300 M. D. HILL. 
Fig. 5. Section of egg twenty-five minutes after fecundation. Male pro- 
nucleus has “fused”? with female, and astrosphere has divided. No 
trace of sperm centrosome. 
Fig. 6. Section of egg forty-five minutes after fertilisation. Centrosome 
has appeared again and divided into two in each astrosphere. Central 
mass has become entirely homogeneous. 
Fics. 7—20.—Phallusia mammillata. 
Fig. 7. Cross-section through ovary of Phallusia mammillata, with 
ovarian ova. 
Fig. 8. Cross-section of an unfertilised ovum directly after leaving ovi- 
duct. Nucleus is enormously reduced in size, and chromosomes are 
eight in number. 
Fig. 9. Shows changes in nucleus in forming first polar spindle. 
Fig. 10. Shows extension of first polar body. 
Fig. 11. The first polar body has divided into two, and the second polar 
body has also been formed. (Slightly less magnification than Fig. 10.) 
Fig. 12. Shows the changes through which remaining chromosomes from 
second polar spindle pass during formation of female pronucleus. 
Fig. 13. Cross-section of egg a few minutes after fertilisation. Sperm 
head has rotated through 90°. 
Fig. 13a. Sperm head enlarged. 
Fig. 14. Cross-section of egg ten minutes after fertilisation. Sperm 
head and astrosphere have increased in size, and centrosome is evident 
in the granular central mass. 
Fig. 15. Cross-section of egg fifteen minutes after fertilisation, Sperm 
head has split into two. 
Fig. 16, Cross-section of egg twenty minutes after fertilisation. Sperm 
head has broken up into eight chromosomes, and central mass has 
(unusually early) become homogeneous. 
Fig. 17, Sperm head has passed into the resting stage of the male pro- 
nucleus. The astrosphere is in act of division, and central mass 
reticular. 
Fig, 18. Sperm astrosphere has divided into two, and two pronuclei are 
in contact. 
Fig. 19, First segmentation spindle with sixteen chromosomes. Central 
masses of atrospheres have become nearly homogeneous, and centro- 
somes divided into two. 
Fig. 20. Cross-section of an unfertilised ovum, taken from oviduct of a 
specimen living in aquarium. 
Fic. 21. Shows transformation of middle piece of spermatozoon into central 
mass of astrosphere. Slightly diagrammatic. 
