Study of Reproduction in the Genus Acer 119 



There is no spiral twisting of the paired threads such as is 

 so characteristic of the stage in the lily (Fig. 11). Instead, they 

 break up into a number of segments equal to the reduced chro- 

 mosome count. These may appear to collect somewhat to one 

 side of the nucleus in a second contraction, but the writer is rather 

 inclined to believe that this appearance is in the nature of an 

 artifact. The halves of each pair now separate more or less, 

 forming very conspicuous rings (Figs. 12, 13), which by separa- 

 tion at the ends and contraction give rise to the chromosome 

 pairs, seeming to pass through a twisted stage during the short- 

 ening. Finally all elements contract to very short rods closely 

 associated in pairs lying around the periphery of the nucleus 

 (Fig. 14). The number of these is readily counted when few, 

 as in this case, but in those forms with many chromosomes, as 

 described later, the task becomes much more difficult. 



The nucleolus has from the earliest beginnings of the division 

 kept its large size and quality of strongly retaining the stain. 

 Although for the most part apparently connected by strands 

 with the spireme, the writer would not consider that it con- 

 tributed chromatin material by bodily transfer, as has been 

 suggested (4), but rather through the intermediary of products 

 dissolved in the plasma and which are recombined in the spireme 

 into stainable chromatin. This is not an hypothesis readily 

 capable of demonstration, but it fits the observed conditions 

 better than the other. 



With the approach of diakinesis as described, there appears 

 gradually a denser interior cytoplasmic zone surrounding the 

 nucleus, which eventually resolves itself into a complex system 

 of filaments that show signs of aggregation into sheaves by the 

 time the breakdown of the nuclear membrane occurs. At this 

 time the cytoplasm rapidly encroaches on the nuclear cavity, 

 strands pass into it, and the chromosomes become forced toward 

 the center (Fig. 14). The filaments then rapidly swing around 

 into a few sheaves showing as a multipolar spindle, and then 

 into two in the typical fashion. All this while the nucleolus 

 has been rapidly decreasing in size so by the time the bipolar 

 condition has been reached, the nucleolus has disappeared 

 (Figs. 17, 18). The chromosome pairs then become arranged 

 in the nuclear plate, the small rounded elements of each pair 

 being directed toward the poles, not lying side by side in the plane 



