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NEURULATION AND EARLY ORGANOGENY 



Epithelial 

 ectoderm 



Neural fold 



Neural groove 



Neural crest 



Notochord 



Neural crest 



Epithelial 

 ectoderm 



Nerve cord 



Neural canal Notochord 



Formation of the neural tube. 



Early Organogeny 



Surface Changes 



The oval-shaped gastrula quickly becomes elongated and the 

 medullary plate provides a slightly elevated (convex) dorsal surface. 

 This soon changes to a flattened and then a concave upper surface, as 

 the development of the central nervous system proceeds. At this 

 time the embryo acquires a distinct head and a body which is ovoid 

 because of the mass of contained yolk. 



The appearance of the thickened and elongating medullary plate 

 and the subsequent formation of a neural tube is the main causal 

 factor in the change in shape of the embryo, which follows upon 

 gastrulation. In fact, it may well be the autonomous powers of 

 elongation of the presumptive notochord that are responsible for 



