The Development of Phascolosoma. I57 



Fig. 11. Chromosomes of the second polar spindle in metaphase, 

 as seen in a section oblique to the long axis of the spindle. 2500: 1. 

 Fig. 12. Second polar spindle in anaphase. 2500 : 1. 



Fig. 13. Rotation of spermatozoon. Sixteen minutes after the 

 approach of the spermatozoon. 1460 : 1. 



Fig. 14. Path of modified protoplasm left by the sperm nucleus. 

 Twenty-five minutes. 670 : 1. 



Fig. 15. Egg of the same age as in Fig. 14. 670 : 1. 



Fig. 16. Forty minutes after fertilization, showing enlarged sperm 

 aster and nucleus. 670: 1. 



Fig. 17. Egg of Ph. gouldii, immediately before the union of the 

 two pronuclei. 670 : 1. 



Fig. 18. Similar stage in Pli. vulgare. Fifty minutes after fertili- 

 zation. 670 : 1. 



Fig. 19. Conjugation of pronuclei. Fifty-five minutes. 670:1. 



Plate 5. 



The figures were drawn from the living egg of Ph. vulgare, with a 

 magnification of 328 diameters. 



Fig. 20 — 25 show the changes in quadrants A and Z? in a living 

 egg, seen from the side. The chief axis in the eggs in this series of 

 figures is not precisely vertical, the active pole lying in each 10^'— lö" 

 to the left of the mid-vertical line. Fig. 20 two cells; Fig. 21 four cells; 

 Fig. 22 eight cells; Fig. 23 sixteen cells; Fig. 24 twenty four cells, six 

 in each quadrant, of which two apical cells and two trochoblasts are in 

 the active hemisphere. Fig. 25. Trochoblasts, ia^-i and W^--, about to 

 divide to form primary cells of the prototroch. 2a and 2b are about to 

 give off minute cells to the left. 



Fig. 26. Side view of living agg to show quadrants C and D in the 

 four-cell stage. 



Fig. 27. The same. Eight-cell stage. 



Fig. 28. (' quadrant. 16- cell stage. 



Fig. 29. The same, 28-cell stage ; posterior trochoblast flattens out 

 and covers 2c. 



Fig. 30. Posterior hemisphere, 16 — 28-cell stage. 



Fig, 31. The same egg, 28 cells. 



Fig. 32. Active pole, 8— 16-cell stage. 



Fig. 33. 15-cell stage, active pole. 



Fig. 34. 16 — 28-cell stage, active pole. The same egg as that 



shown in Fig. 32 and 35. 



Fig. 35. 32 — 48-cell stage, showing the beginning of the formation 



of the rosette by the laeotropic division of la^-^ — Id^-'^. This has been 



