20 ON THE SPERMATOGENESIS OF STENOBOTHRUS YIRIDULUS. 



which their individuality is lost: the heterotropic chromosome is seen in its 

 characteristic form as a darkly staining body apposed to the nuclear membrane. 

 The nucleus is here at its sinallest volume. 



Fig. 7. The same at a later stage. The growth of the nucleus is very noticeable. 



Fig. 8 Beginning of primary spermatocyte prophase. The reticulum has become con- 

 verted into a highly convoluted spireme, the heterotropic chromosome remaining 

 as a homogeneous mass apposed to the nuclear wall. 



Fig. 9. Later prophase of primary spermatocyte. The spireme has broken into filaments, 

 which have become more darkly stained by the closer association of their com- 

 ponent granules. These filaments are shortening and thickening and have begun 

 to assume the characteristic ring and boomerang shapes. A nucleolus is shown 

 above the heterotropic chromosome. 



Plate 2. 



Fio-s. 10-19. Chromatin filaments twisted into crosses, rings, and single or double loops ; 

 they have become shorter and more condensed than they appeared in the last 

 figui'e, and will shortly transform themselves into the smooth and compact 

 chromosomes of the metaphase complex. 



Fio-. 20. Polar view of the primary spermatocyte metaphase — the first maturation division, — 

 showing the nine chromosomes, again divisible into large, small, and medium- 

 sized chromosomes. The heterotropic is still the fourth largest and is marked 

 with a cross. 



Fig. 21. Lateral view of the primary spermatocyte metaphase. The ordinary chromosomes 

 are arranged on the mitotic spindle, and the heterotropic chromosome has already 

 passed to the lower pole. 



Fig. 22. Ditto. 



Fig. 23. Lateral view of the later anaphase of the primary spermatocyte division. The 

 ordinary chromosomes have begun to move towards the opposite poles, but are 

 still attached to one another by the connecting fibrils. The lieterotropic chrnmo- 

 some has passed to the upper pole without division. All the chromosomes are 

 shown. 



Fig. 24. Lateral view of the primary spermatocyte telophase, showing the massing of 

 chromosomes at the two poles of the spindle. 



Fig. 25. Polar aspect of the secondary spermatocyte metaphase. The nine chromosomes' 

 are exhibited, and are again divisible into three small, three large, two medium, 

 and the odd heterotropic chromosome. Two of the small chromosomes are 

 spherical and the third ovoid ; the remainder are seen as a pair of arms joined at 

 one extremity and closely apposed to one another. 



Figs. 26, 27. Ditto. It must not be forgotten that the heterotropic chromosome only occurs 

 in 50 7o of these cells. 



Plate 3. 



Fig. 28. Lateral view of the secondary spermatocyte mitosis — the second maturation 

 division. The heterotropic chromosome is seen on the spindle, with its halves 

 attached to one another by the connecting fibrils, thus appearing as the " lagging " 

 chromosome. The ordinary chromosomes are assembled at the two poles. 



Fig. 29. Resting-stage of the spermatid. The ordinary chromosomes have become resolved 

 into their component chromatin particles and have lost their identities ; the 

 heterotropic chromosome has become ragged through the same process, but can 

 still be recognized. It is marked X- 



