Early embryogenesis. Descriptions of the four stages of the early-embryogenesis period, along with examples for each stage. (A) Animal-pole view of a zygote of N. ophidion ca. 45 min after mating. (B) Animal-pole view of a N. ophidion blastula during early cleavages. (C) Embryonic-shield stage in N. ophidion; the white circle represents the germ ring. (D) Primitive-streak embryo of S. abaster (dechorionated). Scale bars are 0.5 mm.
Sommer et al. BMC Developmental Biology 2012 12:39 doi:10.1186/1471-213X-12-39