<unk>  is a phenomenon occurring during prenatal development. Prior to this event, the predecessor of the embryo, in the form of a blastocyst, is surrounded by a glycoprotein sphere called the<unk>  pellucida. To be able to perform implantation on the uterine wall, the blastocyst first needs to get rid of the<unk>  pellucida. This lysis of the<unk>  pellucida is called the<unk> . In humans, this happens at the end of day four.
Zona hatching
Primordial germ cell migration
Peritomy
Tooth gemination
Fetal resorption
Neural clique
Eutherian fetoembryonic defense system 
Ectogenesis