Practice question

Spemann and Mangold transplanted dorsal lip of blastopore containing prospective chordamesoderm, which normally involutes as notochord, into ventral marginal zone of host gastrula opposite side. Donor tissue retained organizer program expressing Chordin and Noggin, antagonizing ventral BMP4 and eliciting host ventral ectoderm to form secondary neural plate marked Sox2. Host also contributed somites around graft forming second axis. Resulting conjoined twins with secondary dorsal axis bearing notochord and neural tube demonstrated induction not self-differentiation alone. No ventralization occurs; secondary embryo formation proves organizer instructive capacity and neural induction capability.