[Zbrafish] embryos with loose cells?

[Padnos.Beth from epamail.epa.gov]

When collecting and sorting embryos on Friday I observed something I
have not seen before.  Admittedly I am a newer user so this may or may
not be a common occurrence that I am unfamiliar with.
As the cells split and developed many of them appeared to be breaking
off of the developing embryo and coming loose inside the chorion.
Sometimes the loose cells would adhere to the embryo in a different
location often lower on the yolk sac. Another version  (in the same
batch of embryos) was what appear to be areas of cells developing then
dying but remaining attached to the embryo.
All embryos appeared to be developing late Friday. Saturday only 5 or
192 had died. Sunday more had died (unsure of exact number) and the
remaining appear to be developing but slowly.
As background the embryos were collected from 2 tanks of fish, then
mixed together. One tank was received in 10/06 and the other 2/07. All
of the fish are on the same recirculating rank with water quality
testing weekly and all parameters in normal ranges.
Beth Padnos