Ingo Arndt / Sea trout (Salmo trutta trutta) alevin just hatched with full yolk sac, Europe

Tony Wu / Hairchin goby (Sagamia geneionema) eggs. Though still early in development, individual transparent fish embryos are visible attached to the yellow-orange yolk sacs. Miho, Shizuoka Prefecture, Japan, P...

Tony Wu / Flamboyant cuttlefish (Metasepia pfefferi) close up of egg with the developing juvenile attached to its yolk sac and visible inside the egg, Lembeh Strait, North Sulawesi, Indonesia

Andy Sands / Large white butterfly (Pieris brassicae) close up side view of newly laid eggs showing yellow coloured yolk inside, UK, captive . Focus stacked image - sequence 1 of 2

Laurie Campbell / Common / Viviparous Lizard (Zootoca vivipara) young breaking free from egg membrane it was born in. This species can both give birth to live young in a thin membrane and lay eggs. Burnmouth, Berwicks...

Laurie Campbell / Common / Viviparous Lizard (Zootoca vivipara) young breaking free from egg membrane it was born in. This species can both give birth to live young in a thin membrane and lay eggs. Burnmouth, Berwicks...

Laurie Campbell / Common / Viviparous Lizard (Zootoca vivipara) young breaking free from egg membrane it was born in. This species can both give birth to live young in a thin membrane and lay eggs. Burnmouth, Berwicks...

Tui De Roy / North Island Brown Kiwi (Apterix australis mantelli) captive newly hatched chick still carries large yolk reserve after 70 days incubation. Otorohanga Breeding Centre, New Zealand.

Nick Upton / Yolk and developing embryo visible in Mermaid's purse egg case of Lesser spotted catshark / Dogfish (Scyliorhinus canicula) entangled with Coralweed (Corallina officinalis) in a rockpool. Rhossil...

Nick Upton / Yolk and developing embryo visible in Mermaid's purse egg case of Lesser spotted catshark / Dogfish (Scyliorhinus canicula) entangled with Coralweed (Corallina officinalis) in a rockpool. Rhossil...

Nick Upton / Yolk and developing embryo visible in Mermaid's purse egg case of Lesser spotted catshark / Dogfish (Scyliorhinus canicula) entangled with Coralweed (Corallina officinalis) in a rockpool. Rhossil...

Michael D. Kern / California gull (Larus californicus) predating an egg, San Francisco Bay, USA, SFBBO Gull Count SFBBO Gull Count, May 2009

Tim Laman / Antarctic / Subantarctic / Brown skua (Stercorarius antarcticus) feeding on Gentoo penguin egg, Antarctica, January

Tim Laman / Antarctic / Subantarctic / Brown skua (Stercorarius antarcticus) feeding on Gentoo penguin egg, Antarctica, January

Ingo Arndt / Amu-Darya Trout (Salmo trutta trutta) egg with alevin already visible through egg-shell, Europe

Ingo Arndt / Brown Trout (Salmo trutta) alevin hatching from egg with full yolk sac, Europe

Tui De Roy / North Island Brown Kiwi (Apteryx australis mantelli) newly-hatched chick still carrying a large yolk reserve after 70 days of incubation, Kiwi House, Otorohanga Breeding Facility, New Zealand

Fred Bavendam / Egg-yolk Jellyfish (Phacellophora camtschatica), Saanich Inlet, British Columbia, Canada

Shane Gross / Plainfin midshipman (Porichthys notatus) fry, still attached to yolk sacs in nest in the intertidal zone, Vancouver Island, British Columbia, Canada, Pacific Ocean.

Shane Gross / Plainfin midshipman (Porichthys notatus) fish fry still attached to their yolk sacs hidden under a rock in the intertidal zone, Vancouver Island, British Columbia, Canada, Pacific Ocean.

Shane Gross / Plainfin midshipman (Porichthys notatus) fish fry still attached to their yolk sacs hidden under a rock in the intertidal zone, Vancouver Island, British Columbia, Canada, Pacific Ocean.

Shane Gross / Plainfin midshipman (Porichthys notatus) fish fry still attached to their yolk sacs hidden under a rock in the intertidal zone, Vancouver Island, British Columbia, Canada, Pacific Ocean.

Shane Gross / Plainfin midshipman (Porichthys notatus) male, guarding fish fry still attached to their yolk sacs, hidden under a rock in the intertidal zone, Vancouver Island, British Columbia, Canada.

Shane Gross / Plainfin midshipman (Porichthys notatus) male, guarding fish fry still attached to their yolk sacs, hidden under a rock in the intertidal zone, Vancouver Island, British Columbia, Canada.

Shane Gross / Plainfin midshipman (Porichthys notatus) fish fry still attached to their yolk sacs hidden under a rock in the intertidal zone, Vancouver Island, British Columbia, Canada, Pacific Ocean.

Shane Gross / Plainfin midshipman fish (Porichthys notatus) fry attached to yolk sac under a rock in intertidal zone, Vancouver Island, Canada, Pacific Ocean.

Fred Bavendam / Pacific giant octopus (Enteroctopus dofleini) larvae, one carrying the remnants of egg yolk that nourished it during incubation, floating around egg strand, British Columbia, Canada, Pacific Ocean.

Tui De Roy / Olive ridley turtle (Lepidochelys olivacea) hatchling just hatched from egg with yolk sac still partly exposed, Oaxaca, Mexico.

Shane Gross / Fried egg jellyfish (Phacellophora camtschatica), portrait, Vancouver Island, British Columbia, Canada, Pacific Ocean.

Yves Lanceau / Domestic crossbreed hen (Gallus gallus domesticus), chick embryo developing in egg, embryo developing feathers, yolk volume greatly reduced, 18 days, sequence 9/10

Yves Lanceau / Domestic crossbreed hen (Gallus gallus domesticus), chick embryo developing in egg, embryo developing feathers, yolk volume greatly reduced, 18 days, sequence 9/10

Yves Lanceau / Domestic crossbreed hen (Gallus gallus domesticus), chick embryo developing in egg, embryo developing feathers, yolk volume greatly reduced, 15 days, sequence 8/10

Yves Lanceau / Domestic crossbreed hen (Gallus gallus domesticus), chick embryo developing in egg, more blood vessels visible and the embryo begins to digest the yolk, 3 days, sequence 3/10

Yves Lanceau / Domestic crossbreed hen (Gallus gallus domesticus), chick embryo developing in egg, embryo developing feathers, yolk volume greatly reduced, 18 days, sequence 9/10

Nick Upton / Lesser spotted catshark / Dogfish (Scyliorhinus canicula) embryo attached to yolk inside egg case of in a rockpool, The Gower, Wales, UK, September.

Shane Gross / Fried egg jellyfish (Phacellophora camtschatica) off Vancouver Island, Canada

Nick Upton / Close up of yolk and developing embryo visible in Mermaid's purse egg case of Lesser spotted catshark / Dogfish (Scyliorhinus canicula) in a rockpool low on the shoreline, The Gower, Wales, UK, Se...

Nick Upton / Yolk and developing embryo visible in Mermaid's purse egg case of Lesser spotted catshark / Dogfish (Scyliorhinus canicula) in a rockpool low on the shoreline, The Gower, Wales, UK, September.

David Hall / Fried Egg Jellyfish (Phacellophora camtschatica) viewed from below; Browning Pass, Queen Charlotte Strait, British Columbia, Canada. September.

David Hall / Fried egg jellyfish (Phacellophora camtschatica) Nigei Island, Queen Charlotte Strait, British Columbia, Canada. October.

David Hall / Fried egg jellyfish (Phacellophora camtschatica) Nigei Island, Queen Charlotte Strait, British Columbia, Canada. October.

David Hall / Fried Egg Jelly (Phacellophora camtschatica) with juvenile Rockfishes (Sebastes spp.) Nigei Island, Queen Charlotte Strait, British Columbia, Canada, September.