Study of Reproduction in the Genus Acer 121 



(Fig. 19). The open space is filled with a clearer and less vacuo- 

 late plasma than the general neighboring region, clearly differ- 

 entiated from the spindle which it terminates. The nuclear 

 membrane first becomes evident over the external face of the 

 bowl, gradually extending around the sides and contracting the 

 open face, accompanied by the chromatin which becomes dis- 

 tributed over the entire periphery by the time the membrane 

 is complete. The reticulum then rapidly loses its definiteness, 

 but it does not seem to pass through a series of comminuting 

 stages that are in effect a reversal of the prophase stages. The 

 nucleolus in its earliest appearance is difficult to distinguish 

 from the large chromosome-derived bodies, but these stain 

 more lightly and become smaller the later the stage studied, 

 while the nucleolus gradually becoming larger, can soon be rec- 

 ognized (Fig. 21). This nucleolus in many cases seems to be the 

 only chromatic body at interkinesis, but deep staining shows 

 that there remain angular structures on the periphery about 

 equal in number to the chromosomes that went into the nucleus 

 (Fig. 22). 



The second or homotypic prophase begins with the estab- 

 lishment of dark-staining angular bodies on the nuclear periph- 

 ery, seemingly from the centers just mentioned (Fig. 23). As 

 these increase in size the nucleolus shrinks, retaining its dark 

 staining capacity. They can often be counted, but the contour 

 of the nucleus and its small size reduces greatly the number of 

 favorable countable cases. After the break-down of the mem- 

 brane (Fig. 24) they pass from an irregular distribution into 

 the flat plate preparatory to division. The nucleolus, still 

 present, passes out from the forming spindle and takes up its 

 position where the apex of the heterotype spindle had formerly 

 been (Fig. 25). It retains its staining capacity up to this point, 

 and is a very conspicuous object. The equatorial part of the 

 heterotype spindle remains distinct through the homotype divi- 

 sion (Fig. 26). The nucleolus disappears suddenly, shrinking 

 and paling without evidence of fragmentation. 



The axes of the two homotype spindles may lie in the same 

 plane, or in two planes at right angles to each other, or even in 

 any intermediate position, thus giving rise to tripartite or cruci- 

 ate tetrads, or intermediate forms. 



After separation of the tetrad the elements enlarge consider- 

 ably, and each forms its individual wall that, as the pollen grain 



