Amongst the forms with alternations of generations and a fixed larval form Chrysaora and Cassiopea have been most fully investigated. The ovum of the former undergoes the first embryonic phases while still in the ovary. In the latter it is enclosed amongst the oral processes. A complete and more or less regular segmentation leads to the formation of a single-walled blastosphere with a small segmentation cavity. The wall of the blastosphere next becomes invaginated, giving rise to an archenteron (fig. 78 A). The blastopore soon closes up, and the archenteron is converted into a closed sack completely isolated from the epiblast (fig. 78 B). The surface of the larva becomes in the meantime covered with cilia. The free larval stage thus reached is similar to the ordinary Hydrozoon planula. After the closure of the blastopore the larva becomes elongated, and one end becomes narrowed. By this narrowed extremity the larva soon attaches itself, and at the opposite and broader end a fresh involution of the epiblast appears (fig. 78 C); this gives rise to the stomodæum, which is placed in communication with the archenteron on the absorption of the septum dividing them. The relation of the stomodæum to the original blastopore has not been determined. At the point of attachment there is developed a peculiar pedal disc, and around the mouth there appears a fold of epiblast which gives rise to an oral disc (fig. 78 D). Two tentacles first make their appearance, but one of these is primarily much the largest, though eventually the second overtakes it in its growth. A second pair of tentacles next becomes formed, giving to the larva a 4-radial symmetry. Between these four new tentacles subsequently sprout out, and in the intermediate planes four ridge-like thickenings of the hypoblast, projecting into the cavity of the stomach, make their appearance. They imperfectly divide the stomach into four chambers, to each of which one of the primary tentacles corresponds; they may be regarded as homologous with the mesenteries of the Actinozoa. The number of tentacles goes on increasing somewhat irregularly up to sixteen. All the tentacles contain a solid hypoblastic axis. Muscular elements are developed from the epiblast. [FIG. 78. FOUR STAGES IN THE DEVELOPMENT OF CHRYSAORA. (After Claus.) A. Gastrula stage. B. Stage after closure of blastopore. C. Fixed larva with commencing stomodæum. D. Fixed larva with mouth, short tentacles, etc. _ep._ epiblast; _hy._ hypoblast; _st._ stomodæum; _m._ mouth; _bl._ blastopore.] With the above changes the so-called Hydra tuba or Scyphistoma form is reached (vide fig. 85). The peculiar strobilization of this form is dealt with in the section devoted to the metamorphosis. Aurelia is stated by Kowalevsky to develop in the same way as Cassiopea; and the one stage of Rhizostoma observed is that in which it has a (probably invaginate) gastrula form.