cleavages. 6. Soon after fertilization, showing fertilization membrane 

 and hyaline layer. 7. Male nucleus. 8. Monaster. 9. Amphiaster and 

 cleavage furrow. 10 to 13. Cleavages. 14. Hatching from the fertiliza- 

 tion membrane. 15. Free-swimming blastula. 16. Pluteus, well formed. 

 17 to 20. Less regular development with fertilization membranes rup- 

 tured, so that cells are more scattered. 21 to 24. Division planes have 

 failed to come in; nuclear division without cell division. 24. Egg pep- 

 pered with small nuclei without cell division. Magnification about 

 270 X. (Biol. Bull. 79:177, 1940c. Photograph 16 has been replaced by 

 a later photograph.) 



PLATE XI. 



Development of red half-egg, parthenogenetic; parthenogenetic 

 merogone. Times (at 23°) after activation are given under each 

 photograph. Note the similarity of these photographs and those of the 

 fertilized merogones on Plate X, facing this. Compare picture for 

 picture. Photographs 1 to 4. Elongate eggs. 5 to 16. Spherical eggs. 

 1 and 5. Unactivated eggs immediately after removal from the centri- 

 fuge. 2 to 4. Early cleavages. 6. Soon after activation, showing ferti- 

 lization membrane and hyaline layer. 7. Clear sphere, simulating a 

 nucleus. 8. Monaster. 9. Amphiaster and cleavage furrow. 10 to 13. 

 Cleavages; note especially 13, quite regular many celled organism, 

 similar to the same stage of a fertilized merogone on preceding plate. 

 14. Hatching from the fertilization membrane. 15. Blastula. 16. Non- 

 cellular but healthy -looking parthenogenetic merogone of four weeks; 

 there has been no differentiation. 17 to 20. Cleavages without fertili- 

 zation membranes; cells are scattered. In Photograph 20, right, cell 

 divisions have taken place in the light but not in the pigmented por- 

 tion. 21 to 23. Multi-astral eggs with small spheres associated with the 

 asters. 24. Many small spheres resembling nuclei. Magnification about 

 270 X (Biol. Bull. 79:179, 1940c.) 



PLATE XII. 



Development (at 23°) of clear quarter-eggs and of eggs centrifuged 

 after fertilization. Magnification about 270 X. 



Photographs 1 to 13. Development of clear quarter. 

 Photograph 1. Fertilized clear quarter, showing fertilization mem- 

 brane. 2. Monaster stage, 20 minutes after fertilization. 3. Two cells, 

 two hours after fertilization. 4. Two cell stage of the clear quarter and 

 about a 32-cell stage of the white half, two hours after fertilization. 

 5 and 6. Four cells, three hours after fertilization, 7. Eight cells, four 

 hours after fertilization. 8 and 9. Sixteen cells, 5 hours after fertiliza- 



