THE CELL 17 



Phototaxis. Light, which plays so important a part in 

 directing the movements of the higher plants, also acts 

 positively or negatively on many unicellular organisms. 

 Thus the swarm spores of certain algse are positively at- 

 tracted by moderate illumination, streaming to the source 

 of light, while they are negatively stimulated by strong 

 light, and stream away from it. Light also plays an im- 

 portant part in directing the movements of certain bacteria. 



Thermotaxis. The unilateral influence of temperature is 

 well seen in the plasmodium of sethaliuin septicum which 

 streams from cold water towards water at a temperature of 

 about 30 C. 



Gfalvanotaxis. As would naturally be expected from its 

 stimulating action, a current of electricity has a most power- 

 ful effect in directing the movements of many cells. Certain 

 infusoria when brought between the poles of a galvanic 

 battery may be observed to stream towards the negative pole. 



The effects of this unilateral stimulation are of great 

 importance in physiology and pathology, since they explain 

 the streaming of leucocytes to attack micro-organisms and 

 other poisons to the animal body, and since they seem to 

 explain many of the apparently volitional acts of unicellular 

 organisms. Many of these organisms appear to definitely 

 select certain foods, but in reality they are simply compelled 

 towards them by this unilateral stimulation. 



(B) Nucleus. 



(1) Structure. The nucleus, seen with a moderate mag- 

 nifying power, appears in most cells as a well-defined circular 

 or oval body situated towards the centre of the cell. (Figs. 1 

 and 2.) Sometimes it is obscured by the surrounding proto- 

 plasm. It has a granular appearance, and usually one or 

 more clear refractile bodies the nucleoli are seen within 

 it. It stains deeply with many reagents of a basic reaction, 

 such as hsematoxylin, carmine, methylene blue, &c. In 

 some cells the nucleus is irregular in shape (Fig. 2), and in 

 some it is broken up into a number of pieces, giving the cell 

 a multi-nucleated character. 



It is usually composed of a mass of fibres arranged in a 



2 



