380 DONNELL BROOKS YOUNG 



the posterior cirri never regenerated, but were replaced by new 

 ones at the following division. This suggested that possibly 

 the cirri differed from the rest of the cell in their power to re- 

 generate, and this led to further experimentation. In all, 

 eight attempts were made to remove part of the cirri without 

 injm-ing the cell. It was possible to tell whether or not the 

 body was injured, for when the cell was cut the cirri were held 

 together by the protoplasm removed with them. Six of the 

 eight attempts were successful. Without the posterior cirri 

 the animals swam about normally, but did not dart and jump. 

 In no case did regeneration of the missing cut cirri take place, 

 even though the cutting was done from twenty-two minutes 

 to four hours after division. Figure 27 represents a cell whose 

 cirri were removed two hours and forty-five minutes after divi- 

 sion. The drawing was made from life twelve hours later. 

 In this, as in the other five of the six successful experiments, 

 the new cirri formed at the division period to replace the muti- 

 lated ones. 



Stained preparations, both of total mounts and of sections, 

 of normal individuals show that the cirri have a well-developed 

 plate of basal granules imbedded in an area of dense protoplasm, 

 as Maier ('02) has demonstrated for the cirri of Stylonychia 

 histrio. If this basal plate is removed or injured, new cirri 

 will form, but if the cirri themselves are cut without touching 

 the basal granules no new growth takes place. The basal gran- 

 ules therefore do not have the power in themselves to reform the 

 cirri. New basal granules can be formed from which new cirri 

 will develop, but the old basal granules apparently have no 

 power to replace lost parts of cirri. Evidently the development 

 of the cirri is dependent on the activity of the basal granules 

 and once a basal plate has formed a cirrus, its power of causing 

 further growth ceases under normal conditions. In experi- 

 ments in which the micronucleus was injured or destroyed dur- 

 ing the division of the animal, as for instance experiment no. 

 114, figure 25, the cirri did not stop growing when they had 

 reached normal length and so became abnormally long. This 

 may indicate that the micronucleus has some influence over the 

 activity of the basal granules and that if it is injured during the 



