SPIROSTOMUM AMBIGUUM 429 



that the chromatin was gathering at one side of the halo 

 (PI. 23, tig. 10), the rest of the niicronucleus heing ahnost colour- 

 less. At the opposite side of the niicronucleus to the aggrega- 

 tion of chromatin a projection appeared, lengthening until it 

 touched the membrane at the edge of the halo. Threads 

 formed between the chromatin aggregation, which now became 

 broken up into definite granules, and the apex of the projec- 

 tion (PI. 23, fig. 11). This I took to be the beginning of the 

 formation of a spindle ; but, in his work upon Stentor, Mulsow 

 (21) states that such a condition may be preparatory to the 

 formation of the spindle or a stage in degeneration prior to 

 absorption of the micronuclei, both stages being remarkably 

 alike. 



Thus it is not possible to state with certainty that all micro- 

 imclei in this stage gave rise subsequently to spindles. No 

 stages in the formation of the second pole were discovered. 



The formation of the spindle took place inside the micro- 

 nuclear membrane. The structure took up the entire space 

 and no halo was to be seen. Many of the spindles were almost 

 globular in shape (PI. 23, figs. 10 and 15) ; a few, however, w^ere 

 more diamond-shaped (PI. 23, fig. 9). In the globular form the 

 apices were flattened, but in the others they were quite pointed. 

 Whether this difference in shape was accidental or was the 

 expression of different stages in the division process is not 

 certain, but the latter interpretation seems to me more probable. 



No centrioles were ever seen. The spindle-fibres were 

 usually quite obvious. They appeared to fuse and form groups 

 when approaching the apex of the spindle. In the diamond- 

 shaped spindle the fibres appeared to converge at one point, 

 but in the globular spindles the groups of fibres did not all come 

 together at the pole (PI. 23, figs. 10 and 15). 



Spindles were present at the same time in both conjugants, 

 but division did not take place simultaneously in all the 

 micronuclei of the conjugant, since, besides spindles at the 

 equatorial plate stage, micronuclei at the swollen stage prior 

 to division (see PI. 23, fig. 12) were present, as were also micro- 

 nuclei which w^ere not so swollen or pale. Some of these 



