SECT. I STRUCTURE AND PHYSIOLOGY OF ANIMALS 11 



changing its position. This it effects by a kind of streaming 

 motion. A projection forms itself on one side, and the entire 

 substance of the Amoeba gradually streams into it ; a fresh 

 projection appears towards the same side, the streaming move- 

 ment is repeated, and, by a constant succession of such move- 

 ments, an extremely gradual locomotion, which it often takes very 

 close watching to detect, is brought about. In these movements, 

 it is to be noticed, the Amoeba is influenced to some extent by 

 contact with other minute objects ; when the processes come in 

 contact with small grains of sand or other similar particles their 

 movements are modified in such a way that the Amoeba, in its 

 slow progress onwards, passes on one side of them, so that it 

 might be said to feel its way among the solid particles in the drop 

 of sediment. Experiments show that the direction of its move- 

 ments is capable of being influenced from without by other agencies 

 by light and heat, for example. So that we are led to conclude 

 that Amoeba has a fairly wide range of irritability or sensitiveness to 

 external influences or environment. 



Judging from the nature of these movements, we are obliged to 

 infer that the substance of which this remarkable object is com- 

 posed must be soft and semi-fluid, yet not miscible with the water, 

 and, therefore, preserving a sharp contour. The more or less fluid 

 character is evidenced not only by the nature of the movements, 

 but also by the facts that the clear spaces in the interior to be 

 presently noticed assume a rounded shape, and that if the Amoeba 

 be broken up, as it may be by pressure on the cover-glass, the frag- 

 ments become rounded off into droplets. These and other 

 characteristics to be mentioned subsequently enable us to conclude 

 that we have to do with the substance of complex chemical com- 

 position termed protoplasm, which constitutes the vital material of 

 all living organisms whether animals or plants. In Amoeba the 

 protoplasm is in many cases clearly distinguishable into two parts, 

 an outer homogeneous, glassy-looking layer (exoplasm) completely 

 enclosing a more granular internal mass (endoplasm). 



Examination of the Amoeba with a fairly high power of the 

 microscope reveals the presence in its interior of two objects which 

 with a low power we should be likely to overlook. One of these 

 is a small rounded body with well-defined contour, which 

 preserves its form during all the changes which the Amoeba as a 

 whole undergoes. This is termed the nucleus (Fig. 1, nu.) ; it is 

 enclosed in an extremely delicate membrane, and contains a 

 protoplasmic material differing from that which forms the main 

 bulk of the Amoeba in containing a substance which refracts the 

 light more strongly and which has a stronger affinity for certain 

 colouring matters. The other minute object to be distinguished 

 in the interior appears as a clear rounded space (c. vac.) in the 

 protoplasm, or several spaces which subsequently coalesce into one. 



