Fountain of Knowledge

The story that I am working on now has aliens in it. I have most of the details worked out and everything, I just have one question that I thought I could get some interesting thoughts on here. The aliens have three genders; what do the humans call them? I know the aliens have their own terms for each gender, but the humans need to have words for them, too. For example, the aliens' name for themselves is "Gyarken," but the humans usually call them "Three legs," because they have three legs. (Duh, right? Why else? But what I need to figure out is what the humans call each gender. There is one that has something similar to eggs (that's the nearest analogy, anyway), one that has something similar to sperm, and one that has something like a catalyst for the reaction that results in a baby alien. This is the one that actually gives birth. So...yeah. What should the human names for the different genders be? Should the one with the eggs be "female" or the one that gives birth? I guess the one with sperm should be "male," but what about the other one? What should the humans call that? It needs to be a word that is already in existence and that makes sense. Please help!