SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 



1 ."»."> 



cated process of mitosis is meaningless unless iln' chromatin is quali- 

 tatively different in the various regions of the nucleus, and thai the 

 arrangement of the material of the chromosome in the form of a long 



thread prior to its splitting; is a means whereby all these qualities, ar- 

 ranged in a linear series in the thread, are equationally divided and 

 distributed to the daughter nuclei. The theory of Balbiani I L876) and 

 Pfitzner (1881), that the chromatin granules visible in the nuclear rel icu- 

 lum arrange themselves in a series in the chromosome and by their 

 division initiate its splitting, had much to do with the formulal ion of thia 

 hypothesis. That the chromatic granules, or chromomeres (Fol L891), 

 represent the qualities of Roux is a theory which has been widely accepted 

 by cytologists. It was the opinion of Braucr (1893) and many later 

 workers that the granules or chromomeres, rather than the chromosomes 

 themselves, are the significant units in the nucleus, and that their division 

 is an act of reproduction. The division and separation of chromosomes 

 was accordingly regarded as a means of distributing the daughter granules 

 to the daughter cells. That the chromomere is made up of still smaller 

 "chromioles" was held by Eisen (1899, 1900). Strasburger, Allen (1905 

 and Mottier (1907) also found the chromomere to be composed of smaller 

 chromatic granules. 



r\A fee? 





Fig. 54. 

 A, vacuoles in chromosomes at metaphase in Trillium. X 1800. (After Gregoin and 

 Wygaerts, 1903.) B, spiral arrangement of chromatin material within the chromosomes 

 Allium. (After Bonnevie, 1911.) C, D, stages of chromosome splitting in Naja* marina, 



showing chromomeres. X 2250. (After MiOler, 1912.) 



Although a large number of investigators, particularly those interested 

 in the hereditary role of the chromatin, have placed much confidence in 

 the importance of the chromomeres (Strasburger L884, l sss , others 

 have raised serious objections to the theory that they arc significant units 

 or individuals. Gregoire and Wygaerts (1903), .Martins Mano (1904 

 Gregoire (1906, 1907), Marechal (1907). Bonnevie (1908), Stomps 

 (1910), Lundegardh (1912), Sharp (1913, L920), and others have found 

 no such definite behavior on the part of the chromatin granules in the 

 dividing chromosomes studied by them, and have suggested other ex- 

 planations for the appearances observed. According to a modification 

 of the chromomere theory adopted by Muller I L912) the portions of the 

 thread between the chromomeres split first, the division of thechromo- 



