ABSTRACT
In chick embryos with a full-length primitive streak the primitive node has been replaced by a graft from the posterior end of the primitive streak, and the blastoderm subsequently cultivated by Waddington’s technique for 24 hours.
More than half of those embryos which developed at all showed a doubling of the anterior part of the axis, variable in extent but considerably exceeding the original extent of the graft, with convergence to a single posterior part. Prechordal head, parachordal mesoderm, and neural plate were affected by the doubling. Most specimens had two notochords fusing posteriorly into a single one, or occasionally terminating independently at a common primitive streak. Other specimens had a notochord in only one of the two axes, or in neither.
Marking experiments with phosphorus32 on a few specimens showed that the graft, which had become greatly elongated, contributed the mesoderm between the notochords, but did not form any notochordal tissue.
The doubled axes were often asymmetrical, and in the head region probably incomplete. The total amount of neural and notochordal tissue in a doubled embryo was not significantly different in volume from that in a single embryo.
In a few specimens in which the axis remained single the place of the notochord was taken by side-plate mesoderm, probably graft-derived. Posteriorly this midline mesoderm became somitic. The neural plate in such specimens was little if at all reduced.
Other specimens failed to heal and developed a split axis. They were usually devoid of notochord, but occasionally had a notochord along one side of the split, or along both sides.
Finally, a few specimens formed an almost normal single axis.