Simple geometric mitosis

We construct simple geometric operations on faces of the Cayley sum of two polytopes. These operations can be thought of as convex geometric counterparts of divided difference operators in Schubert calculus. We show that these operations give a uniform construction of Knutson-Miller mitosis (in type A) and (simplified) Fujita mitosis (in type C) on Kogan faces of Gelfand-Zetlin polytopes.