Vertebrate anatomy
Nervous tissue is derived from what?
Anatomy
All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal (vertebral column or notochord), with a hollow tube of nervous tissue, the spinal cord above it and the gastrointestinal tract below. Nervous tissue is derived from the ectoderm, connective tissues are derived from mesoderm, and gut is derived from the endoderm. At the posterior end is a tail which continues the spinal cord and vertebrae but not the gut. The mouth is found at the anterior end of the animal, and the anus at the base of the tail. The defining characteristic of a vertebrate is the vertebral column, in which the notochord has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs), which each contain a soft central core (nucleus pulposus), derived from the notochord of the embryo. However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood. These include the sturgeon and the coelacanth. Jawed vertebrates are typified by paired appendages, fins or legs, which may be secondarily lost. The limbs of vertebrates are considered to be homologous because the same underlying skeletal structure was inherited from their last common ancestor. This is one of the arguments put forward by Charles Darwin to support his theory of evolution.