PLATE XVI. 



Fig. 18. — A metaphase of the first spermatocyte mitosis. This figure shows 

 the various forms assumed by the chromosomes during their division. 



Fig. 19. — Same as fig. 18. 



Fig. 20. — One side of the cell, exhibiting a portion of a spindle and the chro- 

 mosomes. These are in different stages of division, the central ones being nearly 

 separated. 



Fig. 21. — Mid-anaphase of the first spermatocyte. Immediately upon the 

 separation of the chromatids along the plane of the longitudinal cleavage, the 

 two remaining chromatids spring apart at the ends unconnected with the fibers, 

 as is here represented. 



Fig. 22. — A series of figures, drawn with the camera lucida, showing the 

 changes experienced by the chromosomes during the first spermatocyte division. 

 The progressive movement of the chromatids along the invisible plane of cleav- 

 age is here conclusively shown; a, b, c, d and e were drawn from one cell, and in- 

 dicate clearly the position of the chromosome with reference to the spindle; h 

 shows the form of chromosome, of the stage represented by a, when viewed from 

 the pole, and i is a ring formed by the union of the free ends of the chromatids 

 seen in h. 



Fig. 23. — Telophase of the first spermatocyte division, exhibiting the close 

 massing of the chromosomes. The persistent character of the spindle fibers is 

 here indicated. 



Fig. 21. — Metaphase of the second spermatocyte. The bipartite structure of 

 the chromosomes, first noticed in the anaphase of the first spermatocyte (21), is 

 here clearly indicated. It will be noticed that in this stage the spindle is much 

 shorter than that of the first spermatocyte metaphase. 



Fig. 25. — Another cell of the same character as shown in fig. 21. Note the 

 persisting spindle fibers of the first spermatocyte division at one side of the mi- 

 totic figure. 



Fig. 26. — Anaphase of the second spermatocyte division. The individual 

 chromatids are here seen, differing much in their form from that of the paired 

 elements found in the anaphase of the first spermatocyte division. In this par- 

 ticular cell each stands nearly opposite the sister element to which it was united. 

 Note the elongated form of the spindle. 



Fig. 27. — A somewhat later anaphase, in which is to be observed the concen- 

 tration of the archoplasmic substance at the periphery of the spindle, and also 

 the variation in the form of the chromatids. 



Fig. 28. — Polar view of a second spermatocyte anaphase, showing the chro- 

 matids moving to the poles. 



