Development of the mouse placenta proceeds rapidly after the allantois and chorion attach through a process called chorioallantoic attachment. As a consequence of chorioallantoic attachment, a subset of chorionic cells in proximity of the allantois differentiate into syncytiotrophoblast cells and form invaginations into which allantoic-derived embryonic blood vessels migrate to form the labyrinth layer. Chorioallantoic attachment and subsequent differentiation events required for placental development have remained poorly investigated partly due to being inaccessible to ex vivo analysis. Here, we report conditions for ex vivo culture of pre-attachment chorion and allantois. Under these culture conditions, explants of pre-attachment allantois and chorion attached and showed extensive mixing of chorionic and allantoic cells. We confirmed that the allantois was required for expression of the syncytiotrophoblast cell marker Gcm1. In addition, we found that maintained expression of the spongiotrophoblast cell marker Tpbpα also depended on chorioallantoic attachment. We tested the efficacy of this ex vivo model by examining the tissue-specific requirement for Tmed2, a member of the transmembrane emp24 domain (TMED) protein family, required for normal placental development. Recombinant cultures of Tmed2 null and wild type chorion/allantois revealed a role for Tmed2 in cell survival and for mixing of chorionic and allantoic cells. Thus we report the first successful ex vivo model of pre-placental tissues before chorioallantoic attachment, and show that this explant system can be used to reveal tissue-specific requirements of genes required for placental development.