76 University of California Publications in Zoology [Vol. 20 



the "resting" phase of the nucleus this occurs has not been deter- 

 mined. The abundance of such stages in what appear to be normal 

 vegetative forms, without other signs of the approach of division, 

 would suggest that it occurs very early in the between-divisiou periods 

 of the life of the organism. 



That we maj^ have here a continuity of chromosomes from one 

 division period to the next is suggested by certain appearances of the 

 resting phases of the nucleus. This is best seen in figures 14, 15, 

 and 17 of plate 6, and figures 26 to 28 of plate 7. These are the 

 nuclear figures which are most frequently met with in the ordinary 

 trophozoite. They present a broken, ragged network with distinctly 

 marked ends of chromatin threads scattered through it. Occupying 

 the nodes of the network or sometimes at the ends of the threads, are 

 chromatin granules. The process of changing from this condition 

 to that of the distinctly marked chromosomes seems to consist in an 

 outmoving of the contents of the granules along the threads to which 

 they are attached (pi. 6, figs. 17, 19). On the completion of this, 

 the fully formed chromosomes become apparent, with an entire 

 absence of large chromatin granules. Differentiation of individual 

 chromosomes other than the heterochromosomes has not been detected. 



In the late prophase of division the chromosomes retain distinct 

 outlines for a considerable period. With the disappearance of the 

 spindle fibers the chromosomes move out to near the center of the 

 nucleus, with the threads lying parallel or nearly so (pi. 12, fi^s. 77, 

 82). Without apparently losing their individuality, these separate 

 threads begin to change their position (fig. 78), and form a loose 

 network by the interweaving of the separate strands (figs. 78, 81, 83). 

 Later granules appear at the intersection of the threads which then 

 may become thinner and we have the same nuclear structure as that 

 shown in figures 27 and 28 of plate 7, and figures 14 and 15 of 

 plate 6. Distinct chromomeres cannot be detected in these stages 

 of the nucleus, but they appear later following the formation of 

 distinct chromosomes. 



The type of division of the chromosomes is apparently transverse 

 in the later stages, but is in reality longitudinal, as shown by the 

 longitudinal splitting of the single threads (fig. D; pi. 7, fig. 25). 

 The contraction of the two, visibly separated halves of the chromo- 

 somes (pi. 9, fig. 49) into single short, thick threads with a V-shape 

 (fig. 51a), produces the units which are unfolded on the spindle as 

 long, single threads (pi. 10, figs. 56, 59). These part in the middle, 



