250 SPELERELLA CHAP, j 



. 



end, where it tapers to a point. The lashing movements 

 are brought about by the flagellum bending successively 

 in different directions (F.). Thus the ciliary movement 

 of Sphaerella, like the amoeboid movement of Amoeba, 

 is a phenomenon of contractility. Imagine an Amoeba to 

 draw in all its pseudopods but two, and to protrude 

 these two until they became mere threads : imagine 

 further these threads to contract rapidly and more or 

 less regularly instead of slowly and irregularly ; the I 

 result would be the substitution of pseudopods by 

 flagella, i.e., of temporary slow-moving protoplasmic 

 processes by permanent rapidly-moving ones. 



To put the matter in another way : in Amoeba the 

 function of contractility is performed by the whole 

 organism ; in Sphaerella it is discharged by a small part 

 only, viz., the flagella, the rest of the protoplasm being 

 incapable of movement. 



Sphaerella multiplies, after becoming quiescent, in the 

 encysted condition (Fig. 68, C, D) ; as in the active 

 Amoeba (p. 236), its protoplasm undergoes simple or 

 binary fission, but with the peculiarity that the process 

 is immediately repeated, so that four daughter-cells are 

 produced within the single mother-cell-wall. By the 

 rupture of the latter the daughter-cells are set free as 

 the ordinary motile form, acquiring their flagella and 

 detached cell- wall before making their escape (D). 



Under certain circumstances the resting form dividesj 

 into numerous smaller daughter-cells, and these when 

 liberated are found to have no cell- wall. Sphaerella 

 therefore occurs, in the motile condition, under two dis- 

 tinct forms, i.e., is dimorphic : the larger or ordinary 

 form with detached cell-wall is called a megazooid, the 

 smaller form without a cell-wall a microzooid. The 

 microzooids, which are all similar to one another, are 

 (in S. btitschlii) liberated as gametes which conjugate in 



