18 



INTRODUCTION, 



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Fig. 2. — Segmentation of the egg of Amphioxus. x 220. (After Hatschek.) 



I, the egg before the commencement of development : only one polar body, PB, has 

 been seen, but from analogy with other animals it is probable that there are really two 

 present. II, the ovum in the act of dividing, by a vertical cleft, into two equal blasto- 

 meres. Ill, stage with four equal blastomeres. 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 : the embryo is represented bisected, to show 

 the segmentation cavity or blastoccel, B. VII, later stage : the blastomeres have in- 

 creased in number by further division. VIII, blastula stage : bisected to show 

 the blastoccel, B. 



