This is the series’ “Nuff Said” entry. The in-story explanation for the silence is that Emma and Jean are trying to “find” Charles Xavier’s mind, which is trapped in Cassandra’s body.
The entire issue is Frank Quitely’s creepily beautiful art across a psychic landscape, telling the story of the twins Charles and Cassandra, in the womb. We see that Charles murdered his sister, and we conclude on the last page with a single word balloon…
“We should talk.” When he is on his game, there’s really nobody who can write comics as great as Grant Morrison.