THE STRUCTURAL BASIS OF THE BODY 27 



In this case the work performed by the muscle was 48 X 3*8 = 182 '4 

 grm. mm., while the potential energy of the stimulus represented only 

 0*5 X 10 '0 = 5'0 grm. mm. Thus the work performed by the muscle 

 was thirty- six times larger than the energy of the stimulus applied to the 

 nerve. 



In the case of unicellular organisms, definite classes of motor reaction to stimulus 

 have been described. The ordinary retraction of a unicellular organism, such as the 

 vorticella, in response to a touch is called thigmotaxis. Certain cells are influenced 

 by gravity, tending to rise or fall in the surrounding medium according to the conditions 

 which favour their existence. A similar sensitiveness to gravity is observed in the 

 growing parts of plants, where the root always grows downwards and the stem up- 

 wards. This reaction to gravity is known as geotaxis, which is distinguished as * nega- 

 tive ' or ' positive ' respectively, according as the plant grows hi opposition or in obedi- 

 ence to the gravitational attraction. If growing plants be placed on the rim of a 

 wheel and rotated so that the centrifugal force is greater than that of gravity, the 

 stems all grow towards the centre of the wheel while the rootlets grow outwards. In 

 the same way the reaction of micro-organisms to light is known as phototaxis, some 

 organisms seeking the light while others shun it. Among the primitive reactions of 

 cells perhaps the most important in the life of higher animals are those grouped under 

 the term chemiotaxis. The fertilisation of the ovum in the prothallus of ferns is effected 

 by the penetration of the antherozoids produced in the male organs at some little 

 distance from the female organs. It was shown by Pfeffer that the movement of 

 the antherozoids towards the ova is effected in response to a chemical stimulus, probably 

 malic acid, since he found that antherozoids suspended in a fluid will always swim 

 towards any locality where there is a gre&ter concentration of this acid. In the same 

 way aerobic bacteria are attracted by the presence of oxygen. If such bacteria are 

 present in a solution with an alga, on exposure of the fluid to light there is an evolution 

 of oxygen by the green alga, and a consequent congregation of the bacteria round the 

 seat of production of the oxygen. The movements of the white corpuscles of the blood 

 of the higher animals are also largely determined by their chemical sensibility, and 

 various substances can be divided into (a) those which exercise positive and (&) those 

 which exercise negative chemiotactic influence on the leucocytes. Thus the intro- 

 duction under the skin of an animal of a capillary tube containing a solution of sub- 

 stances of the first class, such as peptone, tissue extracts, or the chemical products 

 of certain bacteria, leads to an accumulation within the tube of leucocytes which pass 

 to it from all the surrounding tissues. Other substances, such as quinine, exert a nega- 

 tive chemiotaxis. Tubes filled with these, after introduction into the subcutaneous 

 tissue of a mammal, will be found many hours later to contain no leucocytes at all. 



THE RELATIONS OF THE NUCLEUS TO THE CYTOPLASM. The 

 universal existence in living cells of a differentiated nucleus indicates that 

 the life cycle of assimilation and dissimilation must depend on an interaction 

 between the nucleus and cytoplasm, and that each plays a distinct part in the 

 sum of the changes which make up the life of the cell. The different staining 

 reactions of nucleus and cytoplasm suggest a corresponding difference in their 

 chemical composition, a suggestion which is confirmed by analysis. In the 

 building up of protoplasm proteins play an important part. They are not 

 present, however, as simple proteins, but built up with other complex bodies 

 to form conjugated proteins. Whereas in the cytoplasm these conjugated 

 proteins consist chiefly of compounds of protein and lecithin, in the nucleus 

 the chief constituents belong to the class of nucleo-proteins. The nucleo- 

 proteins are of varying composition, and are distinguished chiefly by the 



