300 PROGRESS OF SCIENCE IN THE CENTURY. 



especially to the precise application of chemical and 

 physical methods to physiological problems. On the 

 chemical line, the researches of Wohler, Liebig, 

 Claude Bernard, Pettenkofer and Voit, Ludwig, 

 Pflliger, Kiihne, Hoppe-Seyler, Bunge, Kalliburtou, 

 Kossel, Heidenhain, and many more have been mo- 

 mentous ; on the ph^^sical line we have especially to 

 remember the achievements of Weber, Volkmann, 

 Ilelmholtz, du Bois-Reymond, Marey, Pechner, Lud- 

 wig, Briicke, Pfliiger, Foster, and Burdon-Sander- 

 son. But both lines of work have been prosecuted 

 by so many that it is almost invidious to mention 

 names at all. 



PHYSIOLOGY OF TISSUES. 



The Beginnings of Tissue. — The simplest living 

 creatures are single corpuscles of living matter, 

 structurally comparable to the individual unit-areas 

 or cells which build up the body of a higher plant or 

 animal, but functionally different since each one is 

 necessarily " physiologically complete in itself," 

 while the cell of a more complex creature shows more 

 or less restriction of function as the result of the 

 division of labour in the body. 



Even when we pass a step upwards to the simplest 

 multicellular organisms, such as the beautiful spher- 

 ical colony or community of cells called Yolvox, we 

 do not yet find tissues. The members of the com- 

 munity, though numerous, are almost quite like one 

 another ; there is little or no division of labour. 



A step higher, however, in the more complex Alg£e 

 and Fungi among plants, and in sponges among ani- 

 mals, we find tissues, as it were, a -making. In a 

 sponge, for instance, we may see a number of elon- 



