Researches on the formation of axial organs in the chick embryo. IX. On the development of somites in axial-paraaxial segments explanted to the zona pellucida.

Axial-paraaxial segments (neural tube, chorda, unilateral meso- and endoderm) excized from explanted 36--40-hour-incubated chick embryos at the level of unsegmented mesoderm, after removal of the ectoderm, were grafted onto subectodermal pockets of the zona pellucida. Under these conditions somites develop and differentiate normally. Paraaxial segments (unilateral meso- and endoderm) grafted under the same conditions show (retarded) somitogenesis only in 15% of the cases. Pure paraaxial unsegmented mesoderm grafted under the same conditions develops somites in 14% of the cases. Since in situ, the removal of the axial organs and of the endo- and ectoderm does not inhibit somitogenesis, the above-mentioned results prove that under conditions of grafting, some additionary "factors of realization" necessary for normal somitogenesis are lacking.