138 NEURULATION AND EARLY ORGANOGENY 



side dorso-laterally to the neurocoel as the neural crests. The neural 

 crests are actually part of the original ectodermal neural folds which 

 do not form an integral part of the neural tube. The paired neural 

 crests extend the full length of the central nervous system, lie dorso- 

 lateral to that system, and will give rise to various ganglia of the 

 central and sympathetic nervous systems and to chromatophores. Cov- 

 ering the central nervous system dorsally is the reconstituted ectoderm, 

 derived by the fusion of ectoderm lateral to each of the neural folds 

 which are brought together at the time of closure of the neurocoel, 

 along the mid-dorsal line. 



The continuous and paired neural crests, lodged between the 

 neural folds and the overlying dorsal ectoderm, become metamerically 

 subdivided by the developing somites. These crests are therefore 

 neural from the beginning, but are extra-neural in position in that 

 they are left outside the axial nervous system. These crests retain 

 cellular connections with the dorso-lateral wall of the developing 

 spinal cord and will give rise chiefly to the dorsal root ganglia of the 

 spinal nerves. At the level of the brain they give rise to the ganglia and 

 to the fifth and seventh to tenth roots inclusive. They may also give 

 rise to the visceral and cranial cartilages. At the body level they give 

 rise not only to the paired spinal ganglia but also to the sympathetic 

 nervous system, to the chromatophores of the body, and to the medulla 

 of the adrenal gland. 



The medullary plate at its anterior extremity is the last region of 

 the central nervous system to be closed off from the exterior. The 

 opening from the presumptive brain region to the exterior is the 

 anterior neuropore, which has a homologue in the development of all 

 vertebrates. Due to the original spherical condition of the gastrula this 

 anterior region of the central nervous system curves ventrally at about 

 the level of the future midbrain, and this ventral curvature of the brain 

 persists and is characteristic of all vertebrates. The notochord, which 

 functions as an axial skeleton for the embryo, is ventral to the nerv- 

 ous system, and terminates just at this ventral curvature of the brain 

 (i.e., at the cranial flexure). The anterior neuropore is then found 

 at the anterior extremity of the embryo in the sagittal plane directly 

 in line with the terminated notochord, but in the roof of the brain. 

 Occasionally a sagittal section of a 2 mm. embryo will show a knot of 

 cells in this region which represents the puckered closure of the 

 anterior neuropore. 



