I sowed these on 7/3/12. They are about 25 in a 4" plastic pot. The media is topsoil and perlite 2:1, microwave sterilized. I put the pot with the seedlings into a gallon plastic bag and sealed it. Watered only with distilled water once to saturate the media. The bag has been keeping them humid enough, it almost always has condensation on it. I then put the whole thing in a west facing window that blocked by the sun for part of the day.
I checked on these today 7/24/12. So far 2 have germinated. Not great, but I'd almost be happier if I didn't have 25 baby cacti to deal with.
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According to my photo records, (I take pictures of when things bloom,) the most likely possible candidates for the pollen donor are either Gymnocalycium baldianum or Rebutia muscula. My money is on the Rebutia, but I'm not sure what cacti can pollinate others out of their genus, so it's just a guess. Around the same time Parodia fusca bloomed, but I'm pretty sure it opened after the P. mammulosa had already closed. I'll have to wait a few years to find out, but these might end up being a pretty cool cross.
I'll update about these when they begin to make their spines and actually start looking like they might be cacti one day, hopefully in a month or two.