x1 is an egg/ovum/sperm/pollen/gamete of/from organism [mother/father] x2.
(poorly metaphorical only due to gender- and species- being unspecified): ovoid, oblate (= pevyso'aseltai, but better: claboi); egg, specifically female (= fetso'a), of a bird (= cpifetso'a, cpiso'a), of a chicken (= jipcyfetso'a, jipcyso'a. (but note that Lojban does not require specificity, just as English doesn't for either milk or eggs; "sovda" is fine for most contexts); If fertilized, then tsiju or tarbi. (cf. ganti, gutra, mamta, patfu, rorci, tsiju, lanbi, tarbi; also djine, konju for shape, tarbi)