No. 464.] STUDIES ON PLANT CELL.— VII. 575 
must rest ón the assumption of what is termed the individuality 
of the chromosome. This means that the chromosome is 
believed to be a permanent organ of the cell which never loses 
its organic entity although the form may be frequently obscured, 
as in the resting nucleus, and which reproduces by fission during 
mitosis. We have given in other connections the evidence upon 
which the above view rests, evidence accumulated from the 
studies of the critical periods of gametogenesis, fertilization, and 
sporogenesis (with its reduction phenomena) in plants and of 
gametogenesis and fertilization in animals. All investigations 
indicate that paternal and maternal chromosomes maintain com- 
plete independence in the sexually formed cell or fertilized egg 
and in the mitoses of cleavage so far as these have been fol. 
lowed. Also, descendants of the chromosomes which became 
associated with fertilization have been recognized by their form 
at the end of the life history during the reduction phenomena of 
gametogenesis in certain animals (Sutton, :02, :03; Montgom- 
ery, : 04) and of sporogenesis in the hybrids of Drosera (Rosen- 
berg, :04a, :04b). Furthermore, the entire history of chromo- 
some reduction in both animals and plants finds a satisfactory 
explanation only in the belief that descendants of maternal and 
paternal chromosomes are distributed as organic entities by the 
peculiar mitoses of this period. 
There is a general agreement that the somatic chromosomes 
of animals and the sporophytic of plants become grouped in 
pairs to form bivalent structures before the heterotypic mitosis 
of the reduction division whether this be present in the primary 
gametocyte (animals) or the spore mother-cell (plants). The 
bivalent chromosomes (pairs of chromosomes, dyads) may be- 
come transformed into tetrads before the heterotypic mitosis by 
a division of each chromosome in the pair, as is:characteristic of 
animals, or this division may be delayed until a somewhat later 
period during the heterotypic mitosis, as in plants. We are not 
concerned now with the dispute as to how the pairs of chromo- 
somes come to lie side by side to-form the bivalent structure or 
how tetrads are developed, activities which may indeed be vari- 
ous in different types and which will only be ‘understood by a 
greater body of observations than we have at present (see dis- 
t 
