CAUSES OF STERILITY IN THE MULE

Abstract

1. The parents of the mule are widely different as can be seen from the comparison of the horse and the ass. 2. The mule partakes of both the sire and the dam, but appears to resemble the ass more than the horse, both in structure and habits. 3. The greatest difference seems to lie in the relative number of chromosomes in the cells of these two animals. The horse has thirty-seven and the mule fifty-one. This suggests that the number in the ass is about sixty-five, thus making a difference of twenty-eight chromosomes between the parents of the hybrid. 4. The seminiferous tubules of the mule contain a much smaller amount of germ cells than do the tubules of the horse. Some of the tubules of the mule are entirely devoid of sex cells. 5. The sex cells of the mule are larger than those of the horse in the corresponding stages. 6. All of the fifty-one chromosomes in the spermatogonial cells of the mule enter the spindle for division. The mitotic figures are normal and there are no straggling chromosomes. 7. The accessory chromosome of the hybrid, which is undoubtedly maternal in origin, resembles entirely the accessory of the horse, which fact shows that this sex-determining chromosome retains its individuality. 8. The period of synizesis which is so obvious in the primary spermatocytes of the horse, is lacking in the mule. 9. The spireme of the horse is also lacking in the mule, but is replaced by a continuous network of chomatin threads, parts of which sometimes resemble the spireme to a certain extent. 10. There is no definite time for the pairing of threads or chromosomes in the hybrid; the synaptic period begins at the time the chromatin threads make their appearance and continues through the prophase. 11. The pairing of chromosomes, or pseudo-reduction, is always incomplete and very inconstant. The number of chromosomes in the late prophase of the primary spermatocytes varies from thirty-four to forty-nine. The greatest majority of counts of chromosomes lie between forty and forty-five. The expected number, if reduction was complete, would be twenty-five besides the unpaired accessory. 12. In the late prophase of the primary spermatocytes the bivalent chromosomes can as a rule be readily distinguished from the univalent ones. The number of chromosomes which the various cells lack in order to make the original total of fifty-one, in terms of univalence, can usually be accounted for by the proportional increase in the presence of bivalents in such cells. 13. Up to the early prophase of the primary spermatocytes there seems to be no necessity for the paternal and maternal chromosomes to coöperate in functioning. Each group seems to go on performing its functions normally. The real conflict ensues during the various stages of the primary spermatocyte, and is no doubt occasioned by the necessity for coöperation on the part of the paternal and maternal chromosomes in the process of conjugation or pseudo-reduction. 14. Abnormalities in mitosis occur invariably in primary spermatocytes that attain the metaphase stage. 15. Giant cells are occasionally seen. 16. The chromatoid body which is very conspicuous and constant in the horse is entirely lacking in the mule. 17. There is considerable evidence that primary spermatocytes make an attempt to eliminate some of the chromatin material. 18. The chromosomes expelled by the cells appear to be those which were contributed by the mother of the hybrid. 19. Destruction as well as abnormalities in mitosis seem to be restricted to the primary spermatocytes. Most of the cells disintegrate during the prophase, especially during the period of synapsis. Others meet their fate in the metaphase or early anaphase stages. The remaining few that survive the anaphase succumb soon after, and no secondary spermatocytes nor spermatids, and consequently no spermatozoa are formed and the hybrid remains sterile. 20. There are no authentic cases on record showing that fertility ever occurs in this hybrid.