50 



AMPHIOXUS. 

 -PB 







Fig. 14. — Segmentation of the egg of Amphioxns. x 220. (After Hatschek.) 



I, the egg before the commencement of development. PB, polar body. II, the 

 egg in the act of dividing, by a vertical cleft, into two equal blastomeres ; about one 

 hour after fertilisation. Ill, stage with four equal blastomeres ; about two hours after 

 fertilisation. IV, stage with eight blastonleres : an upper tier of four slightly smallei* 

 ones, and a lower tier of four slightly larger ones. V, stage with sixteen blastomeres, 

 in two tiers, each of eight. "VI, stage with thirty two- blastomeres, in four tiers, each 

 of eight ; about three hours after fertilisation : the embryo is represented bisected ver- 

 tically to show the segmentation cavity or blastocoel, B. VII, later stage : the blasto- 

 meres have increased in number by further division. "VIII, blastula stage : bisected 

 to show the bh^stocoel, B ; about four hours after fertilisation. 



