SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 155 



catrd process of mitosis is meaningless unless the chromatin is quali- 

 tatively different in the various regions of the nucleus, and that 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 (1876) and 

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

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

 division initiate its splitting, had much to do with the formulation of this 

 hypothesis. That the chromatic granules, or chromomeres (Fol 1891), 

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

 by cytologists. It was the opinion of Brauer (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. 



JJI ^2^- V^T ' ^ C 



" A 



FIG. 54. 



A, vacuoles in chromosomes at mctaphase in Trillium. X 1800. (After Gregoirc and 

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

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

 showing chromomeres. X 2250. (After MiUler, 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 1884, 1888), others 

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

 or individuals. Gre"goire and Wygaerts (1903), Martins Mano (1904), 

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

 (1910), Lundegardh (1912), Sharp (1913, 1920), 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 Miiller (1912) the portions of the 

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



