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WILSON. 



[Vol. VI. 



handed, and the third right-handed again. The first two sets 

 of cells thus produced alternate with each other, and thus form 

 a second or inner belt of eight cells {C, D. E), interposed between 



1 



Diagram VII. 



A. Diagram of the polyclade embryo, showing the relations of the micromeres to 

 the macromeres. Mesoblast shaded. The Roman numerals designate the cleavage- 

 plans. 



B. Corresponding diagram of the Nereis embryo. 



C. Twenty-eight-celled stage of polyclade. 



D. MoUuscan embryo after the differentiation of the germ-layers. 



E. Corresponding diagram of Nereis. 



