THIRD PERIOD OF DEVELOPMENT. 125 



closing of the neural canal, this condition conies con- 

 tinually into sharper prominence ; through the consid- 

 erable elongation of the embryo, the neural canal also 

 becomes considerably thinner. The anterior end of 

 the neural canal is now less affected by this elongation, 

 so that proportionately it is continually appearing of 

 greater thickness. While the neural canal, from the 

 second mesoblastic somite on, shows a smaller trans- 

 verse, section the neural plate is found far thicker 

 and considerably widened in the region of the ante- 

 rior half of the first mesoblastic somite, and still fur- 

 ther forward in the region of the prolongation of the 

 anterior end of the mesoblast, where the neural canal 

 is still open outwards (cp. Figs. 113-115 with 116 ; and 

 Figs. 122-124 with 125). The central canal, too, 

 possesses in the neighbourhood of the first meso- 

 blastic somite a considerable diameter. The neural 

 foundation thus shows an unmistakable swelling on the 

 anterior end of the body, especially on from the 

 anterior half of the first mesoblastic somite. 



This is also to be recognised in the side view of the 

 embryo, and is remarkable on a back view, owing to its 

 being wider than the notochord (Fig. 56). 



In an anterior direction the neural plate has not 



