EMBRYOLOGY OF FROG-Development of Neural tube or Neurogenesis in frog

In the gastrula, the presumptive material for nervous system, lies on the mid-dorsal line as a plate called neural plate or medullmy plate. It extends from the dorsal lip of the blastopore to the anterior end. Soon, the edges of the neural plate become thickened and raised above as neural folds or medullary folds. The neural folds of the two sides are continuous anteriorly to form the transverse neural folds. The neural folds enclose a shallow groove called neural groove. The neural folds increase their elevation and bend towards one another until their edges meet and fuse. Thus a tube is formed called neural tube. It encloses at canal  called neurocoel.The fusion first starts just behind  (in the anterior end regio  of future medulla and fmm here the fusion  both ameriorly and postenorly.Anteriorly, the neural tube opens to the exterior for some time by anterior neuropore. It becomes closed soon. Posteriorly the neural folds enclose the blastopore in such a way that the neurocoel communicates with the archenteron through the blastopore. The short narrow canal connecting the archenterons and neurocoel is neurenteric canal. Later it also disappears.After the neural folds have fused in the median line. theneural tube separates itself completely from the overlying epidermis. The free edges of the epidermis fuse together, so that the epidermis becomes continuous over neural tube. As the neural tube is separated from the ectoderm, a certain number of loose cells are liberated from the neural folds in the space between the ectodemt and the neural tube. These cells arrange themselves as two longitudinal bands on the dorso-lateral wall of the neural tube. These cells constitute neural crest. Later the neural crest cells differentiate into the ganglia of the cranial and spinal nerves, melanophores (chromatophores), adrenal medulla and visceral skeleton. The anterior portion ofthe neural tube differentiates into the brain and the posterior part into the spinal cord. The embryonic stage which is having the neural plate or the neural tube, is called neurula.