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; IF, stage with eight blastomeres; F, stage with sixteen blasto- 

 meres ; FJ, stage with thirty-two blastomeres, cut in half vertically ; FIT, blastula or 

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



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