148 INTRODUCTION TO CYTOLOGY 



of a chromosome is occasionally lost or transferred to another chromo- 

 some. Under the proper conditions such altered chromosomes continue 

 as individuals of new types. Since such alterations evidently have 

 occurred with some frequency in the past, a given chromosome should be 

 looked upon as a member of a long series of generations in which structural 

 and functional alterations occur from time to time: it is an assemblage 

 of elements which may remain relatively constant for a long period or 

 perhaps through only a limited number of generations. It may be likened 

 to a persisting society whose membership occasionally changes through 

 losses, additions, and the alteration of personal action. Its persistence 

 as an individual distinct from its neighbors in spite of all such alterations 

 is insured by the spindle-attachment region, a definite "organ" with 

 which the other chromosomal elements are associated. The precise 

 nature of the chromosomal substances which persist and of the chemical 

 transformations which they undergo remains for future investigators to 

 determine. 



