chromosome fixative

[Mark R Wilson]

     I'm an undergrad in a course on human genetics and I was wondering 
if anyone could answer a question for me:  What is the method of action 
of Clarke's fixative (3 parts methanol, 1 part glacial acetic acid) in 
the preparation of chromosomes for microscopical study?
ie. What exactly do the two constituents do to cultured lymphocytes to 
prepare them for rupture for microscopical study of the chromosomes within?

Thanks for any help,
Mark Wilson.