EMBRYOLOGY 391 



similar cleft cuts it off from the ectoderm. In this way the noto- 

 chord is formed. 



We must now go back again to the stage when the blasto- 

 pore is just completed, to consider the formation of another important 

 structure. Here we find that the " nervous " layer of the ectoderm 

 has thickened to form a moderately wide medullary plate which 

 extends forwards irom the dorsal lip of the blastopore to the front 

 end. As the tyastopore closes and the yolk plug is withdrawn, the 

 lateral margins of this plate are elevated slightly to form the lateral 

 neural folds which run from the sides of the blastopore to the anterior 

 end, becoming more marked as they do so. At the front they 

 turn inwards and unite in the middle line, so forming a transverse 

 neural fold marking the front limitation of the medullary plate. The 

 area contained within the folds, termed the neural plate, is somewhat 

 thinner than the margins and a groove appears along its middle 

 line, the neural groove. When this has been laid down a cleft appears 

 beneath the plate and cuts it off completely from the underlying 

 notochord as noted above. 



Although the embryo is now in a triploblastic condition 

 there is as yet no ccelom, and it will be as well to follow the develop- 

 mental processes a little further so as to reach approximately the 

 same stage in which we left Amphioxus. 



The formation of the central nervous system is indicated, as we 

 have seen, by the formation of the medullary plate, neural folds and 

 neural groove. As development proceeds the groove becomes 

 deeper and narrower and the neural folds pass towards the middle 

 line. Finally they bend over and meet, but not simultaneously along 

 their whole extent. They first come into contact at a place that will 

 form the myelencephalon, so that from here onwards the differentia- 

 tion into brain and spinal cord regions can be recognised. From 

 this first point the fusion extends in both directions, but more 

 rapidly towards the posterior end. As the union of the folds passes 

 slowly forwards the transverse folds also meet, their fusion extending 

 backwards until it reaches the fusion of the lateral folds at a point 

 between the future fore and mid brain. This is, therefore, the last 

 region to fuse and so can be considered as the neuropore, but it has 

 only a transitory existence and soon closes. From the manner in 

 which the lateral neural folds come together, the actual union takes 

 place along the edges of the neural plate, so that only this structure 

 takes part in the formation of the neural canal. The neural folds 

 themselves are left outside the canal, between it and the covering 

 ectoderm. Later they become broken up into groups of cells lying 

 along the dorso-lateral region of the central nervous system, thus con- 

 stituting the neural crests which are concerned with the development 



