Nearly every possible method of reproduction occurs amongst the Hydromedusae. In classifying methods of generation it is usual to make use of the sexual or non-sexual nature of the reproduction as a primary difference, but a more scientific classification is afforded by the distinction between tissue-cells (histocytes) and germinal cells, actual or potential (archaeocytes), amongst the constituent cells of the animal body. In this way we may distinguish, first, _vegetative_ reproduction, the result of discontinuous growth of the tissues and cell-layers of the body as a whole, leading to (1) _fission_, (2) _autotomy_, or (3) _vegetative budding_; secondly, _germinal_ reproduction, the result of the reproductive activity of the archaeocytes or germinal tissue. In germinal reproduction the proliferating cells may be _undifferentiated_, so-called primitive germ-cells, or they may be _differentiated_ as sexual cells, male or female, i.e. spermatozoa and ova. If the germ-cells are _undifferentiated_, the offspring may arise from many cells or from a single cell; the first type is (4) _germinal budding_, the second is (5) _sporogony_. If the germ-cells are _differentiated_, the offspring arises by _syngamy_ or sexual union of the ordinary type between an ovum and spermatozoon, so-called fertilization, of the ovum, or by _parthenogenesis_, i.e. development of an ovum without fertilization. The only one of these possible modes of reproduction not known to occur in Hydromedusae is parthenogenesis. (1) True _fission_ or longitudinal division of an individual into two equal and similar daughter-individuals is not common but occurs in _Gastroblasta_, where it has been described in detail by Arnold Lang [30]. (2) _Autotomy_, sometimes termed transverse fission, is the name given to a process of unequal fission in which a portion of the body separates off with subsequent regeneration. In _Tubularia_ by a process of decapitation the hydranths may separate off and give rise to a separate individual, while the remainder of the body grows a new hydranth. Similarly in _Schizocladium_ portions of the hydrocaulus are cut off to form so-called "spores," which grow into new individuals (see Allman [1]). [Illustration: Much modified from C. Chun, "Coelenterata," in Bronn's _Tierreich_. FIG. 43.--Direct Budding of _Cunina_. A, B, C, E, F, In vertical section. D, Sketch of external view. st, Stomach. m, Manubrium. t. Tentacle. s.o, Sense organ. v, Velum. s.c, Sub-umbral cavity. n.s, Nervous system.] (3) _Vegetative budding_ is almost universal in the Hydromedusae. By budding is understood the formation of a new individual from a fresh growth of undifferentiated material. It is convenient to distinguish buds that give rise to polyps from those that form medusae.