Autonomy in the regionalisation of branchial endoderm in amphibia.

1. The branchial endoderm taken from late neurula or tail bud embryos developed into typical pharynx, when left to develop with head-mesenchyme, while in a purely ectodermal environment lacking mesodermal elements, it did not give rise to any identifiable structure. 2. The same endodermal piece was explanted together with lateral mesoderm. In this combination, the frequency of the occurrence of intestine decreased with increasing age of the donor, and the appearance of "pharyngoid" (the amorphous assemblage of pharyngeal cells) increased. 3. The piece of branchial endoderm was removed from an early neurula, and was isolated alone or together with cranial ectomesoderm for 24 or 48 hours, until the synchronous control had reached early or late tail bud stage; then pre-isolated piece of endoderm was combined with lateral mesoderm and cultivated further in an epidermal envelope. A decreased production of intestine and increased production of "pharyngoid" were obtained, according to the duration of the pre-isolation. 4. In the discussion, the establishment of regionalisation of endoderm in normal embryo was interpreted as anautonomous process, which can be accerelated by contact with head mesenchyme but which does not require it, while for the individuation of pharyngeal organisation, the presence of mesenchyme was considered to be indispensable.