PLATES 



PLATE I. 



Test of Arbacia punctulota. Photograph 1. Aboral or dorsal view. 

 2. Oral or ventral view showing the teeth. 3. Side view. 4. Close-up of 

 photograph 1 showing 4 anal and 5 genital plates, with genital pores; 

 including the large madreporic plate. 5. Side view of the test without 

 spines. 6. Aboral view with a light bulb inside the shell; this is ap- 

 proximately natural size. Photographs 3 to 6 with the assistance of 

 G. G. Lower. 



PLATE 11. 



Maturation of the egg; polar body fomiation. Living eggs except 17, 

 18 which are stained sections. 1 to 6. Growth of the oocyte. 7, 8. 

 Breaking of the germinal vesicle. 9, 10. First polar body. 11, 12. Second 

 polar body; note flattening of the cell in 12. Photographs 13, 14, 15. 

 Centrifuged oocytes. 16. Formation of blebs where spermatozoa hit 

 the surface. 17. Stained section, showing a bleb and spermatozoa on 

 the surface and inside the egg. 18. Section showing mitochondria ( dark 

 material ) around the germinal vesicle. Magnified about 270 X except 

 17, 18, about 600 X. 



PLATE III. 



Normal development of living eggs, from one cell to the gastrula. 

 Times after fertilization at 23° C are given under each photograph. 

 Magnification about 270 X, the same throughout as nearly as possible. 

 Photograph 1. Unfertilized egg. 2. Spermatozoa around the egg. 3. 

 Fertilization cone above, center. 4. Fertilized egg. 5. Monaster. 6. 

 "Streak" stage, narrow streak. 7. Broad streak fading out and enlarged 

 nucleus. 8. Amphiaster. 9 to 11. First cleavage; note cells are separate 

 in 10, close together in 11. Photograph 12. Four cells. 13. Eight cells. 

 14. Twelve cell stage, four colorless micromeres below. 15. Sixteen 

 cells. 16. About thirty-two cells. 17 to 20. Blastulae. 21. Hatching 

 from the fertilization membrane. 22. Late blastula. 23. Early gastrula. 

 24. Late gastrula, spicules forming. 



PLATE IV. 



Normal development, continued; gastrula to pluteus. Times after 

 fertilization at 23° C are given under each photograph. Magnification 

 (except 9) the same as Plate III, about 270 X, as nearly as possible the 

 same throughout; Photograph 9 is approximately 180 X. Photograph 1. 

 Late gastrula. 2. Same stage in dark field. 3. "Prism" stage. 4 to 6. Early 



