412 ANN BISHOP 



are unable to reach normal size again before the next division 

 occurs. After this division they are therefore still smaller 

 than half the normal, and this decrease in size is progressive, 

 producing a culture full of small individuals. 



(3n the other hand, in other cultures the stimulus to division 

 was evidently weak or in abeyance, although assimilation and 

 growth were in no way impaired. The Spirostoma divided, 

 therefore, infrequently, and grew to far beyond the average 

 size, sometimes attaining as much as two and a half times the 

 size of the ordinary individuals. 



In starving cultures also, or in tap-water, they become 

 very small. The cytoplasm seems to decrease more rapidly 

 than the nucleus, since in such animals the nucleus is much 

 coiled together. 



A curious phenomenon, observed in most cultures on certain 

 occasions, was the agglomeration of many Spirostoma into 

 balls and strings. In an undisturbed condition of such a 

 culture these clumps were suspended in the fluid, but they 

 sank to the bottom if they were agitated. This condition was 

 very marked at the time of conjugation in nearly all the cultures 

 which contained conjugants. Lebedew (15) describes a similar 

 massing together of individuals of Trachelocerca in the material 

 from which he obtained his conjugants. He believed that the 

 animals congregated around food-bodies. I have seldom been 

 able to find any food forming the nucleus of the balls occurring 

 in my cultures. Calkins (4) states that in Uroleptus 

 mobilis epidemics of conjugation 'are invariably preceded 

 by a characteristic massing or agglomeration of individuals '. 

 By transferring these masses to other dishes containing fresh 

 culture medium, he invariably obtained epidemics of con- 

 jugation. 



In many cultures Spirostoma were observed adhering to 

 the sides of the tubes, evidently by the mucous secretion 

 described in this animal by Jennings (12). It is possible 

 gently to draw them away a little from the solid to which they 

 adhere without severing the mucus. I am of the opinion that 

 the suspended agglomerations are formed in a similar v/ay, by 



