178 SCIENCE FROM AN EASY CHAIR 



living cells, in spite of these difficulties. In the first 

 place, there are a whole host of minute animals and 

 plants of many different kinds which consist of only 

 one cell or nucleated corpuscle of protoplasm (Fig. 

 36 A) ; they are transparent, abound in fresh water and 

 sea water, and can be searched for with the microscope in 

 a drop of water placed on a flat glass plate and covered 

 with a specially thin glass slip. Many of these have 

 been studied for hours^ and even days continuously, 

 and the remarkable internal currents and movements of 

 their viscid " protoplasm," its changes of shape, its feed- 

 ing and growth, and the details of the process of division 

 into two by which it multiplies have been ascertained, 

 as well as the action upon it of light, heat, electricity, 

 and mechanical shock, and of all sorts of chemical sub- 

 stances, carefully introduced beneath the cover-glass. A 

 second fact of great importance is that the " cells " or 

 protoplasmic corpuscles, which build up a complex plant 

 or animal, do not die at once when the plant or animal 

 " dies," that is to say, the animal or plant may be " killed " 

 and fine bits of transparent tissue removed from it and 

 placed beneath the microscope, where, with proper care, 

 the cells may be kept alive for some time. The hairs 

 of many plants are strings of transparent " cells," or 

 boxes, containing living, streaming, active protoplasm. 

 These hairs can be cut off, and the cells will remain alive 

 for a long time whilst they are under the microscope (see 

 Fig. 15 bis). The transparent wall of the eye called 

 the cornea can be removed from a frog after it has 

 been killed, and the still-living cells in the delicate glass- 

 like tissue can be studied with the highest powers of 

 the microscope, and give evidence of their life by their 

 movements and other changes. Most convenient and 

 important for this study is the blood for there the 

 cells are loose, floating in the liquid. The cells in a 



