My guess would be that, because gelatin is the shorter form of the polymer (not entirely sure on that one, but it makes sense to me) it lends itself to manipula

My guess would be that, because gelatin is the shorter form of the polymer (not entirely sure on that one, but it makes sense to me) it lends itself to manipulation much easier. So while it's weaker, it's going to be easier to work with to make something viable for a prototype. It might also be that it made the release of the eggs easier to design in to the artificial organs.