No. 464.] STUDIES ON PLANT CELL.— VII. 575 



must rest on 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 &gg 

 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 ihe 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- 



