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Explanation of Plates. 
All the figures were drawn with a Zeiss 2 mm. oil immersion objective, those of 
pl. XXI, with a Zeiss compensating ocular 6 giving a magnification of 1260, and those 
of pl. XXII, with a Zeiss compensating ocular 12, giving a magnification of 2280. 
Plate XXI. 
Ascaris megalocephala. 
Figs. 1 — 21: bivalens. Fig. 22: univalens. 
Fig. 1. Spermatogonial divisions showing the fifth smaUer idiochromosome. 
Fig. 2. First spermatocyte. Each of the ordinary chromosomes composed of 
four rods; the idiochromosome of two. 
Figs. 3 — 6. Condensation of the ordinary chromosomes to tetrads. The üivided 
idiochromosome lies below in the figures. 
Fig. 7. Beginning of the first division. 
Figs. 8 — 10. First division anaphase. The halves of the idiochromosome üi lin- 
ear arrangement, connected together by protoplasniic processes and also wüth one 
dyad in each migrating group. 
Fig. 9. Showing initial Separation of the halves of the idiocliromosome as the 
cytoplasm divides. 
Fig. 10. Showing divided idiochromosome, caryolymph and metaplasm. 
