14 STUDIES IN SPERMATOGENESIS. 



many other insects. The true synapsis occurs, as shown in figure 

 213, by pairing of like chromosomes side by side. This conjugation 

 of like chromosomes is followed by a stage in which they are massed 

 together at one side of the nucleus (fig. 214). In these latter stages 

 the nucleolus has entirely faded out and nothing suggesting an acces- 

 sory chromosome is present. Figures 215 and 216 are equatorial 

 plates of the first spermatocyte mitosis. There are 5 chromosomes of 

 different sizes and shapes, and figure 216 shows each one double. 

 The first division of the chromosomes, though apparently longitudinal, 

 is evidently a separation of the elements paired in a preceding stage, 

 and is therefore a reducing division . 



The anaphase of the same mitosis is shown in figures 217 and 218 ; 

 it is peculiar in that one chromosome always divides more slowly than 

 the others, the two elements hanging together at one end. In figure 

 219 are sister spermatocytes of the second order, the " lagging " chro- 

 mosomes still connected. The second maturation division is seen in 

 metaphase in figure 220 and in anaphase in figure 221. Figure 222 

 shows a young spermatid, the five chromosomes still preserving their 

 characteristic form. Figure 223 is the equatorial plate of the first 

 maturation division of the winter egg, showing the same form and size 

 relations of the chromosomes as in the spermatocyte divisions. Figures 

 224 and 225 are equatorial plates of a polar spindle (fig. 224) and of 

 a segmentation spindle (fig. 225) of the parthenogenetic egg, where 10 

 chromosomes are present, 2 of each of the sizes found in the sexual 

 germ cells. 



So far as an accessory chromosome or any other visible evidence 

 of a sex determinant are concerned, the results are entirely negative. 

 The conditions shown do, however, support Mendel's conception of 

 the " purity of the germ-cells," and also afford evidence in favor of 

 Boveri's theory of the individuality of the chromosomes. 



Sagitta bipunctata. 



In connection with these insect forms it is of interest to find in the 

 spermatogenesis of Sagitta a body which stains like chromatin and 

 behaves somewhat like the accessory chromosome. It is found in all 

 resting stages of the spermatogonia, closely applied to the nuclear 

 membrane (fig. 226). It divides before each spermatogonial mitosis 

 (fig. 227) and, though not often discernible in the spindle, appears in 

 the next generation. Figure 228 is the last spermatogonial mitosis, 

 and figure 229 shows the element x, and the chromosomes paired at 

 one pole of the spindle. During the various phases of the growth 

 stage (figs. 230-232) the element x is again applied to the nuclear mem- 



