50 



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

 I the e-g before the commencement of development. PB, polar body. II, tlio 

 ess 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 afte 

 fertilisation. IV, stage with eight blastomeres : an upper tier of four slightly 

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

 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 the blustoccel, B ; about four hours after fertilisation. 



