9. All the chromosomes of the second spermatocyte are paired structures and divide in a similar way. The spindle is small and weak as compared with that of the first spermatocyte, and the chromosomes arrange themselves radially on its periphery in such a way that the pairs lie in the plane of the spindle axis with their joined ends inward. The space between the chromatids represents the line of cross-division observable in the prophase segments of the first spermatocyte, and their separation accordingly represents a reduction division. The accessory chromosome, on the contrary, divides along the plane marking the longitudinal cleft of the spermatogonial spireme. 10. From each first spermatocyte there are formed, by two divisions, four spermatids, of which two are distinguished from the remaining pair by the possession of an extra chromosome in addition to the number—sixteen—common to them all. Both classes undergo a like series of transformations by which they become mature spermatozoa. These are necessarily of two kinds; and it is believed that those containing the accessory chromosome, in the act of fertilizing the egg, determine that the germ-cells of the embryo shall be sexually male, or like themselves, while those from which it is absent are unable to impress their sex upon the egg and assist in producing female embryos. VII. BIBLIOGRAPHY. =1=. Atkinson, Geo. F., 1899. Studies on Reduction in Plants, Bot. Gaz., vol. 28. =2=. Baumgartner, W. J., 1902. The Spermatid Transformations of _Gryllus assimilis_, etc., Kans. Univ. Sci. Bull., vol. 1, No. 1. =3=. Brauer, A., 1893. Zur Kenntniss der Spermatogenese von _Ascaris megalocephala_, Arch. mikr. Anat., vol 42. =4=. Carnoy, 1895. La Cytodiérese chez les Arthropodes, La Cellule, vol. 1. =5=. Cholodkovsky, N., 1894. Zur Frage über die Anfangsstadien der Spermatogenese bei den Insecten, Zool. Anz., vol. 17. =6=. Griffin, B. B., 1899. Studies on Maturation, Fertilization and Cleavage of _Thalassema_ and _Zirphæa_, Jr. Morph., vol. 15. =7=. Häcker, Valetin, 1897. Ueber weitere Uebereinstimmungen zwischen den Fortpflianzungsvorgängen der Tiere und Pflanzen, Biol. Cent., vol. 17. =8=. Henking, H., 1890. Untersuchungen über die ersten Entwicklungsvorgänge in den Eiern der Insecten. 2. Ueber Spermatogenese und deren Beziehung zur Eientwicklung bei _Pyrrhocoris apterus_ M., Z. wiss. Zool., vol. 51. =9=. Henking, H., 1892, idem. 3. Specielles und Allgemeines, _ibid._, vol. 54. =10=. Ischikawa, 1891. Studies of Reproductive Elements. Spermatogenesis, Ovogenesis and Fertilization in _Diaptomus_ sp., J. Coll. Sc. Imp. Univ. Japan, vol. 5. =11=. Kingsbury, B. F., 1902. The Spermatogenesis of _Desmognathus fusca_, Am. Jour. Anat., vol. 1, No. 2. =12=. Montgomery, T. H., jr., 1898. The Spermatogenesis in _Pentatoma_ up to the Formation of the Spermatid, Zool. Jahrb., vol. 12.