farr: cytokinesis of pollen-mother-cells 287 



During synapsis the more or less sharp angles become rounded off, 

 so that the cell approaches more nearly to an ellipsoid or ovoid 

 form. At the same time the mother-wall may be seen in crushed- 

 out material to have developed a considerable thickening at either 

 end of the cell. It is frequently noticeable that the mother-wall is 

 flattened on one side parallel to the longitudinal dimension. This 

 flattening may even be so marked that the central optical section 

 appears triangular, or the apex of the triangle may be truncated. 

 When two cells lie contiguous to each other, their flattened faces, 

 which represent the bases of such triangles, are often seen in living 

 material to lie appressed to each other (fig. 4) ; and it is at least 

 suggested that this line of contiguity may mark the plane of the 

 last preceding mitosis. Pairs of mother-cells, in which the proto- 

 plasts have completely divided into the microspores, may even 

 be found in this relation (fig. 14), the mother-wall retaining its 

 original form. 



The nucleus is seen to undergo progressive enlargement during 

 the stages of presynapsis and of synapsis proper. Lubimenko and 

 Maige (40) believe that this is carried to the point of bursting of 

 the nuclear membrane. No evidence of the latter phenomenon 

 was observed in the present study. Sometimes the nucleus would 

 swell and burst, thereupon completely collapsing, but this was 

 thought to be due to the abnormal medium in which the cells had 

 been placed. 



The wall continues to thicken during synapsis, and upon the 

 advent of diakinesis (fig. 2) it is thicker on all sides, but is still 

 more markedly so at either end. The nature of the process of 

 thickening of the cell-wall will be discussed under the observations 

 on fixed material. The heterotypic spindle may lie either in the 

 longitudinal axis of the mother-cell (fig. 21), or transverse to it 

 (fig. 3). As the homoeotypic division takes place (fig. 4) the 

 wall becomes quite uniformily thickened on all sides. The 

 homoeotypic spindles are frequently at right angles to each other, 

 though they may be at a much smaller angle. After the re- 

 organization of the daughter nuclei there appear occasionally in 

 the living material a few central spindle fibers; though these are 

 difficult to make out. One instance (fig. 4) was found in which 

 quite a definite row of granules might be seen in the plane midway 

 between the nuclei. The whole cytoplasm, of course, appears 



