150 



THE CELL 



brane, composed of lipide, protein and water ; (ii) cytoplasm, a 

 water-in-colloid dispersion ; and (iii) the nucleus, a still more 

 complex dispersion within its own membrane. All three elements 

 are easily altered in physical state by alterations in the balance of 

 electrolytes in the protoplasm or in the environment. The whole 

 cell may be regarded as a polyphase crystalloid-colloid complex in 

 unstable equilibrium. " When," says Sharpey-Schafer, " the 

 chemist succeeds in building up this complex it will, without doubt, 

 be found to exhil^it the phenomena which we are in the habit of 

 associating with the term ' life '. " What are the phenomena 

 commonly associated with the term " life," especially as mani- 

 fested by a imicellular animal ? 



{a) Movement is the commonest phenomenon indicative of life. 



Amoeba moves. It extrudes 

 footlike processes, pseudo- 

 podia (Gr. pseudio, false 

 (= similar to), podes, foot), 

 at one part and retracts 

 them at another and so 

 moves along. Similar amoe- 

 boid movements are cha- 

 racteristic of the white 

 cells or leucocytes (Gr. 

 leukos, white) of the blood. 

 Recently, Goodrich has care- 

 fully studied these move- 

 ments of the leucocytes. 

 He produces camera lucida 

 drawings to show that the pseudopodia usually take up the 

 form of expanded motile membranous folds when the living 

 leucocytes are examined suspended freely in the normal fluid 

 which is their habitat. One of his drawings is reproduced in 

 Fig. 36. Movements of a precisely similar character may be 

 produced in substances which are certainly not alive, such as 

 Brailsford Robertson's model amoeba made of camphor, benzene 

 and water (Part II.). These purely physico-chemical reactions 

 are produced by alterations of the surface tension of the fluids 

 under observation. Macallum has shown (pp. 173 and 185) that 

 alterations in surface tension occur in living tissue during motion. 

 Movement can, therefore, not be considered as a specifically vital 

 phenomenon. Certain parts of the cell, e.g. the vacuoles, show a 

 rhythm in their movements. In polycellular organisms, certain 

 organs, e.g. the heart, pulsate. It is comparatively easy to produce 

 rhythmical movement in material which is not living. A globule 





Fig. 36. — Leucocyte of invertebrate. (Redrawn after 

 Goodrich.) 



