ABSTRACT
The experiments were performed on 3·5-day and 5·5-day chick embryo leg-buds. Homo-and heterochronic heteropolar ecto-mesodermal recombinants were prepared in which the dorso-ventral (d-v) axis of the ectoderm was rotated by 180° with respect to that of the mesoderm. The mesodermal component of the associations consisted either of the intact mesodermal core or of a reaggregate of dissociated mesodermal cells. Recombinants were grafted onto a host wing stump and grown to day 16–20 of incubation.
The resulting feet display a d-v polarity in their integument and skeleton. This polarity conforms to that of the mesoderm in the proximal part of the foot (roughly basipod and metapod) and to that of the ectoderm in its distal part (roughly acropod) in the case of recombinants containing 3·5-day mesoderm. With 5·5-day undisrupted mesoderm, the polarity conforms entirely to that of the mesoderm.
In the recombinants containing 3·5-day or 5·5-day disrupted reaggregated mesoderm, the d-v polarity of the resulting feet, on the whole, conforms to that of the ectoderm. However, a high frequency of unpolarized integumentary and skeletal abnormalities occurs with 5·5-day disrupted mesoderm.
It is concluded that the ectoderm is polarized along its d-v axis. It exerts a d-v polarizing activity on the mesoderm during the 48 h period necessary for the segregation of the presumptive territories of the autopod. The ectodermal polarizing effect is expressed in the morphology of the integument and skeleton. At the end of the 48 h period, the d-v axis of the mesoderm is irreversibly laid down.
