372 DONNELL BROOKS YOUNG 



Cells cut in early division 



Summary of table 7. Nine individuals of U. setigera were cut 

 in various planes during the early stages of division. At this 

 period the micronucleus has not begun to elongate in a spindle, 

 and any plane passing just above or just below the center leaves 

 it uninjured. Two such cuttings were made. In experiment 

 no. 3 the anterior end of the dividing cell was removed. Figure 

 23 was drawn from the stained preparation of the regenerated 

 amicronucleate anterior fragment. Aside from the lack of a mi- 

 cronucleus the cell is coniplete. The posterior fragment com- 

 pleted its division before the missing parts were regenerated. 

 This process took about ten hours and the complete sister cell 

 divided twice before the regenerated one did once. In experi- 

 ment no. 74 the animal was cut so that, as in experiment no. 3, 

 the posterior part contained the micronucleus. However, in 

 this case regeneration was completed before division. At the 

 time of cutting the process of division had not progressed as 

 far as in the first case. Evidently up to a certain stage in divi- 

 sion the process can be stopped for the time being, but beyond 

 that stage the momentum of division is sufficient to carry through 

 that process regardless of other influences. Two individuals, 

 nos. 34 and 65, were cut exactly in the center, and although 

 the pieces regenerated in but little longer than it would have 

 taken for the cells to have completed the division which they 

 had begun, they never divided. It is quite certain that the 

 micronuclei were destroyed. Figure 24 shows one of these 

 cells, the anterior from experiment no. 65, which was killed and 

 stained as soon as its sister cell died. No micronucleus was 

 present. In the case of three individuals only small fragments 

 were removed. In these division progressed normally and the 

 lost parts were replaced only after the completion of the division. 

 The time taken for regeneration was about the same as that 

 which was needed for the regeneration of a similar part when the 

 cut was made on a cell within the first few minutes after division. 

 One (no. 83) was cut in a longitudinal plane. Both parts com- 

 pleted division and the two cells with micronuclei regenerated. 



