366 DONNELL BROOKS YOUNG 



It lived for four days and regenerated one of the two left posterior 

 cirri. It may be that the cut left the base of this cirrus attached 

 to the right piece, and if so this accounts for the subsequent re- 

 generation. The cell never became normal. The length of 

 time which pieces lived after cutting seemed to depend on the 

 amount of injury done by the cut and on the amount of food 

 present in the cell. Several instances were noted in which the 

 knife crushed one part of the cell more than the other. This 

 crushing sometimes resulted in immediate death and sometimes 

 in merely delaying the process of regeneration. The amount of 

 food present in a cell could sometimes be told by the number of 

 food vacuoles visible. In one instance (no. 47) there was a large 

 food vacuole, and the piece having it lived for five days as against 

 two to four for the others. Even though in some cases a new 

 mouth may have developed in the amicronucleate piece, ap- 

 parently no new food was taken into the cell. Evidently the 

 pieces died of starvation. 



Summary of table 2. Thirteen individuals of U. setigera cut 

 into pieces of markedly unequal size during the first fifteen 

 minutes after division are hsted in table 2. In only two cases 

 was any regeneration noted in the piece without a micronucleus. 

 Both of these pieces had the posterior cirri attached and the 

 anterior end not only closed over, but new anterior membranelles 

 formed. These pieces lived more than twenty-four hours and 

 shortly before death had the appearance of a very much trun- 

 cated but otherwise normal individual. In ten cases no regen- 

 eration was seen in the small fragment and in most of them 

 death was immediate. Experiment no. 122 is especially interest- 

 ing, for in this case, although the large piece lived for thirty 

 hours, no regeneration took place. The fragment was then killed 

 and stained. No micronucleus was found (fig. 22). The small 

 piece died very soon after cutting, without any signs of regenerat- 

 ing. The cut was recorded as transverse C. If the plane was 

 a little farther forward than C, it seems probable that the mi- 

 cronucleus was injured or destroyed. From experiments which 

 were performed and which will be described later, this is the 

 probable explanation of this case. 



