i 9 4 INDIVIDUALITY OF CHROMOSOMES [CH. 



and that changes from one number to another occur either 

 by reduplication or by nuclear fusion not followed by a 

 "reducing" division. 



The examples of various kinds of numerical abnormality 

 given above thus suggest that the chromosome number is 

 dependent on the behaviour of the chromosomes as indi- 

 viduals, and not on some innate tendency for the chromatin 

 of the nucleus to aggregate itself into a definite number of 

 masses during mitosis, while ceasing to have any such ar- 

 rangement in the resting condition of the nucleus. 



FIG. ^a f . Ascaris megalocephala bivalens. Above, three equatorial plates of 

 first segmentation division showing different arrangements of chromo- 

 somes. Below, nuclei of two-cell stage in reticular condition and in 

 preparation for the next division. From BOVERI (1903 a). 



(2) The persistence of definite chromosomes in the resting 

 stage of the nucleus. 



Generally speaking the chromosomes are only visible as 

 definite bodies during mitosis, and one of the chief objec- 

 tions to the individuality hypothesis is that not only do they 

 disappear in the reticular stage of the nucleus, but observa- 

 tion seems to show that in this stage the chromatin becomes 

 more or less uniformly distributed over the network, and 

 that the chromosomes, as such, cease to exist. Careful 



