tubes. 13. Red halves. Magnification of Photographs 1-9, 270 X. (Photo- 

 graphs 1-9 Biol. Bull. 79:167, 1940c. Photographs 10-13 Biol. Bull. 

 71:105, 1936, modified.) 



PLATE VIII. 



Development of centrifuged eggs (at 23°). Photographs 1 to 8. 

 Elongate eggs. 1. Unfertilized elongate egg stratified as in Plate VII; 

 note cortical granules at periphery of the clear layer. 2. Usual two 

 cell stage, 1)^ hours after fertilization at 23°. 3. Usual four cell stage, 

 1/2 hours. 4. Occasional four cell stage, 1/2 hours. 5. About 16 cells, 

 2/2 hours; note red cells are larger than the white ones. 6. About 32 

 cells, micromeres at left, 3 hours. 7. Slipper blastula, 5/2 hours. 8. 

 Hatching from the fertilization membrane, 7 hours. Photographs 9 

 to 20. Spherical eggs. 9. Unfertilized egg; same layers as in Photo- 

 graph 1. Photograph 10. Monaster. Note pigment being redistributed 

 as the aster forms. 11. First cleavage, usually at right angles to the 

 stratification. 12. First cleavage parallel with the stratification. 13. 

 Four cell stage, 1/2 hours. 14. Eight cells, 2 hours. 15. Micromeres 

 (without pigment) at the oil cap. 16. Micromeres (without pigment) 

 at the centrifugal pole. 17. Blastula. 18. Late blastula, pigment still 

 concentrated. 19, 20. Plutei with pigment still concentrated; in 19, 

 near the mouth, in 20 in one arm, 24 hours. Magnification about 270 X . 



PLATE IX. 



Development of white half -egg. Times (at 23°) after fertilization 

 given under each photograph. Photographs 1 to 4. Elongate egg. 5 to 

 21. Spherical egg. 1 and 5. Immediately after removal from the cen- 

 trifuge. 6. Monaster (not well marked). 7. "Streak" stage, enlarged 

 nucleus. 8. Breaking of nuclear membrane. 9. Amphiaster. 10. Just 

 before cleavage. 11, 12. First cleavage in any plane. 13 to 17. Cleavages 

 to form blastula; blurring in 17 caused by blastula swimming inside 

 the fertilization membrane. 18. Hatching from the fertilization mem- 

 brane. 19. Free-swimming blastula. 20. Gastrula. 21. Pluteus; note 

 lattice-like skeleton in the arms. Magnification about 270 X. (Biol. 

 Bull. 79:171 and 173, 1940c, modified.) 



PLATE X. 



Development of red half-egg, fertilized; fertilized merogone. Times 

 after fertilization at 23° are given under each photograph; 1 to 4, 

 elongate eggs; 5 to 16, spherical eggs. Photographs 1 and 5. Unferti- 

 lized eggs immediately after removal from the centrifuge. 2 to 4. Early 



