This video says about itself:
5 December 2014
… Video displays how it is swimming and waving his tail inside the egg.
Video is created by me, soundtrack is a classic song “Song of the Indian Guest” created by composer Nikolai Rimsky-Korsakov who died in 1908 (more than 100 years ago).
Translated from Dutch NOS TV today:
It is an egg of a small-spotted catshark, a small shark of about one meter long. The small-spotted catshark is common in the North Sea and the discovery of a shark’s egg is not so special, but a living embryo is.
“We sometimes find empty egg capsules on the beach, but I’ve never seen a live shark in an egg,” says Marijke Barhorst of the Visitors Centre of Schiermonnikoog National Park.