6 STUDIES IN SPERMATOGENESIS. 



diamond-shaped rings. The tetrads usually remain connected by- 

 delicate linin threads, as shown in figures 57 and 60, also in figures 

 62 and 63, the latter taken from the metaphase of the first maturation 

 spindle. If these linin connections persist, as they appear to do, from 

 the segmentation of the spireme to metakinesis, the first division of 

 the contracted tetrads must be longitudinal, corresponding to the split 

 in the segments of figures 55, 57, 58, etc. The chromosomes in the 

 metaphase usually appear as dumbbells (fig. 66) or elongated crosses 

 (fig. 67), but occasionally one can be found which still shows its 

 tetrad nature (fig. 64), so clearly indicated in the quadrivalent crosses 

 of figure 59. In the anaphase the chromosomes are often split as in 

 figure 68, and occasionally the two components can be seen as plainly 

 as in figure 65. Figure 61 shows the various shapes assumed by the 

 element x during the tetrad-stage of the chromosomes. This element 

 X almost invariably appears in a vesicle near one pole of the spindle 

 (figs. 67, 68) ; in exceptional cases it is found nearer the equatorial 

 plate, as in figure 66, or even in the same plane with the ordinary 

 chromosomes, but always somewhat isolated from them. In position 

 and form this element resembles the accessory chromosomes described 

 by Baumgartner ('04) for Gryllus domes Hcus ; in its mode of origin it 

 seems to difier from the other accessory chromosomes yet described. 



Figures 69 and 70 show the 23 bivalent chromosomes in metaphase ; 

 in figure 69 the element x is shown partly behind the large chromo- 

 some and at a different level. In figures 66 and 67 the one excep- 

 tionally large chromosome doubtless represents the two larger ones of 

 the spermatogonia. In the anaphase the element x is sometimes as 

 conspicuous as in figfure 71 ; in other cases it is concealed either behind 

 or within the polar mass of chromatin. In this form there is a dis- 

 tinct resting stage between the two maturation mitoses (figs. 72-75). 

 The element x is conspicuous in one-half of the cells (figs. 72, 73) ; it 

 may be included in the nucleus as in figure 72, or it may be partly or 

 wholly outside, as in figures 74, 75, and 76. In the latter case, but 

 not in the former, it is surrounded by its own membrane. As the 

 chromatin begins to condense for the second mitosis, disintegration 

 of the element x becomes apparent. This is most easily made out in 

 cases where the element is isolated, as in figures 75 and 76 ; but there 

 seems to be little doubt that it disappears before the metaphase of the 

 second maturation mitosis. It is not possible to count the chromosomes 

 in this stage, they are so crowded together, but it is not probable that 

 such a conspicuous chromatin element as that seen in the first division 

 could escape detection, even if it were in the equatorial plate among 

 the chromosomes. No aberrant element is ever seen in these spindles ; 



