The study of mitotic chromosomes from mule embryos at the 72-96 hour stage has shown that there is a blockage in the condensation of large regions of the small arms of the inherited number 1 parent chromosomes. Zones of decondensation on chromosomes 2 and 3 were also observed. In some cells there is a blockage of condensation ('prophazing') in most of the microchromosomes. These phenomena could be linked to the inactivation of part of the hybrid genome and explain at least some of the early mortality of these intergeneric embryos.