440 GENERAL PHYSIOLOGY 



two ends of the spermatozoid that determines the appearance of 

 the chemotactic effect. Since the spermatozoid possesses only the 

 minute length of 0'015 mm., we can form an approximate idea of 

 how extraordinarily small must be the difference in concentration 

 at its two poles, and therewith the amount of the stimulus that 

 calls out a chemotactic effect. Thus, chemotactic phenomena and, 

 as we shall see, analogous phenomena caused by other stimuli, 

 give us a better idea than all other reactions of how excessively 

 feeble stimuli produce a remarkable effect upon living substance. 

 Living substance responds to extremely delicate influences. When 

 homoeopathy afSrms the effectiveness of extremely small quantities 

 of certain medicines, its claim in this respect is fairly justified, 

 however much superstition in other respects may attach to the 

 homoeopathic doctrine. 



2. Barotaxis 



All mechanical stimulation of living substance consists in a 

 change of the pressure- relations under which it exists. Every 

 degree of pressure can act as a stimulus, from crushing or cutting, 

 which destroys the continuity of the substance, down to the slightest 

 touch and the most delicate change in the pressure of the air or 

 the water that surrounds the organism. Under the unilateral action 

 of pressure-stimuli — in other words, in all cases where differences of 

 pressure exist upon two different parts of the body of an organism — 

 phenomena appear that correspond to those of chemotaxis. Since 

 these possess in common the one characteristic of being called forth 

 by pressure {^dpo<;) acting unequally on different sides, they may 

 be designated by the term harotaxis. Various kinds of barotaxis 

 can be distinguished according to the kind of pressure ■; and it can 

 be loositive or negative, according as the organism turns toward the 

 side of the higher or the lower pressure. 



Under tliigmotaxis, all those cases of barotaxis can be grouped 

 in which the phenomena are caused by the more or less strong 

 contact of living substance with more solid bodies. Naked proto- 

 plasmic masses, such as rhizopods and leucocytes, exhibit the 

 simplest form of this. These afford, indeed, striking examples of 

 how feeble contact calls out positive thigmotaxis, strong contact 

 negative, and how, analogously to the case of chemotaxis, 

 differences in the intensity of the stimulus are of essential 

 importance. If, e.g., a marine rhizopod, such as the often -mentioned 

 Orhitolites (Fig. 98, p. 238), be left quiet in a glass vessel contain- 

 ing sea-water, after some time pseudopodia begin to be put out 

 from the small openings in the calcareous shell. Consisting at first 

 of very short fibrils, they float freely in the water. Soon, becoming 

 longer and heavier, their ends sink to the bottom and become fixed 

 there by means of a delicate secretion, and the protoplasm begins 



