Developmental Biol neural told Fig 2.17 Development of the neural ube and mesoderm in the chick mbryo. Once the notochord has rmed, neurulation begins, following notochord formation in an anterior to neural posterior direction. The figure shows a foregut series of sections along the antero notochord posterior axis of a chick embryo Neural tube formation is well advanced at the extra-embryonic nterior end(top two sections), where mesoderm the head fold has already separated the head fold future head from the rest of the blastoderm and the ventral body fold has brought neural plate endoderm from both sides of the body gether to form the gut. During neurulation, 神经管沿Am the neural plate changes shape: neural folds rise up on either side and form a P轴线依次闭 tube when they meet in the midline. The will give rise to head structures. Further 合,完成形成 neural fold back(middle section). in the future trunk egion of the embryo, notochord and 过程。 neural groove starting. At the posterior end, behind Hensen s node( bottom section), notochord termediate formation somite formation and neuru- exoderm lation have not yet begun. The mesoderm coelom internalized through the primitive streak somatic starts to form structures appropriate to its sition along the antero-posterior and dorso-ventral axes. For example, in the mesoderm future trunk region, the intermediate mesoderm will form the mesodermal arts of the kidney and the splanchnic mesoderm will give rise to the heart neural folds The body fold will continue down the and also bringing paired organ rudiments Hensen's node that initially form on each side of the midline(e.g. those of the heart and dorsal aorta) together to form the final organs lying ventral to the gut. Blood biood island slands, from which the first blood cel primitive groove are produced, form from the ventral- primitive ridge After Patten, B.M. 1971Developmental Biology 神经管沿A －P 轴线依次闭 合，完成形成 过程