THE STRUCTURAL BASIS OF THE BODY 27 



In this case the work performed by the muscle was 48 x 3-8 = h 



grm. mm., while the potential energy of the stimulus repi 



0-5 X 10-0 -5-0 grm. mm. Thus the work performed by the m> 



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



nerve. 



In the case of unicellular organisms, definite classes of motor reaction to >timulu< 

 have been described. The ordinary retraction of a unicellular organi 

 vorticella, in response to a touch is called thigmotaxi*. Certain e,ll> ;1I . mtlu. 

 by gravity, tending to rise or fall in the surrounding medium according to t he ,n,| 

 which favour their existence. A similar sensitiveness to gravity is ol.-.rv. d , 

 growing parts of plants, where the root always grows downwards and th. ,t, . up 

 wards. This reaction to gravity is known as qeotaxis, which is distinguished as ' i 

 tive' or 'positive' respectively, according as the plant grows in opposition . 



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

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

 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 phatotaxix, MMH- 

 organisms seeking the light while others shun it. Among the primitive reaction-, of 

 cells perhaps the most important in the life of higher animals are those group-d under 

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

 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, prohaNy 

 malic acid, since he found that antherozoids suspended in a fluid will always s\\im 

 towards any locality where there is a greater concentration of this acid. In the same 

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

 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 t lie- 

 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 smsil.iliiy. and 

 various substances can be divided into (a) those which exercise positive and (b) 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 Mil. 

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

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

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

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

 tissue of a mammal, will be found many hours later to contain no leucocyte; 



THE RELATIONS OF THE NUCLEUS TO THE CYTOPLASM. The 

 universal existence in living cells of a differentiated nucleus indicate* that 

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

 between the nucleus and cytoplasm, and that each plays a distinct pai 

 sum of the changes which make up the life of the cell. The Afferent 

 reactions of nucleus and cytoplasm suggest a corresponding differ* 

 chemical composition, a suggestion which is confirmed by analvs, 

 building up of protoplasm proteins play an important part. 

 present however, as simple proteins, but built up with other c 01 

 to form conjugated proteins. Whereas in the cytoplasm the. 



- 



proteins consist chiefly of compounds of protein and k^ , - 

 the chief constituents belong to the class of nucleo-prote, 

 proteins are of varying composition, and are cl,st,n,rish,,l 



