The eggpod is the structure where alien players spawn from. It can be built on walls and ceilings and therefor needs to fit roughly into a cubic bounding box (the proportions of the concept are fine). The structure is made up of two main elements, the slightly flexible outer body and a slimy balloon that grows inside. The balloon should be a seperate mesh and needs to be modeled in a way that allows the animator to have it grow until it fills all the space inside the pod. It will then burst, spawning a new alien (this will be done by hiding the mesh and playing a particle effect) and regrow afterwards.
Have a look at the concept made by Pevel:
It would be nice if the sections of the egg's base would also drift apart a little while the balloon grows. For the balloon itself, note that unlike the concept (which depicted it as a membrane instead), any surface detail shouldn't stop where it touches the egg. Previous attempts have also shown that a glowing surface like this doesn't look as fitting in-game, so feel free to give it a more fleshy look (similiar to an amniotic sac maybe, though less translucent as atleast two different kinds of aliens can spawn inside and you shouldn't be able to tell which one until it bursts).