Pressure caused by 

 thickening of neural plate 



Continued thickening 

 forces greater folding 



/ \ 



Fig. 31. Neural tube formation. 



Fig. 32. Changes in the external form of the amphibian embryo, from the 

 appearance of the neural fold to the completion of the neural tube. 



DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM 



Originally the neural tube is wider at the front end of the embryo. 

 This width is enhanced by local swellings that produce three distinct 

 bulges. These are the forebrain, midbrain, and hindbrain. Now the fore- 

 brain bulges laterally and two cup-like rudiments appear. Simultaneously 

 the surface ectoderm (the epidermis) folds inward to meet the optic cups. 

 Folds are pinched off and ultimately develop into eye lenses. The ears 

 and nostrils also appear at first as epidermal infoldings. In the meantime 

 the spinal cord undergoes considerable growth and cell division. Meso- 

 dermal cells migrate toward the cord and aggregate around it and the un- 

 derlying notochord. They transform into cartilage and ultimately into bone 

 tissue, giving rise to a hollow vertebral column with associated ribs, etc. 



The spinal-cord cells are at first large and rounded. They now as- 

 sume typical neuron shapes, spinning out long fibers. The cells in the 

 ventral part become motor neurons, and their fibers penetrate peripheral 



63 



