THE ASCIDIANS AND AMPHIOXUS 



III 



Figure 7.3 

 Development ofAmphioxus. 



(a) Section through egg one hour after fertilisation, showing the conjugation 

 of the pronuclei, a polar body, and the grey crescent material on the left 

 (posterior) side. 



(b) Eight-cell stage, seen from right side. 



(c) Thirty-two-cell stage. A space is appearing in the middle of the group 

 of cells. 



(d) A left half of an early gastrula, seen from the right side. Note large blasto- 

 coel cavity, the plate of large endoderm cells on the lower right (anterior) 

 side of the drawing and the irregular, rapidly dividing cells of the mesoder- 

 mal (=former grey) crescent on the lower left. 



(e),(/) Similar views of later gastrula stages. 



(g) Gastrulation nearly completed and the blastopore narrowed to quite a 



small hole. 



