MECHANISM IN SEGREGATION 51 



quence for genetics in so far as the chromosomes represent 

 the bearers of genes, for while side-to-side union offers 

 an opportunity for interchange between the paternal and 

 maternal members of a pair, no such interchange could be 

 postulated if end-to-end conjugation took place. So far 

 as segregation is concerned either method supplies all 

 that is called for.^ A discussion of other matters will 

 be left until later. 



Individuality of the Chromosomes 



During the period of cell-division there can scarcely 

 be any question concerning the persistence of the individ- 

 ual chromosomes, because they remain visibly distinct 

 elements in the cell ; but when the nucleus re-forms after 

 each division the chromosomes spin out threads laterally, 

 and these appear to fuse, making a continuous network 

 throughout the nucleus. Whether there is actual fusion 

 between these threads or whether they occupy delimited 

 contact areas, and whether the branches represent the 

 essential part of the chromosome concerned in heredity, 

 are questions impossible to answer at present. The 

 genetic evidence at least consistently shows that no real 

 fusion of the hereditary material occurs even in cells that 

 have passed through many such resting periods. 



From several other sources there are strong indica- 

 tions that the chromosomes retain their individuality dur- 

 ing the resting stage. In Ascaris, where the chromosomes 

 are few and long, they are often drawn out in an irregular 

 way in the cleavage cells as they pass to the poles of the 

 spindle of the dividing cells. Daughter halves of the same 

 chromosomes show the same identical irregularity. 

 Boveri has shown by an examination of a large number of 

 daughter cells (pairs) that are getting ready for the next 

 division, that when the chromosomes of sister cells reap- 



• If the pairs fused end to end and the tetrad arose by two longitudinal 

 divisions, the outcome would not be in harmony with the theory of segrega- 

 tion based on separation of maternal and paternal chromosomes at reduction. 



