Hatching. The pressure in the swollen egg pushes out the end of the mantle. The then hatchling frees himself from the egg, and a fully functional octopus emerges.
From: Wood, J.B., E. Kenchington, and R.K. O'Dor. 1998. Reproduction and embryonic development time of Bathypolypus arcticus
, a deep-sea octopod (Cephalopoda
). Malacologia, 39(1-2): 11-19.