The Alien Egg can be printed at ten different ages using the Customizer. At age 0, there is just the shell. Then the embryo appears at age 1 and grows gradually until it gets big enough at age 9 to outgrow the shell. Along the way, side ports open up so you can see it grow. At each age it can be printed in different variations: without the antennae, without the ribs and even without the shell.
The photos show eggs at Ages 0, 5, 6 and 8. There's also an animation showing all the stages of growth at: https://vimeo.com/101582356

I used the following settings on my MakerBot Replicator 2 to print this thing. Support is needed and may be a challenge to remove for some of the ages. I used Simplify 3D to do the slicing.

Infill=20%
Layer Height=0.200
Shells=3
Feedrate=48
Travel Feed=90
Temperature=230