N. M. Stevens

“ Of the insects considered in this paper Aphis and Termopsis have no "accessory chromosome" or "heterochromosome" of any kind. The fact that no males develop from the fertilized eggs of Aphis might be offered as a reason for its absence, but such an argument would not apply to Termopsis. The sex-character may indeed be represented in the chromatin of some one of the pairs of paternal and maternal chromosomes of the spermatocytes, but there is no evident peculiarity by which one-half of the spermatozoa can be said to be different from the other half. ”

“ In several of the species of Coleoptera and in Aphrophora, it has been shown that a body staining like chromatin develops in the spermatids, increasing in size for a time, then breaking up into granules and disappearing. This body evidently has no relation to the heterochromosomes, as it is the same for all of the spermatids. Its staining qualities suggest that it may be material derived from the chromosomes. ”

“ 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. ”

“ On the whole, the first theory, which brings the sex determination question under Mendel's Law in a modified form, seems most in accordance with the facts, and makes one hopeful that in the near future it may be possible to formulate a general theory of sex determination. ”

“ 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. ”

“ 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 very young follicles are spherical, the older ones ovoid in form. The primary spermatogonia (plate XIV, fig. 237) —very clear cells with a lobed nucleus which stains slightly—occupy the tip of the follicle. Next to these comes a layer of cysts of secondary spermatogonia which are conspicuous for their deeper staining quality (fig. 238) . There appears to be no plasmosome in either class of spermatogonia. Figure 239 is the equatorial plate of a secondary spermatogonium. There are 23 chromosomes, two of which are conspicuously larger than the others and evidently form a pair. ”

“ The question of dominance is a difficult one, especially in parthenogenetic eggs and eggs which are distinctly male or female before fertilization. It may be possible that the sex character of the egg after maturation is always dominant in the fertilized egg, as appears to be the case in these insects (see scheme) . Conditions external to the chromosomes may determine in certain cases, such as Dinophilus, which sex character shall dominate in the growing oöcyte, and maturation occur accordingly. ”

“ 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. ”

“ The larvæ only were collected, as they gave all the desired stages for a study of the spermatogenesis, and also oögonia and synizesis and synapsis stages of the oöcytes. In the first collections the testes were dissected out, but the many free follicles break apart so easily that the later material was prepared by cutting out the abdominal segments which contained the reproductive organs, and fixing those without dissection. The same methods of fixation and staining were employed as for the Coleoptera. ”

“ 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 dumb-bells are so bent at the ends (fig. 52) that the spindle fibers, here also, are attached at or near the center of each univalent component of a bivalent chromosome, and the separated, univalent chromosomes go to the poles of the spindle in the form of Vs. As in Tenebrio the heterochromosome pair is late about coming into the equatorial plate (figs. 47-48) , but it does finally take its position with the others (fig. 49) and separates into its component parts somewhat earlier than the other bivalents (figs. 52, 53) . ”

“ 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. ”

“ 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) . ”

“ Sections of male pupæ were examined for equatorial plates of somatic mitoses. Figure 1 is a specimen of such plates. As might be expected, this figure resembles quite closely the spermatogonial equatorial plate (fig. 3) in number, form, and size of chromosomes, the small one being present in both. ”

“ 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. ”

“ Here Morgan's ('06) suggestion as to degenerate males seems much to the point. The male sex [Pg 56] character, having become dominant in certain eggs at an early stage, may, from that time on, determine the kind of development. ”

“ 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. ”