Animals die during embryonic development. In C. elegans, often assayed as refractile eggs that fail to hatch; when applied to large-scale RNAi screens in C. elegans, more than 10% of embryos die.
The morphology of pronuclei and nuclei are defective in the early embryo. In C. elegans, this defect is often characterized by pronuclei and nuclei that are small or missing altogether and are often accompanied by spindle defects.