THE vertebrates: the amphibia and birds 165 



while between them the surface is somewhat flattened to make a wide 

 shallow depression. The ridges wliich mark the boundary of the area are 

 known as the neural folds, while the depressed area between them is the neural 

 plate. Sections show that the neural plate is thicker than the remainder of 

 the ectoderm (Fig. 20.21, p. 452). As time goes on this thickening increases, 

 the plate simultaneously becoming narrower and the folds higher, until the 

 whole neural area becomes more appropriately referred to as the neural 



Figure 9.9 



Semi-diagrammatic drawings of newt gastrulae sectioned through the 



dorso- ventral plane. The ridge on the wall of the archenteron inc, d and e 



shows where the endoderm and mesoderm are separating from one another. 



(From Spemann 1938.) 



groove rather than the neural plate. Eventually the neural folds approach so 

 closely that their upper margins touch and fuse with one another; neural 

 material joins on to neural, and skin to skin, so that the neural groove be- 

 comes converted into a tube lying beneath an unbroken covering of epi- 

 dermis. Clearly the tendency of material in this region of the egg to 

 converge towards the mid-dorsal line, which we noticed during the gastru- 

 lation movements, has continued even after the stretching in length has 

 become less marked. The same is true of the underlyuig mesoderm. The 

 central strip which overlies the primitive gut and forms its roof condenses 

 together into a single median strand, the notochord; while the tissue 



