[Development of the cloaca and its ulterior transformations in the polypterids (Pisces)]

Generally spoken cloacae have been but little studied. In Polypterus, a brachiopterygian fish, a cloaca only exists during parts of the embryonal and larval life and it gets replaced by structures consisting of an anus and an urinary sinus. In the beginning of embryonal life the posterior opening of the body seems to be nothing else than the original blastoporus. The wall of this opening acquires evaginations; the anlagen of the excretory ducts come in contact with them, establishing in that way a structure, that merits the name of cloaca. During the elongation of the post-vitellin body, the latter gets shifted from the level of metamere XII to that of metamere XXX. Another migration of the cloaca occurs when the digestive system acquires its development. Finally the cloaca may be found under the 47th and 48th muscular segments (at least in the species Polypterus senegalus senegalus Cuvier). During these stages the excretory ducts have build up an urinary sinus that gets separated from the gut. In adults no communication exists any more between the gut and the urinary sinus; the latter is a very destinct organ that may have an impair or pair aspect according to its being filled up. Because of the establishing of a communication with the genital ducts the sinus becomes an uro-genital sinus in the adult.