CH. xm] HYPOTHESIS OF INDIVIDUALITY 189 



provision the most convinced adherents of the indivi- 

 duality theory no longer hold it in this extreme form. For 

 in what has gone before it has already been seen that in 

 several cases the chromosomes cannot be regarded as indi- 

 viduals in any such sense, for bodies which at some stage 

 behave as one chromosome may sometimes break up at 

 another stage into several or many smaller units. A con- 

 spicuous example of this is seen in the segmentation of the 

 eggs of Ascaris, in which the two long chromosomes of the 

 fertilised egg not only throw off their thickened ends, but 

 also break up into many small portions, in the cells which 

 do not give rise to the germ-track. Similar division of chro- 

 mosomes that at some stage behave as units has been ob- 

 served in several animals, and other reasons for regarding 

 the chromosomes as compound will be considered in the 

 chapter on their relation to hereditary characters. 



In consequence of facts of this nature, the original 

 individuality hypothesis has been modified in such a way 

 as to assume that the chromosomes as they ordinarily appear 

 are not indivisible units, but are composed of smaller por- 

 tions to which the theory of individuality applies strictly. 

 According to this view the chromosomes are definite aggre- 

 gations of individual units arranged in a constant manner; 

 the units persist as individuals from one mitosis to the next 

 during the resting stage, and since they are arranged along 

 the chromosomes in a definite order, when the chromosomes 

 divide in the mitotic figure, their component units are 

 similarly divided, so that each daughter chromosome is 

 composed of a similar series of individual units. Although, 

 therefore, the units are the ultimate individuals, the chro- 

 mosomes themselves have also a definite individuality, for 

 they are composed of a constant series of these smaller 

 individuals. As a rough analogy, the units may be com- 



