No. 464.] STUDIES ON PLANT CELL. VII. 577 



divisions. If this is true then by the law of chance the propor- 

 tions of germ cells of the hybrid which are absolutely pure (con- 

 taining chromosomes entirely from one parent) woukf 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 Button, Montgomery, Moenk- 

 haus, Baumgartner, and Rosenberg. 



Sutton (:O2, : 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 1 1 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 1 1 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 1 1 threads appear which form 1 1 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 



