EXPLANATION OF PLATE XV. 



The original camera lucida drawings are at a magnification of 3000 

 diameters, and appear here at 2000. The optical equipment with which 

 the preparations were studied consists of an apochromatic objective of 

 2 mm. focus and 1.40 N. A., and compensating oculars. The light of the 

 Welsbach mantle was delivered to the objective through a Watson oil- 

 immersion condenser of 1.30 N. A. 



Fig. 1. — Telophase of secondary spermatogonium, showing the ends of 

 the divided accessory chromosome almost in contact. 



Fig. 2. — Telophase of similar cell, showing the accessory chromosome 

 en face. 



Fig. 3. — Same stage as figure 2. 



Fig. 4. — Same as figure 2, but with the halves of the accessory chromo- 

 some more separated. 



Fig. 5. — Various forms of the accessory chromosome in the prophase of 

 the first spermatocyte. Those represented at a, b, c, were fixed in Flem- 

 ming's fluid and stained in iron-hsematoxylin ; the remaining ones were 

 fixed in Hermann's fluid and stained with safranin-gentian violet. The 

 latter combination causes the elements to appear larger than the 

 former. Variations in the form of the accessory chromosome, at about 

 the same stage, are shown at d, e, f, g, h. The elements shown at /, ;, 

 k, I, m, n, are of later stage. Chromosomes in and n are from the same 

 cyst, the former being a lateral view, the latter en face. 



Fig. 6. — Polar view of a metaphase of the first spermatocyte, showing the 

 sixteen ordinary chromosomes and the accessory chromosome beneath. 



Fig. 7. — A slightly later stage than that represented in figure 6. No ac- 

 cessory chromosome shown. 



Fig. 8. — Polar view of the first spermatocyte metaphase, showing the six- 

 teen ordinary chromosomes. 



Fig. 9. — Similar to figure 8. 



Fig. 10. — Lateral view of a first spermatocyte metaphase, showing a few 

 of the chromosomes. The one in the form of a ring lies in the plane of 

 the spindle axis. 



Fig. 11. — Same stage as represented in figure 10, but showing the acces- 

 sory chromosome at one pole of the spindle. 



Fig. 12. — Telophase of the first spermatocyte, showing the accessory chro- 

 mosome in only one daughter-cell. 12a and 126 are views of the acces- 

 sory chromosome in the same stage but with different degrees of chro- 

 matid separation. 



Fig. 13. — Polar view of second spermatocyte metaphase in which the ac- 

 cessory chromosome is present. 



Fig. 14. — Similar to the cell shown in figure 13. 



Fig. 15." — Polar view of second spermatocyte without an accessory chro- 

 mosome. 



