Tenebrio molitor.
In the metaphase of all spermatogonial mitoses where it was possible to count accurately, 20 chromosomes were found, 19 large ones of approximately equal size, and 1 small spherical one (figs. 169, 170). There is nothing in the resting nucleus of the spermatogonia which would suggest either a nucleolus or an accessory chromosome. The chromatin stains well during the whole growth period of the spermatocytes, but it is impossible to separate the period into so definite stages as in most other forms.
In the youngest spermatocytes one finds occasionally a cyst containing cells with nuclei like those of figures 171 and 172, indicating that a brief "synapsis" or condensation stage occurs at the close of the last spermatogonial mitosis. During the greater part of the period the chromatin forms a heavy, irregular, and often segmented spireme (figs. 173, 174). Shortly before the first maturation division, such split segments as appear in figure 175 are sometimes found; some of these simulate tetrads with slender connecting bands between the paired elements. Again, one finds a few cases like figure 176, where the spireme is segmented into bivalent chromosomes, each component showing a longitudinal split. This figure also shows the small chromosome. Usually, however, the irregular and much tangled spireme (figs. 173, 174) condenses into a heavy segmented band variously disposed in the nucleus (fig. 177). This band soon separates into the bivalent chromosomes shown in figures 178 and 179, giving 9 symmetrical pairs and 1 unsymmetrical one (fig. 179 s) composed of the small chromosome and a much larger mate. In the prophase of the spindle, in rare cases, some of the chromosomes are longitudinally split and transversely constricted, forming tetrads (fig. 180), but more often they appear as in figure 181. The unequal pair appears in each figure at s. In the metaphase (fig. 182) it is the last to come into the equatorial plate, possibly because of its lack of symmetry. The smaller component of this pair is always directed toward the equator of the 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 1 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 chromosomes 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 hæmatoxylin. 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 chromosome. 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 corresponds fairly well to the large or to the small component of the unsymmetrical 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 b), 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-tubes were sectioned and the chromosomes counted in the dividing cells of the egg-follicle (♀ somatic cells), and in dividing oögonia. In both cases 20 large chromosomes were found. Figure 207 is the equatorial plate from a female somatic cell of a young egg-follicle. Figure 208 a and b shows two sections of an oögonium in the prophase of mitosis. In order to determine the number and character of the chromosomes in the male somatic cells, several male pupæ were sectioned. As in the spermatogonia, 19 large chromosomes and 1 small one were found. Figure 204 shows the equatorial plate of a dividing male somatic cell, and figures 205 to 206 are daughter plates from a similar cell. (Three large chromosomes of the plate shown in figure 206 are in another section.)
From these facts it appears that the egg-pronucleus must in all cases contain 10 large chromosomes, while the spermatozoön in fertilization brings into the egg either 10 large ones or 9 large ones and 1 small one. Since the somatic cells of the female contain 20 large chromosomes, while those of the male contain 19 large ones and 1 small one, this seems to be a clear case of sex-determination, not by an accessory chromosome, but by a definite difference in the character of the elements of one pair of chromosomes of the spermatocytes of the first order, the spermatozoa which contain the small chromosome determining the male sex, while those that contain 10 chromosomes of equal size determine the female sex. This result suggests that there may be in many cases some intrinsic difference affecting sex, in the character of the chromatin of one-half of the spermatozoa, though it may not usually be indicated by such an external difference in form or size of the chromosomes as in Tenebrio. It is important that related forms should be studied in order to ascertain whether the same chromatic conditions prevail in other species of this genus or possibly in the Coleoptera in general.[A]
[A] Prof. E. B. Wilson has recently found a similar dimorphism in the spermatozoa of Lygæus and other of the Hemiptera heteroptera.