(11) 



only state that rr.y cultures have repeatedly been interrup- 

 ted, usually in January or February, by what I take to be 

 epidemics of sexual reproduction. At the beginning of 

 such epidemics, the protoplasts of nearly every indivi- 

 dual in the cultures divide into a nurrber of small amebae, 

 which show no trace of a formed nucleus, but contain 

 scattered chrom.atinic granules. These amebae escape from, 

 the shell, leaving it empty, I was unable to follow their 

 further history. 



Methods . 



Cultural technique. In keeping Centropyxis 

 in cultures in the laboratory, my aim was to keep the 

 environmental conditions as near like the natural ones 

 as was possible. Each individual was isolated in the de- 

 pression of a hollow-ground slide, in about five drops of 

 pond water. To th.is water was added, at the time of iso- 

 lation, a few strands of the Oedcgonium, in the mats of 

 which this rhizopod was living, as well as seme debris 

 taken from among the matted filaments of the alga. i'his 

 debris furnished diatoms, the principal constituer t of 

 the food of the rhizopod, as well as sand grains to be 

 used in the construction of new shells. 



In these miniature aquaria the water was changed every 

 two or three days, and once a week all the individuals 

 were transferred to freshly prepared slides, to avoid any 

 possibility of an injurious accumulation of either ex- 

 cretion products or bacteria. Whenever a fission occur- 

 red, the new individual was removed with a capillary 



