THE EVOLUTION OF THE CELL. II 



By the Late PROFESSOR E. A. MINCHIN, F.R.S. 



Even more remarkable than the relation of the chromo- 

 somes to cell-reproduction is their behavior in relation to 

 sexual phenomena. In the life-cycles of Metazoa the 

 sexual act consists of the fusion of male and female pro- 

 nuclei, each containing a definite and specific number of 

 chromosomes, the same number usually, though not al- 

 ways, in each pronucleus. It has been established in 

 many cases, and it is perhaps universally true, that in the 

 act of fertilization the male and female chromosomes re- 

 main perfectly distinct and separate in the synkaryon or 

 nucleus formed by the union of the two pronuclei, and, 

 moreover, that they continue to maintain and to propa- 

 gate their distinct individuality in every subsequent cell- 

 generation of the multicellular organism produced as a 

 result of the sexual act. In this way, every cell of the 

 body contains in its nucleus distinct chromatinic elements 

 which are derived from both male and female parents and 

 which maintain unimpaired their distinct and specific in- 

 dividuality through the entire life-cycle. This distinct- 

 ness is apparent at least in the germ-cell-cycle of the or- 

 ganism, but may be obscured by secondary changes in the 

 nuclei of the specialized tissue-cells. 



( hily m the very last stage of the life-cycle do the group 

 of male and female chromosomes modify their behavior in 

 a most striking manner. In the final generation of oogo- 

 nia or spermatogonia, from which arise the oocytes and 

 spermatocytes which in their turn produce the gamete- 

 cells, it is observed that the male and female chromosomes 

 make a last appearance in their full number, and then 

 fuse in pairs, so as to reduce the number of chromosomes 

 to half that previously present. 



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