21. Notochord flexion occurs at around 4 mm standard length in " R . loricata ".22. Von Baer studied the embryonic development of animals, discovering the blastula stage of development and the notochord . 23. Notochord -derived Shh signals to the floor plate, and induces Shh expression in the floor plate.24. Loss of Shh from the notochord and / or floor plate prevents proper specification of these progenitor domains. 25. In the fish and all subsequent vertebrates, the notochord forms the backbone ( or vertebral column ). 26. A notochord is formed early in development, and always consist of a row of exactly 40 cells. 27. Instead of true vertebrae, they have a series of cartilaginous structures called arcualia arranged above the notochord . 28. One of the characteristics linking the specimen to chordates is its notochord , the precursor of a spinal column. 29. The ectoderm is instructed to become the nervous system by the notochord , which is typically positioned above it. 30. The lower tube surrounds the notochord , and has a complex structure, often including multiple layers of calcification.
