Fig. 161. Early cleavage in Echidna. (Courtesy, Flynn and Hill, '39.) (A) Four-cell 

 stage. (B) Eight-cell stage. (C) About 16-cell stage. Two meridional and four vertical 

 cleavages have occurred. (D) About 32-cell stage. Observe marginal and central cells. 

 (E) Section through blastodisc of 4 to 8-cell stage. (F) Section through blastoderm 

 of 16-cell stage. See (C). (G) Section through blastoderm of 32-cell stage, showing 

 central and marginal cells. 



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