No. 464.] STUDIES ON PLANT CELL— VN: 577 
divisions. If this is true then by the law of chance the propor- 
tions of germ cells of the hybrid which are adsolutely pure (con- 
taining chromosomes entirely from one parent) would be small. 
Likewise there would be a small proportion of germ cells in 
which the paternal and maternal chromosomes are equally dis- 
tributed. And in contrast to these two groups the great major- 
ity of germ cells would have a marked preponderance of chromo- 
somes derived from one parent or the other and this condition 
may be termed one of relative purity. 
We shall now summarize the cytological evidence for the con- 
clusions of the paragraph above, first with respect to the actual 
distribution of the somatic and sporophytic chromosomes as 
entities during the mitoses of reduction, and second as to the 
probability of the bivalent chromosomes consisting of a pair of 
maternal and paternal elements. The evidence on the first 
point has been treated as regards plants in our own account of 
* Reduction of the Chromosomes ” and need not be repeated. 
With respect to the possibilities of distinguishing maternal and 
paternal chromosomes throughout a life history and especially 
at the period of chromosome reduction we must consider briefly 
the remarkably favorable studies of Sutton, Montgomery, Moenk- 
haus, Baumgartner, and Rosenberg. 
Sutton (:02, :03) discovered in the “ lubber grasshopper ” 
(Brachystola magna) a form in which the somatic chromosomes, 
23 in number, are markedly different in size, presenting a graded 
series with respect to pairs in which the two elements are ap- 
proximately equal. There are then 11 types of chromosomes in 
two groups, a pair of each type, and in addition an accessory 
chromosome which remains apart from the rest in a special 
vesicle of its own. These two sets of 11 chromosomes appear 
with regularity throughout the mitoses leading up to the reduc- 
tion divisions of spermatogenesis. Previous to the reducing 
divisions the chromosomes of each pair become closely asso- 
ciated end to end so that 11 threads appear which form 11 biva- 
lent chromosomes (dyads) that later become tetrads through the 
division of each chromosome in the pair. Sutton concludes that 
the somatic chromosomes which make up each bivalent structure 
conjugate during synapsis and that the transverse fission which 
