34 AMCEBA. PHYLUM PROTOZOA 



investigated in some other minute organisms, as, for instance, in 

 Paramecium (p. 55). 



ENCYSTMENT 



In certain circumstances Amoeba withdraws its pseudopodia 

 and becomes a rounded mass which secretes about itself a tough 

 case or cyst. In this it lies dormant and can survive the drying 

 or freezing of the pond in which it lives or be transferred in mud 

 to other ponds. We have here an instance of a widespread pheno- 

 menon known as suspended vitality, which is found, for instance, 

 in seeds and in frozen tissues. The exact condition of the 

 protoplasm in such cases is a mystery, but no vital processes 

 can be detected, and it has been shown by experiments on 

 seeds that, if they be kept perfectly dry, not even respiration 

 takes place. We must conclude that life, regarded as a process, 

 has slowed down and, at least in some cases, ceased, but that the 

 protoplasm retains the power of resuming it in certain circum- 

 stances. At death, on the other hand, the protoplasm passes into 

 a condition in which it will indeed remain intact in suitable 

 circumstances (as when it is frozen) but has lost the power of 

 resuming life. 



REPRODUCTION 



Amoeba reproduces by the process known as binary fission, in 

 which first the nucleus and then the cytoplasm parts asunder into 

 two halves, each of which appears, at all events, to differ from the 

 parent in nothing but size. The division of the nucleus is a peculiar 

 kind of mitosis (p. 688) in which the place of centrosomes is 

 taken by a mass of clear protoplasm at each end of the nucleus. 

 These masses are known as pole plates and arise within the 

 nuclear membrane, which does not break up during division as 

 in ordinary mitosis. After the division of the nucleus the cyto- 

 plasm flows apart into two bodies, each of which contains one 

 of the daughter nuclei. The new bodies are in some species at 

 first connected by a bridge of protoplasm, but this becomes 

 narrower until it breaks through and two new individuals come 

 into being, the whole process having taken about one hour. The 

 part where the break occurs becomes the ' tail ' of each daughter 



