46 OUTLINES OF EVOLUTIONARY BIOLOGY 



FIG. 13. Early Development! of Amphioxus. (Adapted from Ziegler's 

 Models, after Hatschek.) 



I, the fertilized egg ; II, two blastomeres formed by the first cleavage ; III, stage with 

 four blastomeres; IV, stage with eight blastomeres; V, stage with sixteen blasto- 

 meres ; VI, stage with thirty-two blastomeres, cut in half vertically ; VII, blastula or 

 blastosphere stage, cut in half; VIII, early stage of gastrulation, cut in half; 



