Plate II 



Pyrrochoris apterus 



26 to 31. Spermatogonia! groups, each showing twenty-three chromosomes, including the large 

 unpaired idiochromosome; 30, 31 illustrate the rare case in which the latter appears double, owing to 

 marked sigmoid curvature. These photos are copied in Text-figs. 2, c, d, e, j, k and /, respectively. 



32 and 33. Post-phases shortly following last spermatogonia! division; the chromosomes still distinct, 

 idiochromosome recognizable by its large size and deeper color. 



34 and 35. Presynaptic stages following the last, showing "caterpillar'' stage of idiochromosome 

 and small nucleoli. In the last two the shortening has begun. 



36 to 38. Further condensation of the idiochromosome; initial stages of synizesis; apparent duality 

 of the idiochromosome in two of the cells. 



39 to 42. Synizesis, showing various forms of the chromosome nucleolus. 



43 and 44. Early post-synaptic stages. 



45 and 46. Polar metaphases, first spermatocyte division. 



47 to 49. Side views of second division. 



50 and 51. Polar metaphases, second division. 



