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 üner 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 (1) 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 zoölogists although we have now a much more 
extended knowledge of each type than when Flemming first 
proposed the classification in 1887. We described fhe 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 
