It's been 6 days since I planted the cactus seeds, and I'm happy to say that there's progress to report. Although the package said 7-56 days before germination, the first seed sprouted after only 4 days, most likely because I soaked the seeds and kept them nice and moist.
I don't know which of the types of seeds have germinated, except that some of them are the small black ones. The large ones that I put in the separate container have not emerged yet.
They even have two little cotyledons -- the fat first leaves that open and hold the seed's food reserves:
The difference starts becoming apparent after that. Instead of elongating and growing upward, the stems start fattening up:
So do the cotyledons. I think the color change is due to too much light, but I'm not sure. At this stage I'd rather have too much light than too little.
There are so many seeds germinating now, and I can't tell if they're all the same type or not.
I'm keeping a close eye on these guys, as I want to see how they transform from these short smooth stubs into a prickly miniature cactus. It's quite exciting!