The phrase “ontogeny recaptitulates phylogeny”
expresses the idea that the development of embryos
repeats the evolutionary stages of the species;
however, evolution of a new creature
is more interesting than the accumulation
of changes to the end of its ancestor’s development.
Eggs are single-cells but animal eggs are not the same as single-celled organisms;
they contain only half the chromosomes of the mother.
See also in The book of science:
Readings on wikipedia: