N. M. Stevens

“ In both somatic and germ cells of the two sexes there is a difference not in the number of chromatin elements, but in the size of one, which is very small in the male and of the same size as the other 19 in the female. The egg nuclei of the female must be alike so far as number and size of chromosomes are concerned, while it is absolutely certain that the spermatids are of two equal classes as to chromatin content of the nucleus—one-half of them have 9 large chromosomes and 1 small one, while the other half have 10 large ones. ”

“ It becomes separated from the other chromatin in the tetrad-stage, remains nucleolus-like in form, and later appears in the first maturation division either among the chromosomes or in a more or less aberrant position. It passes into one of each pair of spermatocytes of the second order, persists during the rest stage, appears in the second mitosis as a dyad and then divides, going into one-half of the spermatids. The spermatids, however, as in Stenopelmatus, all have the same appearance: each has in the center—not against the nuclear membrane—a small element that stains like chromatin. ”

“ 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. ”

“ Usually the two daughter nuclei are formed in an undivided cell. The resting-stage between the two divisions is only partial. The nucleolus appears and divides into two (figs. 33-36) , and the chromosomes change into the dyad form (fig. 36) , in which they come into the second maturation spindle (figs. 37, 38) . The equatorial plate again shows 26 chromosomes (fig. 39) . The formation of the spermatozoa is peculiar in that the original spermatocyte cell-body, as a rule, does not divide; but the four nuclei resulting from the two maturation divisions develop into sperm-heads in one cell. ”

“ The youngest spermatocytes closely resemble the spermatogonia (fig. 212) . There is no bouquet stage and no such marked spireme stage as in [Pg 14] 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 accessory chromosome is present. Figures 215 and 216 are equatorial plates of the first spermatocyte mitosis. ”

“ The forms selected for study have been taken from several groups of insects, and are all species whose spermatogenesis has not been previously worked out. They are (1) a California termite, Termopsis angusticollis; (2) a California sand-cricket, Stenopelmatus; (3) the croton-bug, Blattella germanica; (4) the common meal-worm, Tenebrio molitor; and (5) one of the aphids, Aphis œnotheræ.
A brief account of a chromatin element resembling the accessory chromosome in Sagitta has been added for comparison. ”

“ If 23 is the somatic number in both sexes, it must be maintained by union of sex-cells containing 11 and 12 chromosomes, respectively, the same unequal number occurring in the maturated eggs as in the sperm. Under such conditions it is difficult to see how the odd chromatin element of the spermatozoa can determine sex. ”

“ In the spermatogonium there is a very large nucleolus (plate I, fig. 1) , which in the iron-hæmatoxylin preparations is very conspicuous, but does not stain like chromatin with thionin or other anilin stains, nor does it behave like an accessory chromosome during the maturation mitoses. Before each spermatogonial division it divides as in figures 2 and 3, and the same is true for each maturation mitosis. Figure 4 shows the 52 chromosomes of a spermatogonial division in metaphase. Figures 5 and 6 are young spermatocytes, showing the division of the nucleolus. ”

“ That of Miss Wallace, who suggests that in the spider only the one out of each four spermatids which contains the accessory chromosome is capable of developing into a functional spermatozoön, while the other three degenerate, as do the polar bodies given off by the egg. McClung is inclined to believe that the accessory chromosome is an element common to all of the male reproductive cells of Arthropods, and probably to vertebrate spermatocytes as well ('02) . ”

“ In figure 108 similar segments may be seen, one of them showing a longitudinal split. The element x is present, but the nucleolus has disappeared. In many cases the split, if it appears at all, closes quickly and the chromosome bends in U-shape, as in figure 109, plate IV. This figure also shows two centrosomes (c) . In other cases the split persists as in figure 110 and leads to the formation of crosses of a tetrad character (figs. 111, 112, 113) , as in Stenopelmatus and many other insects. Figures 114 to 117 show later stages of the U-shaped chromosomes. Perfect rings are rare. ”

“ The testes were fixed in various fluids—Flemming's strong solution, Hermann's platino-aceto-osmic, Gilson's mercuro-nitric, Lenhossek's alcoholic sublimate acetic, and corrosive acetic. Flemming's and Hermann's fluids followed by safranin gave good results in most cases. ”

“ Second spermatocyte division—metakinesis.
79. Same; late anaphase. [Pg 23]
[Pg 24]
Plate III. Stenopelmatus.
Fig. 80. Telophase of second maturation mitosis.
81. Young spermatid, showing spindle-remains at s.
82. Spermatid showing a conspicuous chromatin element in nucleus, and spindle-remains (s) elongated.
83. Spermatid, showing centrosome (c) and divided spindle-remains (s and a) .
84. Older spermatid, showing centrosome (c) , axial fiber of tail, and spindle-remains (s) .
85. Spermatid, showing acrosome material (a) migrating to side of nucleus opposite centrosome. ”

“ On the ovogenesis and spermatogenesis of Sagitta bipunctata. Zool. Jahrb., vol. 18.
Sutton, W. S.
'02. On the morphology of the chromosome group in Brachystola magna. Biol. Bull., vol. 4, no. 1.
'03. The Chromosomes in heredity. Biol. Bull., vol. 4, no. 5.
Wallace, L. B.
'00. The accessory chromosome in the spider. Anat. Anz., vol. 18.
'05. The spermatogenesis of the spider. Biol. Bull., vol. 8, no. 3.
Wilcox, E. V.
'95. ”