Today, the seeds I saw sprouting yesterday have grown even more and there are other seeds germinating left and right. According to this picture, the small arcs I saw pushing up out of the soil yesterday were the hypocotyl. Today, those 2 cosmos have broken free from their shells completely and I can see their cotyledons.
Here is a shot of the 2 cosmo seedlings we saw yesterday. The green things are the cotyledon and will become the first basic leaves of the plant.Here, you can see more cosmos have sprouted, completing the row:Here are some more seedlings. This cosmo seems to have been planted the right way up, so it's cotyledon is pointing straight into the air, and the shell is still on top. The marigolds are very thin, just like their seeds.When I saw 2 sprouts coming from an area with just 1 spinach seed, I had to investigate. Gently uncovering the seed, I found that at least 3 sprouts had grown from the single seed. I don't know if they are all 1 plant, or if multiple plants can grow from a spinach seed. I just hope they're yummy: