232 Vertebrate Embryology 



notochord, probably as a product of the periph- 

 eral cells. During the fourth day, all the 

 cells of the notochord become vacuolated, and 

 the vacuoles continue to increase in size until, 

 on the sixth day, they make up almost the 

 entire cell, the protoplasm being reduced to 

 an extremely thin wall around each cell ; the 

 nuclei are very indistinct if not quite invisible. 

 Thus is the notochord converted into a spongy 

 network, the fine meshes of the network being 

 the remains of the walls of the originally solid 

 cells (Figs. 55, nek, 73). The notochord 

 reaches its greatest development on the sixth 

 day ; and after that time it is gradually en- 

 croached on by the growth of the vertebrae, as 

 has already been described, until it finally 

 disappears. 



The notochord is the most characteristic 

 structure of the great group of animals known 

 as the chordata, being found in all represen- 

 tatives of this group, either during the embry- 

 onic period only, or throughout life. In the 

 higher members of the group, as in the chick, 

 it is a very prominent and characteristic em- 

 byronic structure, but disappears in the adult : 

 in some of the lowest members of the group, 

 as in the little fish-like Amphioxus, it persists 



