66 University of California Publications in Zoology [Vol. 20 



from the undifferentiated mass of chromatin-encrusted network, lienee 

 the number of threads originally formed can not be determined. The 

 separation of the two parts thus formed seems to take place immedi- 

 ately, since in slightly later stages the number of threads appears 

 much greater, with no evidences of the splitting of a single thread 

 (pi. 6, figs. 20-22; pi. 7, figs. 23, 29). In these stages also many 

 of the threads or chromosomes are arranged in pairs which are nearly 

 equal in size and length. In a later stage, but still one which precedes 

 the rounding up of the body, this pairing of the chromosomes becomes 

 more pronounced (pi. 6, fig. 18). The threads thus sorted out in 

 pairs are evidently the products of the splitting of the original threads 

 or chromosomes. 



During the time these changes in the central nuclear mass are 

 taking place, the small vesicle with its single coiled, chromatin thread 

 or chromosome remains intact, with no apparent change beyond an 

 enlargement of the vesicle itself. This forms a large clear area with 

 the chromosome contained within it loosely coiled or V-shaped (pi. 7, 

 fig. 29; pi. 6, figs. 16, 20). In common with the remainder of the 

 chromatin of the nucleus, it seems to increase somewhat in bulk dur- 

 ing the early prophase, though this is not invariably the case (pi. 9, 

 fig. 46). The vesicle disappears before the end of the prophase, 

 leaving the single chromosome lying in a clear space apart and 

 detached from the remainder of the chromatin threads (pi. 6, fig. 22.; 

 pi. 7, fig. 29). This isolation is apparently retained throughout the 

 subsequent stages, though this can be satisfactorily demonstrated only 

 when the nucleus is oriented so that it is seen near the lateral margin 

 (pi. 9, figs. 46, 47). In other positions its relations are obscured by 

 the great number of chromosomes. 



The occurrence of a definite, continuous spireme stage is not 

 certain, the short chromatin threads or chromosomes apparently being 

 formed directly from the breaking up of the reticulum (pi. 6, figs. 

 14-22), before the body of tlie flagellate has begun to round up or 

 give other evidences of the approach of division (pi. 7, fig. 23). The 

 exact number of the threads thus formed seems to be fifty-two. In 

 the stages represented in figures 18 to 22, plate 6, these could not be 

 counted, but in the later stages, represented by figures 44, 47 and 

 49, plate 9, with the chromosomes more fully organized, this could 

 be done with a considerable degree of accuracy, and the number of 

 chromosomes given is based on counts made on fifteen different 

 individuals. 



