RESPONSE TO STIMULI 



usually arranged in a distinct network, confluent into a " parietal 

 layer" within the nuclear wall; the meshes traversing a. cavity 

 full of liquid, the nuclear sap, and containing in their course the 

 granules ; while in the cavity are usually found one or two droplets 

 of a denser substance termed " nucleoles." These differ slightly 

 in composition from the chromatin granules l (see p. 24 f.). 



The movements of the leucocyte or Amoeba are usually 

 most active at a temperature of about 40 C. or 100 F., the 

 " optimum." They cease when the temperature falls to a point, 

 the " minimum," varying with the 

 organism, but never below freezing- 

 point ; they recommence when the 

 temperature rises again to the same 

 point at which they stopped. If now 

 the temperature be raised to a certain 

 amount above 40 they stop, but 

 may recommence if the temperature 

 has not exceeded a certain point, the 

 "maximum" (45 C. is a common 

 maximum). If it has been raised to 

 a still higher point they will not 

 recommence under any circumstances 

 whatever. 



Again, a slight electric shock will 

 determine the retraction of all pro- 

 cesses, and a period of rest in a 

 spherical condition. A milder shock 

 will only arrest the movements. But a stronger shock may 

 arrest them permanently. We may often note a relation of 

 the movements towards a surface, tending to keep the Amoeba 

 in contact with it, whether it be the surface of a solid or that 

 of an air-bubble in the liquid (see also p. 20). 



If a gentle current be set up in the water, we find that the 

 movements of the Amoeba are so co-ordinated that it moves up- 

 stream ; this must of course be of advantage in nature, as keeping 

 the being in its place, against the streams set up by larger 

 creatures, etc. (see also p. 21). 



If substances soluble in water be introduced the Amoeba will, 



Fig. 2. Ovum of a Sea-Urchin, 

 showing the radially striated cell- 

 membrane, the cytoplasm con- 

 taining yolk-granules, the large 

 nucleus (germinal vesicle), with 

 its network of linin containing 

 chromatin granules, and a large 

 nucleole (germinal spot). (From 

 Balfour's Embryology, after 

 Hertwig.) 



1 The chromatin and nucleoles are especially rich in phosphorus, probably in tli 

 combination nucleinic acid. 



