^24 embryology: lancelet 



invagination. Meanwhile the cells of the ectoderm develop cilia, by 

 means of which thegastrula revolves within the vitelline membrane; 

 and the cells of the flat dorsal side become more columnar and 

 form a distinct strip known as the neural or medullary plate. 

 The ectoderm at the sides of this plate now becomes detached 

 from it and grows over it, enclosing a small space (Fig. 481). 

 This process begins at the hinder end, so that the blastopore is 

 covered and opens into the space in question (Fig. 482, 2a). 

 The sides of the neural plate then fold upwards and meet above 

 the space, so as to form a tube which will become the nerve cord. 

 Its hollow is the neural canal, and the blastopore, which is now 

 known as the neurenteric canal, leads from it to the gut. Even- 

 tually the neurenteric canal closes. An opening, known as the 

 neuropore, long remains at the front end of the neural canal 

 and puts it into communication with the animal's olfactory pit 

 (p. 307). While these things are happening the ceiling of the 

 archenteron gives rise to the notochord and to the mesoderm. 

 The notochord is formed by a median longitudinal strip (Fig. 

 483) which becomes grooved and separated from before back- 

 wards, its cells eventually rearranging themselves to form a rod 

 and becoming vacuolated. Its front end grows forwards to the 

 end of the snout. The hind end is for a long time connected with 

 the endoderm in front of the neurenteric canal. The mesoderm 

 arises as hollow outgrowths (Figs. 481, 482, 483). One of these 

 is median and unpaired in front. Behind it hes a pair of dorso- 

 lateral grooves at the sides of the notochord. The median pouch 

 soon separates from the gut, but the grooves, as fast as they close 

 off in front, are prolonged backwards. As growth progresses 

 the separated anterior part of the groove becomes segmented 

 into a series of pouches. These pouches are the mesoderm segments 

 or mesoblastic somites. They will presently spread between ecto- 

 derm and endoderm and give rise to the mesoderm of the adult. 

 When the notochord and mesoderm have separated, the dorsal 

 edges of the endoderm close in under them to form a complete 

 tube, the enteron, which will become the ahmentary canal of 

 the adult. The rudiments of all three layers of a triploblastic 

 animal are now present. Ectoderm, endoderm, and mesoderm 

 are known as the germ layers, and though, as we shall see, they 

 arise in different ways in different animals, they are present at 

 an early stage in the embryos of all Triploblastica. Before these 

 processes are complete, hatching takes place by the throwing 



