Changes in the differentiation tendencies of the hypoblast-free Hensen's node during "Gastrulation" in the chick embryo.

Hensen's node was isolated from chick blastoderms of medium-streak to headfold stages from which the endodermal layer had previously been removed. The isolates were culturedin vivo by means of the intra-coelomic grafting technique. Node pieces with the endodermal layer intact served as controls.Endodermal differentiation tendencies gradually decreased from the medium-streak to the pre-head-process stage and completely disappeared at the head-process stage, whereas the controls gave rise to endodermal structures throughout all stages. "Cranial" structures such as oesophagus and trachea, often together with thyroid, parathyroid and/or thymus, were only found in grafts of younger stages, while gizzard, intestine and/or pancreas were observed in many gut-containing grafts throughout all stages.There was a constant high incidence of notochord, muscle, and cartilage formation. The incidence of mesonephric structures, sometimes accompanied by adrenal gland, rose steadily throughout all stages both in experimentals and controls.Neural differentiation tendencies (rhombencephalon and/or spinal cord) were always present in the nodes isolated (with or without endoderm) from the definitive primitive-streak stage onwards, but in nodes from earlier stages the incidence of neural differentiation was significantly lower.The results are discussed in relation to the possible location and determination of the prospective endoderm and mesoderm.