i;6 SCIENCE FROM AN EASY CHAIR 



or makers of the tissue. The cells in one tissue may 

 form a honeycomb of boxes ; in another a jelly-like 

 mass or a fibrous network, with the cell-substance 

 scattered as nucleated particles in it (Fig. 39). Or the 

 cells may be elongated and contractile (Fig. 37, E, F). 

 They may be more or less fused with one another, as 

 in flesh or muscular fibre ; but we can always recognise 

 the presence of the individual cells under the microscope 

 by their distinct and separate " nuclei." 



Schwann's most important conclusion from this uni- 

 versal presence of soft corpuscles of cell-substance, each 

 with its globular nucleus, in all the tissues and most 

 varied parts of animals as well as plants, was that the 

 life of a living thing, the chemical and physical changes 

 which go on in it from birth to death, consist in chemical 

 and physical changes in each of these microscopic, nu- 

 cleated bodies, and that the life of the whole animal or 

 plant is the sum of the lives of these microscopic units. 

 If we wish to know more about the real nature of the 

 growth and activities of living things, said Schwann, we 

 must thoroughly study and ascertain the chemical and 

 physical changes, and the properties of the cell-substance 

 in all the different varieties of tissue. That is the cele- 

 brated " cell-theory " of Schwann. And this examina- 

 tion of, and experiment with, the cells of all kinds of 

 tissues of plants and animals has been going on ever 

 since Schwann made his historic statement more than 

 seventy years ago. The branch of science called " his- 

 tology " is the outcome of that study. 



Microscopes have been immensely improved since 

 Schwann wrote, first in England by the father of the 

 present Lord Lister, then later in Germany by Abbe 

 and Zeiss, of Jena. A variety of methods have been 

 devised for making the " cells " in thick, solid tissues 

 visible. Very thin sections thin enough to be trans- 



