(a) _The Polyp._--The buds that form polyps are very simple in mode of formation. Four stages may be distinguished; the first is a simple outgrowth of both layers, ectoderm and endoderm, containing a prolongation of the coelenteric cavity; in the second stage the tentacles grow out as secondary diverticula from the side of the first outgrowth; in the third stage the mouth is formed as a perforation of the two layers; and, lastly, if the bud is to be separated, it becomes nipped off from the parent polyp and begins a free existence. (b) _The Medusae._--Two types of budding must be distinguished--the direct, so-called, palingenetic type, and the _indirect_, so-called coenogenetic type. The direct type of budding is rare, but is seen in _Cunina_ and _Millepora_. In _Cunina_ there arises, first, a simple outgrowth of both layers, as in a polyp-bud (fig. 43, A); in this the mouth is formed distally as a perforation (B); next the sides of the tube so formed bulge out laterally near the attachment to form the umbrella, while the distal undilated portion of the tube represents the manubrium (C); the umbrella now grows out into a number of lobes or lappets, and the tentacles and tentaculocysts grow out, the former in a notch between two lappets, the latter on the apex of each lappet (D, E); finally, the velum arises as a growth of the ectoderm alone, the whole bud shapes itself, so to speak, and the little medusa is separated off by rupture of the thin stalk connecting it with the parent (F). The direct method of medusa-budding only differs from the polyp-bud by its greater complexity of parts and organs. [Illustration: FIG. 44.--Diagrams of Medusa budding with the formation of an entocodon. The endoderm is shaded, the ectoderm left clear. A, B, C, D, F, Successive stages in vertical section. E, Transverse section of a stage similar to D. Gc, Entocodon. s.c, Cavity of entocodon, forming the future sub-umbral cavity. st, Stomach. r.c, Radial canal. c.c, Circular canal. e.l, Endoderm lamella. m, Manubrium. v, Velum. t, Tentacle.] The indirect mode of budding (figs. 44, 45) is the commonest method by which medusa-buds are formed. It is marked by the formation in the bud of a characteristic structure termed the _entocodon_ (_Knospenkern_, _Glockenkern_). [Illustration: FIG. 45.--Modifications of the method of budding shown in fig. 44, with solid Entocodon (Gc.) and formation of an ectotheca (ect.).]