50 



AMPHIOXUS. 



Flo. 14. Segmentation of the egg of Amphioxus. x 220. (After Hatscbek.) 



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 blastomeres : an upper tier of four slightly smaller 

 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 blastoccel, B. VII, later stage : the blasto- 

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

 to show die bhistocot'l, B ; about four hours after fertilisation. 



