AMCEBA 



117 



In certain circumstances Amoeba withdraws its pseudo- 

 podia 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 

 phenomena known as suspended vitality, and found, for in- 

 stance, in seeds and in frozen tissues (see p. 86). 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 cer- 

 tain circumstances. At death, 

 on the other hand, the pro- 

 toplasm passes into a con- 

 dition in which it will indeed 

 remain intact in suitable cir- 

 cumstances (as when it is 

 frozen) but has lost the power 

 of resuming life. Many other FlG . 72 ._ A diagram of the fission 

 small freshwater animals can of Amaba. The dark spots 



pass into a state of suspended represent nuclei. 

 vitality, and dried mud 



brought to England from places thousands of miles away 

 will often, on being placed in water, give rise to numerous 

 creatures that thus lay dormant in it. The artificial sus- 

 pension of vitality in tissues by freezing has been made use of 

 with very striking results in surgery, portions of living tissue 

 removed from one body being preserved for days in a state 

 of suspended vitality by cold and afterwards successfully 

 grafted in to repair injuries in another body. In this way, 

 for instance, the cornea taken from an eye that had for 



1 It is doubtful whether the resting cyst of A. proteus has been seen. 

 Such cysts are known in other kinds of Amceba, and in spore formation 

 (see p. 118) A. proteus shows that it has the power of encystment. 



