THE LATER EMBRYONIC DEVELOPMENT. 63 



In the later stages the neural canal deepens, owing to 

 longitudinal folding of the neural plate ; at the same time 

 the cells at the free margins of the plate grow in towards one 

 another from the two sides, and meeting in the median plane 

 complete the wall of the neural canal (Fig. 32). 



The nervous system is now a tube (Figs. 30 and 33), with 

 proper walls of its own, extending along the dorsal surface of the 

 embryo. It opens in front to the exterior, at the neuropore, op- 

 posite the anterior border of the first somite; and it communicates 

 posteriorly with the archenteron, through the neurenteric canal. 

 The wall of the tube consists of a single layer of cells, which 

 bear flagella at their inner ends. 



The anterior end of the neural tube, close to the neuropore, 

 has, almost from the first, thicker walls than the rest of the 

 tube. This thickening, which affects especially the ventral 

 wall of the tube (Fig. 33), becomes much more marked in 

 the later stages ; partly owing to actual increase in the thick- 

 ness of the wall itself; and partly to a great diminution in 

 the diameter of the hinder part of the tube, as the embryo 

 becomes drawn out into the elongated form characteristic of the 

 later larval condition. 



In the ventral wall of the neural tube, opposite the fifth 

 pair of somites, a black pigment spot, possibly a sense organ, 

 appears at about the stage represented in Fig. 33 ; and 

 much later, towards the end of the embryonic period, another 

 pigment spot, the eye, is formed in the anterior wall of the 

 brain swelling (cf. Fig. 36). 



2. The Notochord. 



The notochord is developed, as already noticed, from the band 

 of hypoblast cells which forms the dorsal wall of the archenteron, 

 and lies between the two lateral mesoblast folds. 



Its earliest appearance as a distinct structure is seen in a 

 larva with three pairs of somites, i.e. immediately after the 

 time of hatching ; and the successive stages in its formation 

 are shown in Figs. 28, 29, and 32, CH. 



The median plate of hypoblast cells, fonning the roof of the 

 archenteron, first becomes marked off, by a difference in mode of 

 arrangement of the cells, from the lateral mesoblast folds, and 



