  Biologically, for any organism life does not start at birth but at
fertilization of the embryo. Embryonic development is of great importance
because it determines congenital anomalies and influences their severity.
Whereas there is detailed qualitative knowledge of the successive steps of
embryonic development, little is known about their probabilities of success or
failure. Embryonic mortality as a function of post fertilization time provides
a simple (albeit crude) way to identify major defects. We find that, in line
with the few other species for which data are available, the embryonic
mortality of zebrafish has a prominent peak shortly after fertilization. This
is called the early embryonic mortality (EEM) effect. Although a number of
immediate causes of death (e.g. infection, excess of carbon dioxide or of
lactic acid, chromosomal defects) can be cited, the common underlying factor
remains unknown.
  After reviewing embryonic mortality data available for chicken and a few
other farm animals, we explain that zebrafish are particularly suited for such
a study because embryogenesis can be followed from its very beginning and can
be observed easily thanks to transparent egg shells.
  We report the following findings. (i) The mortality peak occurs in the first
15% of the 75-80 hours of embryogenesis and it is about 50 times higher than
the low plateau which follows. (ii) The shape of the age-specific death rate is
largely independent of the death level.
  Presently, little is known about the nature of embryonic defects. However, by
reviewing two special cases we show that even small initial defects, e.g.
spatial cellular asymmetries or irregularities in the timing of development,
carry with them lethal effects in later stages of embryogenesis.
