Section 2. the Vertebral Column Is Devoid of Vertebrae; It Is

throughout life a rod of gelatinous tissue, the notochord (Figure 1, n.c.), surrounded by a cellular sheath. Such a rod is precursor to the vertebral column in the true vertebrates, but, except in such lowly forms as the lamprey, is usually replaced, partially (e.g., dog-fish) or wholly (as in the rabbit) by at first cartilaginous vertebrae whose bodies are derived from its sheath. Further, while in all true vertebrata the notochord of the developing young reaches anteriorly at most to the mid-brain, and is there at its termination enclosed by the middle portion of the skull, in Amphioxus it reaches far in front of the anterior extremity of the nervous system, to the end of the animal's body.* On this account the following classification is sometimes made of those animals which have a notochord:--

-Chordata_ (= Vertebrata, as used by Lankester).

1. Having the notochord reaching in front of the brain. Cephalochorda = Amphioxus.

2. Having the notochord reaching anteriorly to the mid-brain, a brain of three primary vesicles and a skull. Craniata = all "true vertebrata": fishes, amphibia, reptiles, birds, and mammals (Vertebrata of Balfour).

3. Having the notochord confined to the tail. Urochorda = the ascidians, or sea-squirts, certain forms of life only recently recognised as relatives of the vertebrata.

* The anterior end of the notochord in the developing rabbit or dog lies where the middle of the basisphenoid bone is destined to be.