Plate III. Fertilization and segmentation of the egg of the sea urchin Arbacia, 

 as seen in eggs sectioned and stained for microscopic examination. A, sperm cell 

 about to enter egg. B, sperm nucleus (small black-stained object) approaching egg 

 nucleus. C, sperm nucleus (black object) fuses with egg nucleus, 10 minutes after 

 entry of sperm cell into egg. D, nucleus of fertilized egg begins to divide. E, divi- 

 sion in progress. The black-stained objects in a row at center are the chromosomes. 

 F, nuclei of the two daughter-cells re-forming. G, stage of two cells. H, the first two 

 cells now divide again. I, stage of four cells. All magnified 375 diameters. Prepared 

 and photographed by Ethel Browne Harvey. 



