What's going on with my frogspawn?

I have a big frogspawn that has recently formed a couple of large 1/2" diameter bubbles. You can see two in this pic - one at the top center and one at the bottom left. Does anyone know what's going on and if this is anything to worry about?
 
The bubble is usually where one head is splitting into two (very slowly).
 
Thanks for the replies! You can see from the pic where a head just died a couple of days ago, and I was worried the bubbles were a precursor to more heads dying. I've seen new heads form from little buds that branch off below the existing heads, so I didn't know these bubbles are another form of growth. Certainly glad to hear it though :D !
 
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