AbstractsBiology & Animal Science

The members of the class Hydrozoa are the only coelenterates that are really diploblastic (Hyman ‘4o). The general body plan, polyp and medusa alike, consists of an outer epidermis and an inner gastrodermis, separated from each other by a thin non-cellular mesolamella. The epidermis is derived from the embryonic ectoderm,while the gastrodermis is formed of the endoderm. Whether the sexual products originate from any particular layer, or whether there exists a sexual polarity between the two layers, has long attracted the attention of zoologists since the enunciation of von Baer’s germ-layer theory. Many papers have dealt with the topographical origin of the germ cells in this group of animal, the results of most of which are summarized below in chronological order.[...]