1)Where do the extraembryonic membranes of bird embryos and human embryos come from? Are they made of embryonic tissues or maternal tissues or both? Do the extraembryonic membranes of bird embryos have the same kind of origin and developmental processes as the extraembryonic membranes of human embryos?
2) Let's examine that identicalness among organisms by focusing on one gene called Sonic Hedgehog (the SHH gene)--James' mention of hedgehogs sparked this line of thought for me. What is the SHH gene? What kinds of organisms have the SHH gene? What does the SHH gene do in those different organisms?
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