62. 



other is continuous, and forms a part of the cells constituting 

 the walls of the whole body. This new cavity or incomplete 

 tube is the rudiment of the embryonic nervous system, and 

 may, therefore, be called the neural tube. Between it and 

 the alimentary tube at the hind part of the body a new 

 structure has now made its appearance — the row of large 

 cells s, the immediate origin of which is uncertain. This 

 row of cells is the rudiment of the axis band (or notochord) 

 of the tail. 



The embryo now becomes bent at its hind part, the bend- 

 ing takes place more on the left side than the right ; and the 

 bent portion grows more rapidly than the rest of the body, 

 thus giving rise to the tail. 



The stage represented in fig. 4 is now soon reached. The 

 digestive tube h, with its thick walls, occuj)ies the fore part 

 of the body ; immediately above it is the neural cavity n, 

 wider and more capacious in front than it was, but filled up 

 with cells behind m. It is still open at d (thus presenting a 

 very close correspondence with Amphioxus, in which the front 

 opening of the neural tube is long patent). The cells previ- 

 ously forming the general body wall have now become differ- 

 entiated into an epidermis, underneath which are, at certain 

 spots, numerous loose rounded cells. The most conspicuous 

 feature, however, is the row of cells, x, each possessing a 

 nucleus stretching from behind the neural cavity to the tip of 

 the tail. (The figure also shows the gelatinous envelope 

 omitted in the previous figures.) A transverse section taken 



at this stage through the forepart of embryo present, as fig. 

 5, with a neural tube 7i placed above a visceral or alimentary 



