Oögenesis, fertilization and early cleavage in Habrobracon

Since differential maturation has been used in explaining sex determination in Hymenoptera, the pertinent stages were studied cytologically in Habrobracon by using whole mounts of timed eggs stained by the Feulgen reaction. The egg is laid with the maturation figure in an arrested first anaphase close to the ventral egg surface, and with the spindle horizontal to the latter. Eggs from different stocks show differences in position of maturation figure. A single sperm enters during oviposition, the maturation spindle revolves to a vertical position and normal maturation and change of the sperm ensues. Nuclei of unfertilized eggs behave similarly. Ten chromosomes enter each of the three polar and the one matured egg nuclei, the latter being always the innermost of the four. Unfertilized eggs have ten chromosomes at first and eleventh cleavages; fertilized eggs have around the expected twenty. Chromosomes in maturation divisions are too small and round to show individual differences. There is no good evidence of chromosome elimination.