16 VETERINARY PHYSIOLOGY 



Thermotaxis. The unilateral influence of temperature is 

 well seen in the plasmodium of sethalium septicum which 

 streams from cold water towards water at a temperature of 

 about 30 C. 



Galvanotaxis. 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 definitely to 

 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 (c) 

 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 haematoxylin, carmine, methylene blue, etc. In some cells 

 the nucleus is irregular in shape, 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) fibres arranged in a complicated 

 network (fig. 2). These fibres appear to be of two kinds : (1) 

 forming a network similar to the cytomitoma the linin net- 

 work (G) ; and (2) forming generally a coarser network, the fibres 

 of which have a special affinity for basic stains the chromatin 

 network (F). The chromatin substance contains a large amount 

 of nucleic acid, and its richness in phosphorus has been de- 

 monstrated by treating the cells with ammonium molybdate 

 and pyrogallol, which colours parts rich in phosphorus of a 

 brown or black tint. 



