Giard suggests that in some cases of apparently normal fertilization one of the pronuclei may degenerate, the resultant embryo being the product of one pronucleus only. In this way he explains certain cases of hybridization in which the paternal (rarely the maternal) type is exclusively reproduced. For instance, in the batrachiate Amphibia, Héron Royer succeeded in 1883 in rearing, out of a vast number of attempts, a few hybrids between a female _Pelobates fuscus_ and a male _Rana fusca_; the product was a _Rana fusca_. He also crossed a female _Bufo vulgaris_ with a male _Bufo calamita_; in the few cases which reached maturity the product was obviously a _Bufo calamita_. Finally, H.E. Ziegler (_Arch. f. Ent.-Mech._, 1898, p. 249) divided the just-fertilized ovum of a sea-urchin in such a way that each half had one pronucleus; the half with the male pronucleus segmented and formed a blastula, the other degenerated. It is said that in a few species of animals males do not occur, and that parthenogenesis is the sole means of reproduction (a species of Ostracoda among Crustacea; species of Tenthredinidae, Cynipidae and Coccidae among Insecta); this is the thelytoky of K.T.E. von Siebold. The number of species in which males are unknown is constantly decreasing, and it is quite possible that the phenomenon does not exist. Parthenogenesis, however, is undoubtedly of frequent occurrence, and is of four kinds, namely, (1) that in which males alone are produced, e.g. honey-bees (_arrhenotoky_); (2) that in which females only are produced (_thelytoky_), as in some saw-flies; (3) that in which both sexes are produced (_deuterotoky_), as in some saw-flies; (4) that in which there is an alternation of sexual and parthenogenetic generations, as in Aphidae, many Cynipidae, &c. It would appear that "parthenogenesis does not favour the production of one sex more than another, but it is clear that it decidedly favours the production of a brood that is entirely of one sex, but which sex that is differs according to circumstances" (D. Sharp, _Cambridge Natural History_, "Insects," pt. i. p. 498). In some Insecta and Crustacea exceptional parthenogenesis occurs: a certain proportion of the eggs laid are capable of undergoing either the whole or a part of development parthenogenetically, e.g. _Bombyx mori_, &c. (A. Brauer, _Arch. f. mikr. Anat._, 1893; consult also E. Maupas on parthenogenesis of Rotifera, _Comp. rend._, 1889-1891, and R. Lauterborn, _Biol. Centralblatt_, xviii., 1898, p. 173). Determination of sex.