Metroid Egg

A Metroid Egg, laid by a Queen Metroid, is the first (amniotic) stage of a Metroid, following birth. The first "natural" egg encountered in the series is during the end of Metroid II: Return of Samus, where the Baby hatches in front of Samus Aran. Artwork depicts a blue-green egg, covered in a membranous substance as well as several green vein-like structures; the appearance differs greatly from the in-game sprites due to the Game Boy's monochrome color scheme. The introduction in Super Metroid gives the egg considerably more detail and even more so in Metroid: Other M. The Infant Metroid hatches via violent shaking of the egg, and forcibly erupting from it. The embryonic liquid released from the egg appears to rapidly harden.

Several unnatural Metroid eggs appear in the Prime games being Phazon-infused, and are the first eggs Samus had ever witnessed. These anomalies can be found within the Metroid Processing room on the Pirate Homeworld, and on Phaaze in Metroid Prime 3: Corruption. They presumably produce Miniroids and Phazon Metroids. In Metroid Prime 2: Echoes, Space Pirates engineered artificial eggs called Metroid Cocoons designed to carry Infant Tallon Metroids.

In Metroid: Other M, Metroid Eggs are seen twice. The first instance is in a flashback of the Baby's hatching in Metroid II. The second time, Samus sees several hatched eggs in Room MW of the Bottle Ship shortly before facing another Queen Metroid. The Baby's egg also appears in TV Commercial :60 Spot, the commercial for the game. The way they hatch is also different, in that they simply crack and explode, with the Metroid emerging.

Scan
Metroid Prime 3: Corruption (Phaaze)
 * "Target identified: Metroid Egg. Completely saturated with Phazon. Faint life signs detected within."

Matt Manchester
"I made the metroid eggs specific for this room. I watched the intro to Super Metroid again, and noticed that the fluid inside the egg dries almost instantly when exposed to the air. I created some "hatched" eggs similar to this, with the embryonic fluids hardened immediately in their dripping state."

Unused scan
"EGGS CHAMBER"

''Exposure of unhatched metroids to alien sustentative chemicals and levels of radiation has been insightful as to the adaptability and survivability of the organism, even at this early latent state.  Specimens that survive have been observed with greater scrutiny as to their cumulative behavioral augmentations, as well as any affect exposure has to their ability to drain life energy.''

Trivia

 * When Samus first lands on the Pirate Homeworld, extremely large structures can be seen which bear an identical resemblance to the saturated Metroid eggs. It is unknown if these structures actually contain Metroids of any kind or are simply Phazon growths.
 * Curiously, there are incredible amounts of mutated Metroid Eggs on Phaaze, along with hives containing Phazon Metroids (and presumably Miniroids). While it can be assumed that Pirates had brought to the planet a good portion of both Metroids and eggs, the presence of hives (which would logically lead to the production of more eggs) is truly a mystery. Whether or not these eggs were laid by a mutated Metroid providing a role similar to a queen was never revealed.
 * In several areas where the mentioned eggs are located, unknown pulsating cocoons can be found near them.