fixing young embryos

Hi all !

I have a small problem ! when i fix my 6 and 12 somite stage zebrafish

with 4%PFA overnight at 4degrees, the yolk sac becomes dark brown

when i look it under the microscope the next day. I was  them in PBT (PBS+

Tween) and store in 100% MeOH for ISH. The embryo however, remains

clear. When i fix the 6 somite stage embryos, both the embryo and the yolk becomes brown in color. Can anyone please tell me why this happens and how can I

prevent it ? How do i obtain clear yolk an embryos ??? I have checked my PFA solution, and it seems to be fine.

please help !