What layed the egg in the cave?

from my understanding of it that flying muto had hatched and traveled all the way to janjira to cocoon itself there feeding on the radiation until it matured. I hope this helps out some. :)

Well the Muto eggs looked alot different from the cocoons in the cave. So perhaps those cocoons were designed for hibernation, to keep the Muto alive while it matured. The male may not have had wings until it fully matured, perhaps making it look more like a larva of some sort.

what bothers me is that the cocoon in the cave doesnt seem so big, but when it escaped, the trail made it seem like it grew 4x in a matter of seconds? ugh i m so confused

@Borake

I notied that too, but it's pretty common in all Godzilla films for them to adjust the scale of the monsters to fit the scene. 

BORAKE - This is a very interesting set of questions you've raised! Hmm, allow me to expound a little. If I were to take a guess at what laid the eggs within the ancient ribcage/cave, I'd say it was likely a primitive form of MUTO - perhaps something similar to the ones we saw in the film, but somewhat less refined in appearance and capability. Consider this: is it possible that the higher concentrations of artficially-refined nuclear energy upon which the MUTOs fed might have altered them in some way? Perhaps they evolved along a new and more destructive line after feeding upon such concentrated atomic energy - thus making them much more formidable foes for Godzilla! That might also explain why the female was carrying so many eggs; she'd undergone a strange and divergent metamorphosis which made her the first of her kind with regards to procreative ability. As for what the male MUTO might have looked like before it emerged as an adult in Janjira: I've a feeling it looked something like a larval cicada just after having emerged from the ground - large, bulky, clumsy and single-mindedly driven to feed until it could pupate into its final, adult form. I do hope this helps to clarify some of your confusion. If anything, this was a very fun topic you've presented. Thank you ever so much for bringing this to us! :)

I agree with Kaijujira. It wasn't an actual egg. I think it was a cocoon for hibernation.