19 a 



20 a 



20 b 



No. 21 Egg o{ Chaetopterus, X 8oo, compensated. Metaphase of first maturation 

 division. Chromosomes are visible at the equator of the spindle. Photograph kindly 

 lent by S. Inoue. 



No. 22 Egg of the sea-urchin Psammechinus miliaris, X 450, compensated. Late 

 anaphase. Photograph taken at high numerical aperture to show astral rays. 

 Birefringence largely obscured by light scattering. 



