1923] Hall: Binary Fission of Menoidium incurvum 455 



Prophase 



With the initiation of the prophase, the seattered chromatin 

 granules appear to become organized into fine thread-like structures, 

 so that soon the semblance of a fine spireme is produced (pi. 40, fig. 8). 

 It has not been determined with any degree of certainty how many 

 threads this 'spireme' contains, but in figure 8 of plate 40 are shown 

 what appear to be the ends of several threads, indicating the presence 

 of more than one. The fine chromatin threads gradually shorten and 

 thicken until, finally, distinct chromosomes are formed. Although 

 it is possible that such structures exist, there is no evidence whatever 

 for the emergence of chromosome pairs from tfie resting stage into 

 the prophase, as described by Tschenzoff (1916) for Euglena. At 

 first these chromosomes are coiled in a seemingly haphazard fashion, 

 but they soon assume a somewhat radial arrangement with respect 

 to the endosome (pi. 40, figs. 9 and 10). The number of these chromo- 

 somes of the prophase has not been definitely determined, since the 

 exact endings of all of them cannot be made out ; tentatively their 

 number may be stated as approximately twelve. Except in a very 

 few instances the chromosomes have not appeared granular, as 

 described for Euglena (Tschenzoff, 1916; Dehorne, 1920) and Hetero- 

 nema (Rhodes and Kirby, MSS). It is quite possible, however, that 

 such is the structure of the chromosomes of Menoidium; their com- 

 paratively small size renders critical observation difficult. 



With the advance of the prophase, the finer threads become trans- 

 formed into the thicker chromosomes of the later stages (pi. 40, 

 fig. 11). In the meantime, the endosome has begun to elongate in 

 preparation for the metaphase, while the chromosomes still appear 

 radially arranged with respect to the axis of the endosome. 



Metaphase 



With further elongation of the endosome, splitting of the chromo- 

 somes apparently begins (pi. 40, figs. 12 and 13 ; pi. 41, figs. 14 and 

 15). In such stages the chromosomes appear thinner and more 

 numerous than those of the late prophase. Although the evidence is 

 not entirely conclusive, the split apparently begins at one end of a 

 thick chromosome and continues toward the other, resulting in the 

 formation of a V-shaped structure (pi. 40, fig. 13; pi. 41, fig. 15). 

 During the splitting process the free ends of the Vs begin to migrate 



