If this were a tomato plant rather than a tomatillo, I'd say you have adventitious roots forming.

When a tomato plant is lying on the ground, the stem grows roots into the soil; it's natural for them to do so. Often in humid weather, tomato stems that aren't in contact with the soil will sprout adventitious roots. Usually the stem doesn't actually split, but sometimes it will.

In rarer cases, adventitious roots can be an indication the plant has a serious disorder affecting its lower stem or roots.

This site says tomatillos do grow adventitious roots; be sure to scroll down for the second photo.
http://plantdiagnostics.umd.edu/level3.cfm?causeID=404

The plant is growing like crazy and is covered with flowers. Congratulations! You have a happy, healthy tomatillo.

With tomatillo, the bumps are still adventitious roots.

That's it! Thanks, Obvious, for pointing out the obvious. I never knew tomatillo plants grew roots like tomatoes do. Now I hope something grows as I also didn't know that you need two for cross pollination:~/

missingtheobvious - Thank you so much for this answer. I was worried there were eggs growing inside the stems and I was going to have to burn the plants right as they are starting to set fruit.

Glad to help, Ginny. Adventitious roots have scared most of us at one time or another. May you harvest many huge tomatillos!