…into what is called the embryonic disk. The embryonic disk soon acquires three layers: theectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). Within the mesoderm grows the notochord, an axial rod that serves as a temporary backbone. At what location does the amniot...
(1) a brain subserves the entire body, not just restricted segments; (2) it has functionally specialized parts; (3) it is bilobar; (4) commissures and neurons form the surface with axons in the central core; (5) interneurons are more numerous than primary motor or primary sensory ...
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Amphioxus may represent an intermediate stage in the evolution of the vertebrate nervous system, but the anatomic relationship between the notochord and neural tube is more complex. The decussating interneuron of amphioxus, to mediate a primitive coiling reflex away from any stimulus, provides a ...
They had no jaws, primitive spinal columns called notochords, and primitive gills. Haikouichthys is one of the primordial fish. It had a distinct head and tail, and its body was bilaterally symmetric, with the left side mirroring the right. It had two eyes and a mou...
notochord (a cylindrical rod of cells). The sheet of notochordal cells is laterally in contact with the roof of the primitive gut and dorsally in contact with the midline cells of the neural plate. The notochordal plate folds off from the roof of the primitive gut to form the notochord. ...