6 ESSENTIALS OF ZOOLOGY 



has rarely been seen, that two Amoebae are attracted to one 

 another, fuse, and as a result of the fusion a fresh series of 

 ordinary reproduction divisions takes place. This process of 

 fusion of two individuals is termed conjugation, or syngamy. 



Encystment has been frequently observed. Provoked by 

 some such external influence as advancing cold, drought, want 

 of food, or probably occurring as a necessary rhythm of life, 

 the Amoeba becomes spherical and secretes around itself a 

 mucous coat within which first one and then a second thin 

 coat is formed. Thus invested, it remains for a variable period, 

 but usually about three months. In some cases the Amoeba 

 appears to escape as it becomes encysted as a single Amoeba, 

 but usually the nucleus undergoes fragmentation and a large 

 number of small Amoebae is liberated. 1 



It will be noted that under ordinary conditions the Amoeba 

 reproduces the same form with remarkable fidelity. But some 

 degree of modification is possible under stress of conditions. 

 One of these is encystment ; another is the production of 

 flagella. It has been demonstrated by Wilson that the soil 

 Amoeba, which has been described under the name Naegleria 

 gruberi, may become flagellated, that this state is assumed to 

 escape rapidly from such bad conditions as too high a tempera- 

 ture or too intense light, and he was able to produce the change 

 by adding distilled water to his cultures. 2 This method had 

 already been followed by Wherry 3 in producing flagellates 

 of soil Amoebae. 



Wilson also found that the nucleus has a structure and 

 undergoes processes during division which are remarkably 

 similar to those of the cells of Metazoa. The stainable material 

 of the resting nucleus is gathered into a part of the nucleus, 

 forming a nucleolus or karyosome. When division is about to 

 take place (prophase) the more firm or gel constituents of the 

 nucleus are arranged in the shape of a spindle of eight fibres, 

 which presents two opposite poles and an equator. The 

 karyosome then becomes dumb-bell shaped and divides into 

 two masses, which are attracted one to each of the poles. The 



1 1915. Carter, Proc. Roy. Phys. Soc., Edinburgh, vol. 19. 



2 1916. Wilson, Univ. of California Publ. in Zoology, vol. 16. 



3 1913. Wherry, Arch. f. ProtistenJcunde, Bd. 30. 



