The evolutionary advent of uterine support of embryonic
growth in mammals is relatively recent.
Nonetheless, striking differences in the earliest steps
of embryogenesis make it difficult to draw parallels
even with other chordates. We suggest that use of
fertilization as a reference point misaligns the earliest
stages and masks parallels that are evident when
development is aligned at conserved stages surrounding
gastrulation. In externally deposited eggs
from representatives of all the major phyla, gastrulation