Yes, they are juvenile spines. Mammillaria tend to get them almost straight out of the seed. Next step will be baby tubercles :)

Rewarding eh, Davide? I hope to eventually be set up for seedlings..
Bill in SC

...but when the seed will finished to germinate what should I do? Just putting the pots in another place without heating or what else? Help!
This is another pic of the seedlings:
{{gwi:489601}}
Bill: Sure! but how to send you the seedlings?

Davide, I sowed seedlings under artificial lights last January and kept them under the lights for about three or four months. The lights also kept them quite warm. After that they were all large enough to go outdoors and get used to the sun. All except a few slow Mammillaria are now large enough to go through winter without trouble.

I think you should keep your's above 15C at night and up to 25C or even a bit more during the day, moist, and in the brightest light that you have. Even direct sun is probably safe at this time of year, it is here in the north anyway :)

SnB: thank you for your helpful infos. The original idea was to build another box without heater but with 4 neon just to keep the temp. around 25°C. In this way the acquarium is ready for other seeds. I'm now using two neon tubes (OSRAM L18W/77 FLORA-OSRAM L18W/860 LUMILUX DAYLIGHT). Are they ok in your opinion?
Davide

Those lamps should be fine. Two of them should be enough for up to about 60cm x 30cm. You should probably add more tubes after the first couple of months. Watch the seedlings and add more light if they seem too pale.

I start the seeds under just warm white (3000K) and then use half warm white and half daylight (6500K) about six weeks after germination.