332.
Background
Many species form extraembryonic tissues during embryogenesis, such as the placenta of humans and other viviparous mammals.
Extraembryonic tissues have various roles in protecting, nourishing and patterning embryos. Prior to gastrulation in zebrafish,
the yolk syncytial layer - an extraembryonic nuclear syncytium - produces signals that induce mesoderm and endoderm formation.
Mesoderm and endoderm precursor cells are situated in the embryonic margin, an external ring of cells along the embryo-yolk
interface. The yolk syncytial layer initially forms below the margin, in a domain called the external yolk syncytial layer
(E-YSL).
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