I 



MUSEUM OF COMPARATIVE ZOOLOGY. 619 



PLATE L 



Figs. 1-20. Formation of polar globules. 



Figs. 1-9. Successive views of the same egg at 6:00, 6:02, 6:04, 6:06, 6:30, 

 7:40, 9:30, 10:45, and 11 o'clock. 



Figs. 10-14. Formation of second polar globule. Another egg seen at 11:08, 

 11:09, 11:10, 11:12, and 11: 15 o'clock. 



Figs. 15-20. Formation of second polar globule as seen in another egg, at 6:08, 

 6: 33, 7: 04, 7: 05, 7: 06, and 7: 15. First segmentation of this egg nearly completed 

 at 10:00 o'clock. 



Fig. 21. Egg showing an irregular zone of clear protoplasm and two pronuclei. 



Fig. 22. The deeper of the lateral zones of fibre thickenings has reached the 

 border of the well-defined central area of the internal aster. Optical meridional 

 section. 



Fig. 23. Optical section in the plane of the polar globule, showing the second 

 archiamphiaster ; the peripheral aster more sharply outlined than the deeper one. 



Fig. 24. Same seen along the axis of the spindle. Focused a little above the 

 centre of the superficial aster ; the polar globule "projected." (Its outline has been 

 made too irregular and ragged in lithographing. ) 



Fig. 25. Another egg of about the same stage, and seen in the same position as 

 Fig. 22. 



Fig. 26. Yolk elements from the vitellus of a crushed egg. 



Fig. 27. Peculiar appearance, as of decussating fibres, seen at the animal pole 

 after the formation of the first polar globule. 



Figs. 28, 29. The fourth and third respectively of five successive sections of the 

 egg, Fig. 21, put in acid during the first segmentation. Slightly distorted by the 

 traction of the knife in cutting. The plane of section is not quite parallel with the 

 plane determined by the polar axis and the line joining the centres of the two asters, 

 but cuts both these lines. From its obliquity to the polar axis, it results that the 

 polar globules, and the curved remnant of the spindle (interzonal filaments), which 

 both lie in this axis, are found not in the same but in successive sections. From its 

 obliquity to the line joining the astral centres, it results that the nucleus of one of 

 the two segmentation spheres is cut, as shown in Fig. 29, while the other remains 

 untouched, and also that the interzonal filaments {spf") are cut across in the sphere 

 marked " y" (Fig. 29). Chromic acid preparation. 



Figs. 30-32. Three successive views of an egg, at 8, 8:54, and 9 o'clock. 



Fig. 30. Polar globules already formed. The two pronuclei with very clear cir- 

 cular outlines of nearly equal size. No change from the spherical form observed. 



Fig. 31. The egg has changed form slightly, and two oval, ill-defined spots are 

 visible, at some distance apart, the pronuclei having disappeared. The yolk shows 

 a faintly expressed radial arrangement of granules about these two spots. 



Fig. 32. The spots are farther apart ; the radiate arrangement more distinct ; the 

 cleavage furrow at the animal pole of the yolk is quite pronounced. 



Figs. 33-35. Three views of an egg, at 7: 54, 8: 35, and 9: 12 o'clock. 



Fig. 33. A clear spot at the animal pole, and deeply penetrating narrow zone of 

 clear protoplasm (pz) near the equator. 



