Studies in Spermatogenesis (Part 1 of 2)

by N. M. Stevens

GENERAL DISCUSSION.

THE "ACCESSORY CHROMOSOME."

The literature bearing on the "accessory chromosome" of McClung, the "small chromosomes" of Paulmier, and the "chromatin nucleoli" of Montgomery has been fully discussed by McClung in the paper entitled, "The accessory chromosome—sex determinant?" ('02), and will therefore be considered here only in its relation to the several forms studied. The present status of the question has been well summarized more recently by Montgomery under the heading "Heterochromosomes" in the paper, "Some observations and considerations upon the maturation phenomena of the germ cells."

Three theories as to the function of the "heterochromosomes" have been advanced: (1) That of McClung that they are sex-determinants, since in the forms which he has examined these chromatin bodies occur in only one-half of the spermatozoa, and the sex-character is the only character which divides the individuals of a species into two approximately equal groups. (2) That of Paulmier and Montgomery that they are degenerating chromatin. Montgomery regards them as "chromosomes that are in the process of disappearance in the evolution of a higher to a lower chromosome number." (3) 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).

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. As to McClung's statement ('02) "that if there is a cross-division of the chromosomes in the maturation mitosis, there must be two kinds of spermatozoa, regardless of the presence of the accessory chromosome," it appears to me that in a case like the aphid, where the paired elements of the five bivalent chromosomes are separated in the first maturation mitosis, there may be as many as seventeen kinds of spermatozoa instead of two. If, however, we suppose that the sex characters are segregated in the first maturation mitosis, there would, of course, be two equal classes of spermatozoa with reference to that character.

In Stenopelmatus the element x in certain stages closely resembles the "accessory chromosome" of McClung, and especially that described by Baumgartner for Gryllus domesticus, but its origin and fate are different. It first appears attached to an end of the spireme in the growth stage of the young spermatocytes, where it is much smaller than in later growth stages. It gradually increases in size, is a conspicuous element in the first maturation spindle, goes into one of each pair of spermatocytes of the second order, and there degenerates during the rest stage between the two maturation mitoses. The whole history of this element suggests that it may be rejected chromatin analogous to that observed in the ovogenesis of many forms. In Sagitta, for example, a considerable quantity of chromatin granules is given off by the chromosomes and cast out into the cytoplasm near the close of ovogenesis (Stevens, '03). Rückert ('92) has described a similar casting out of chromatin material by the chromosomes of the oöcytes of Pristiurus.

The spermatogenesis of Stenopelmatus, therefore, differs from that of the other Orthoptera which have been described in having (1) a larger number of chromosomes (46), (2) an even number in the spermatogonia, (3) an accessory chromatin structure in the spermatocytes of the first order, which disappears before the second maturation division.

In Blattella we have a typical "accessory chromosome," according to McClung. It appears (1) in all resting spermatogonia closely associated with a nucleolus, (2) in the spermatogonial mitoses as an odd chromatin element, making 23 in all, (3) in the growth stage of the spermatocytes connected with an end of the spireme and also with the nucleolus. 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. Occasionally a mass of chromatin is found outside the nucleus, but this is not constant enough to support the contention of Moore and Robinson ('05) that the "nucleolus" of the related form, Periplaneta americana, is fragmented and cast out into the cytoplasm. The spermatids all appear to develop equally well for some time, but as they approach maturity a varying proportion of them become degenerate. This can not, however, be due to absence of the accessory chromosome, as Miss Wallace supposes, in the spider; for in some follicles no degenerate spermatozoa are found, and in others more than half may be degenerate. All attempts to study fertilization stages of the egg have so far failed, and the chromosomes in the female somatic cells have not proved favorable for counting. Twenty-three have been counted in several cases, but there was always some chance of error. 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 brief description of the chromatin element x in Sagitta, introduced here because it behaves like the accessory chromosome in many particulars, serves as an example of the occurrence of such an element in the spermatogenesis of a hermaphrodite form, where it can not possibly be conceived of as a sex determinant. In Sagitta it is known to be confined to the male germ-cells. No such element occurs in the ovogenesis, in the sperm nucleus in the egg, or in the first segmentation spindle. Its function must, therefore, be confined to the process of spermatogenesis.

From the standpoint of sex determination, we have in Tenebrio molitor the most interesting of the forms considered in this paper. 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. Since the male somatic cells have 19 large and 1 small chromosome, while the female somatic cells have 20 large ones, it seems certain that an egg fertilized by a spermatozoön which contains the small chromosome must produce a male, while one fertilized by a spermatozoön containing 10 chromosomes of equal size must produce a female. The small chromosome itself may not be a sex determinant, but the conditions in Tenebrio indicate that sex may in some cases be determined by a difference in the amount or quality of the chromatin in different spermatozoa. This is much the most suggestive part of the work, and it will be followed up by the study of related forms.