The true asexual modes of reproduction amongst the Metazoa consist of fission and gemmation. Gemmation is by far the most widely disseminated of the two. Various as are the methods in which it takes place, it seems nevertheless that cells derived from all the germinal layers, and very frequently from all the important organs of the adult, assist in forming the bud. Into the details of the process, which require in many points a fuller elucidation, it is not my purpose to enter. Gemmation is a far commoner occurrence amongst the simpler than amongst the more highly organised forms. It appears to have been superadded to the sexual mode of reproduction quite independently in a number of different instances. While there is no difficulty in understanding how gemmation may have started in such simple types as the Coelenterata, the manner in which it first originated in certain highly organised forms, as for instance the Ascidians, is somewhat obscure, but it seems probable that it began with the division of the developing germ into two or more embryos, at a very early stage of growth. Such a division of the germ is, as has been shewn by Kleinenberg, normal in Lumbricus trapezoides[7] and Haeckel has shewn that an artificial division of the germ in the Siphonophora leads to the development of two individuals. It has been pointed out by various naturalists that the production of double monsters is often a phenomenon of the same nature. While it is next to impossible to understand how production of a bud could commence for the first time in the adult of a highly organised form, it is not difficult to form a picture of the steps by which the fission of the germ might eventually lead to the formation of buds in the adult state. [7] The case of Pyrosoma, which might be cited in this connection, is probably secondary. The coexistence of sexual reproduction with normal asexual multiplication, or with parthenogenesis, has led to a remarkable phenomenon in the animal kingdom known as alternations of generations[8]. [8] For an excellent account of this subject, _vide_ Allen Thompson's article Ovum in Todd's _Cyclopædia_. The metamorphosis of the Echinoderms included under this head in Thompson's article is now known not to be a proper case of alternations of generations.