I THE PROTOZOA 9 



tion appears in the protoplasm, which gradually deepens and 

 separates it into two halves, each of which contains half of 

 the original nucleus. In this way, by " simple fission " of the 

 parent organism, rep-odmtion has taken place. 



Each daughter Amoeba lives exactly as did the parent 

 cell, finally dividing to form two new Amoebae, and so 

 the life of the first individual never ceases, but becomes 

 extended in the separate lives of its descendants. There seems 

 no necessary natural end to the life of such a simple form, 

 though death must frequently occur through an accident, 

 such as the drying up of the necessary water environment, or 

 the digestion of the Amoeba by another higher form. 



An Amoeba can only live under certain condi- 



Stage^ tions of life as to temperature, moisture, and air 

 supply. If external conditions become unfavourable 

 to its active life, it can protect itself for a time by secreting 

 fi'om the superficial layer of its protoplasm a horny or 

 chitinous substance, which hardens and forms a thin shell or 

 " cyst " over it ; within this it can remain passive for a time, 

 until conditions are favourable again and it can once more 

 resume its normal activity ; it then ruptures the cyst and 

 escapes from it. 



Two Amoebae, on meeting, have been observed to 



^sfon^ fuse together for a time and then to separate, and 



it is thought that as a result of the stimulus given 



by such fusion, they may afterwards carry on their separate 



lives more vigorously, for this is known to be the case in 



some other forms. 



From the above observations we see that this very simple 

 unicellular form, which has no definite sense perceptions 

 and no organs of any kind, though to a certain degree 

 sensitive to external stimuli, yet exhibits all the main pheno- 

 mena of life, and the very simplicity of its structure and 

 meagreness of its requirements seem to further its success in 

 the struggle for existence. 



There are several species of Amoebae dififering 



8^601^' ^" ®^^® *"*^ ^^ *^® chai-acter of the nucleus and 

 in other small points, but all so similar in their 

 main characters that they are called by the same generic name. 

 Amoeba. 



The species of Amoeba described above is Amoeba proteus, 



