120 AN INTRODUCTION TO ZOOLOGY 



producing a stippled appearance in the stained nucleus, is a constant 

 and essential constituent of all nuclei in plants and animals alike. 

 The endoplasm also contains certain spaces with a more or less fluid 

 content termed the vacuoles. When the food is taken in it imme- 

 diately becomes surrounded by fluid, thus constituting a food 

 vacuole in which the food is carried freely round in the endoplasm 

 while it gradually becomes digested. Certain other of these spaces 

 appear to contain nothing but a watery fluid, and so are called the 

 water vacuoles. Finally, we have a vacuole that is concerned with 

 the elimination of the liquid waste of the body. This may appear 

 quite small, but as we watch it, it gradually gets larger and larger, 

 a process termed diastole, until a limit is reached. At this point it 

 suddenly expels its contents to the outside by a sharp contraction 

 known as systole, and disappears temporarily to reappear later and 

 repeat the process. From this constant and fairly regular diastole 

 and systole it is known as the contractile vacuole, and it discharges its 

 contents through a temporary break in the ectoplasm. 



The various granules in the endoplasm are probably minute 

 quantities of digested food on their way to being built up into proto- 

 plasm, or stores of reserve material, or particles of protoplasm that 

 are being broken down into waste matter. Collectively they may be 

 called the metaplasmic granules. 



When the environmental conditions become unfavourable, 

 Amoeba assumes a different form. The pseudopodia are withdrawn, 

 the contractile vacuole is often much reduced, and the animal rounds 

 itself off and secretes around itself a tough cyst of chitin or a closely 

 allied substance. Inside the cyst the ectoplasm is more marked 

 than when it is moving about freely. The Amoeba remains quiescent 

 until upon the restoration of favourable conditions the cyst wall 

 ruptures and the animal comes out again. This encystment is to 

 be regarded as a protective adaptation enabling the animal to tide 

 over periods of stress, etc., and incidentally in this condition it can 

 be more readily conveyed from place to place. 



Let us turn now to consider the physiological activities of 

 the Amoeba. Movement takes place by the formation of pseudo- 

 podia, a process that can easily be watched in the living animal. 

 The ectoplasm bulges out to form a small knob, which gets larger 

 and larger, and then all at once the endoplasm bursts into it. This 

 streaming movement may continue until the whole protoplasm 

 has flowed into the pseudopodium, and thus a certain amount of 

 ground has been covered. On the other hand, after pushing out the 

 pseudopodium a short way, it may be withdrawn again. Certain 

 of the different species of Amoeba can be recognised by the shape of 

 their pseudopodia, which may be single or many, short and blunt, 



