According to the classical hypothesis of Marriosis De' Abrtona, derived from the results of the expedition of the famous British research vessel RRS "Discovery", egg development then proceeds as follows: gastrulation (development of egg into embryo) sets in during the descent of the eggs on the shelf at the bottom, in oceanic areas in depths around . The egg hatches as a nauplius larva; once this has moulted into a metanauplius, the young animal starts migrating towards the surface in a migration known as developmental ascent.