a, To examine factors affecting nuclear size, Levy and Heald2 mixed sperm chromatin from either Xenopus laevis (X.l.) or X. tropicalis (X.t.) with cytoplasmic extracts from the eggs of either X. laevis or X. tropicalis. They find that, regardless of the chromatin used, X. laevis extracts promote formation of large nuclei, whereas X. tropicalis extracts promote formation of smaller nuclei. (The drawings of X. laevis and X. tropicalis are to scale). b, Two proteins, importin-α and Ntf2, account for the differences between the X. laevis and X. tropicalis extracts: addition of importin-α and inhibition of Ntf2 activity in X. tropicalis extracts lead to formation of larger nuclei.