EMBRYOLOGY 



479 



blastopore lip. This growth is at first more rapid on 

 the side which corresponds to the future dorsal side of 

 the body, but afterwards the ventral side grows the 

 more rapidly. As a result the blastopore becomes 

 a small opening at what will be the hinder end of 

 the body. 1 One (the dorsal) side becomes flattened, and 

 the blastopore comes to lie at the extreme hinder end of 

 this. Meanwhile the cells of the epiblast develop cilia, by 



Fig. 358. — Stages in the cleavage of the ovum of Amphioxus. — After 

 Hatschek. 



1, Unfertilised egg ; 2, stage of two blastomeres ; 3, stage of four blastomeres ; 4, 

 stage of eight blastomeres ; 5, stage of seventy-two blastomeres ; 6, blastula ; 

 j>.b., polar body. 



means of which the gastrula 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 epiblast at the sides of this plate now 

 becomes detached from it and grows over it, enclosing a 

 small space (Fig. 360). This process begins at the hinder 

 end, so that the blastopore is covered and opens into the 

 space in question (Fig. 361, 14 v.). The sides of the neural 

 plate then fold upwards and meet above the space, so as to 



1 The details of the narrowing of the blastopore are a matter of 

 dispute. 



