No. 463.] STUDIES ON PLANT CELL VI. 477 



feature of that period when the number of chromosomes is 

 reduced by half. 



Synapsis is followed very shortly by the two mitoses charac- 

 teristic of sporogenesis. These nuclear divisions have given 

 rise to a lengthy literature in which well known investigators 

 have shifted their positions more than once. The discussions 

 have centered on the methods of fission and distribution of the 

 reduced number of bivalent chromosomes which appear in the 

 first mitosis following synapsis. Assuming that the chromatin 

 is organized into smaller units, represented by the chromatin 

 granules (chromomeres, Fol, 1891), which compose the chromo- 

 somes, it is at once apparent that these finer elements may 

 become variously distributed according to the structure of the 

 bivalent chromosomes and the character of the mitoses of sporo- 

 genesis. Each fusion bivalent chromosome is composed of two 

 chromosomes joined (i) end to end or (2) side by side or (3) it 

 is possible that the chromatin is intricately mixed in the struc- 

 ture. With respect to the mitoses a transverse division of the 

 fusion chromosomes might be expected to give a very different 

 proportionate arrangement of the maternal and paternal chroma- 

 tin from longitudinal divisions. Should the chromatin granules 

 differ qualitatively from one another then different parts of a 

 chromosome might be expected to have different characteristics 

 which would be distributed by the mitoses of sporogenesis in 

 various proportions or ratios. 



It has long been known that the mitoses of sporogenesis pre- 

 sent peculiarities in the mode of division and arrangement of 

 the chromosomes at the nuclear plate which make them unlike 

 the typical mitoses of cell division. These peculiarities have 

 led to the designation of the first mitosis as heterotypic and the 

 second as homotypic, terms which are now applied by both bot- 

 anists and zoologists although we have now a much more 

 extended knowledge of each type than when Flemming first 

 proposed the classification in 1887. We described the charac- 

 ters of the heterotypic and homotypic mitoses in Section III, 

 "The Spore Mother-cell" {Amer. Nat., vol. 38, p. 740, Oct., 

 1904), and will presently treat them further since some papers 

 of the past year have opened again a discussion which seemed 



