why the embryos are broken into fragments?

[kathy]

I tried the ZF embryos whole mount antibody staining according to
the procedure on Zbook, after permeablizing with acetone -20
centigrade , 7min ,the embryos were completely broken into
fragments.In fact, even during last step--washing with PBS, I had 
found some of the embryos become so fragile that the fixed 
crystal-like yolk granules are very easy to broken down just as 
pearls dispersed in the dish if observed under dissect microscope. 
why ? ( the samples are all before gastrulation).

It is really a very urgent problem to me.