great penetrative power. Between the 

 years 1830 and 1850 a number of acute observers applied themselves 

 with much energy and enthusiasm to the examination of the minute 

 structure of the tissues and organs in plants and animals. 



CELL THEORY. 



It had, indeed, long been recognized that the tissues of plants were 

 to a large extent composed of minute vesicular bodies, technically called 

 cells (Hooke, Malpighi, Grew). In 1831 the discovery was made by the 

 great botanist, Robert Brown, that in many families of plants a circu- 

 lar spot, which he named areola or nucleus, was present in each cell; 

 and in 1838 M. J. Schleiden published the fact that a similar spot or 

 nucleus was a universal elementary organ in vegetables. In the tissues 

 of animals also structures had begun to be recognized comparable with 

 the cells and nuclei of the vegetable tissues, and in 1839 Theodore 



