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Plate IV. Segmentation of the egg of the sea urchin Arbacia, photographed 

 while living by Ethel Browne Harvey. A, fertilized egg. B, beginning of segmenta- 

 tion. C, two-cell stage. D, four cells. E, eight cells. F, sixteen cells. G, "mulberry" 

 (morula) stage. H, hollow embryo (blastula) . Ij young adult sea urchin. A to H, 

 magnified 290 diameters; /, almost natural size. 



