12 STUDIES IN SPERMATOGENESIS. 



spindle. Figure 183 shows a small tangential section of a spindle in 

 metaphase, containing the unequal pair and one equal pair. In figure 

 184 a polar view of a metaphase is shown, the unequal pair, which 

 was somewhat below the others, being indicated by stippling. Figures 

 184 a and 185 show that the unequal components of the unsymmetrical 

 pair, as well as the equal components of the symmetrical pairs, are 

 separated in metakinesis, making this clearly a reduction division. 

 Two polar plates are shown in figures 186 and 187, one containing 10 

 equal elements, the other 9 equal ones and i small one. The telophase 

 is shown in figure 188. There is no resting stage, but the new spindle 

 is formed from the remains of the old one, and the spindle-shaped 

 mass of chromatin seen in figure 188 either passes into the center of 

 the new spindle or becomes enveloped by it. The double chromo- 

 somes separate as in figures 189 and 190. Figure 190 shows the small 

 dyad, and figure 189 an aberrant one which may be its mate. The 

 spindle in both divisions is peculiar in having outside of the spindle 

 proper a dense mass of fibers which, in osmic material, stain deeply 

 with iron hsematoxylin. These fibers are shown in all the figures from 

 174 to 196. Figures 191 and 192 are equatorial plates of the two kinds 

 of spermatocytes of the second order, figure 191 showing the small 

 chromosome. An early anaphase appears in figures 193 and 194, 

 which show both the small and larger chromosomes in metakinesis. 

 Figure 195 is a later anaphase containing the divided small chro- 

 mosome. In figure 196 are shown the two polar plates of a spindle 

 corresponding to that of figure 195, and in figure 197 the polar plates 

 of a spindle in which 10 equal chromosomes have been divided. In 

 Tenebrio molitor the spermatids are therefore certainly of two distinct 

 kinds, so far as the chromatin content is concerned. 



In most of the young spermatids, after the nuclear membrane has 

 formed, there appears an isolated chromatin element, which corre- 

 sponds fairly well to the large or to the small component of the unsym- 

 metrical pair, separated in the first mitosis and divided in the second. 

 The clear portion of the nucleus containing this isolated element is at 

 first turned toward the spindle-remains (fig. 198), but before the tail 

 appears either the whole nucleus or its contents have rotated 180 

 (fig. 199). Various stages in the development of the spermatid are 

 seen in figures 200 to 203. The clear region and the isolated element 

 finally disappear (fig. 202 6), and the chromatin breaks up into coarser 

 and then into finer granules within the sperm-head. In the later stages 

 the centrosome is clearly seen at the base of the head (fig. 203). 



In order to determine, if possible, the value of the unsymmetrical 

 pair of chromatin elements, very young ovaries and ovaries with egg- 



