NERVOUS 

SYSTEM) . The nerve cells are arranged in three main columns, ventral, intermedio-lateral and posterior vesicular . The ventral cell column has the longest cells, and these are again sub-divided into antero-mesial, antero lateral, postern-lateral and central groups . The intermedio lateral cell column is found in the lateral horn of the thoracic region . compressed central canal (see fig . 5) . The original ectodermal cells elongate and, radiating outward from the canal, are now known as spongioblasts, while the inner ends of some of them bear cilia and so the canal becomes ciliated . A number of round. cells, known as germinal cells, now appear close to the central canal, except at the thin -mid-dorsal and mid-ventral laminae (roof-plate and floor-plate) .