Pre-Streak (PS): Advanced egg-cylinder stage with the first evidence
of of an embryonic axis. There is clear morphological distinction
between the embryonic and extraembryonic ectoderm. The ectoplacental
cone is further invaded by maternal blood and the original lumen
of the uterine crypt has disappeared.
Early Streak (ES): Late in this stage gastrulation begins, producing the first
6.5 dpc (range 6.25-7.25)
Downs & Davies PS - ES: prestreak to early streak