8PIR0ST0MUM AMBIGUUM 411 



The chief differences between the two varieties, therefore, 

 can be summarized as difference in size, in colour of proto- 

 plasm, in position of mouth and length of peristomial area, 

 and finally in the shape and relative size of the contractile 

 vacuole and in its periods of contraction. 



7. Reproduction. 



A. Observations on the Growth and Eeproduc- 

 tion of Spirostomum ambiguum during Cul- 

 tivation. . 



In many cultures of the large variety of Spirostomum 

 ambiguum (see p. 404) made by both the culture methods 

 described above, it was observed that the size of the individuals 

 in different cultures varied enormously. Observations made 

 upon animals grown in a rich wheat or leaf culture medium 

 and kept at a constant temperature of 16° C. in the incubator, 

 showed that they were larger than those grown in a similar 

 culture at a higher temperature. Even when the culture 

 was not very rich in food and was kept at a temperature of 

 about 16° C. the individuals were very large. That these 

 differences in size were not due to differences of race in the 

 Spirostoma was proved by reversing the conditions, when 

 the animals altered in size correspondingly. 



Further, it was observed that individuals in the cultures 

 which were kept at 20° C. divided more rapidly than did those 

 in cultures kept at 16° C. or lower. This was confirmed by 

 starting two cultures similar in every way and containing the 

 same number of Spirostoma but keeping one at 20° C. and 

 the other at 16° C. The former increased more rapidly than 

 the latter. 



This is probably the whole reason for the variations in size 

 found in Spirostoma ambiguum major in the various 

 cultures. When the cultural conditions favour rapidly repeated 

 divisions the individuals become smaller than the normal size 

 of the species. At each division their size is halved and the 

 intervals between successive divisions are so short that they 



