[The development of the mycetome in the embryo of Euscelis plebejus Fall. (Homoptera-Cicadina)].

The egg of the leafhopper Euscelis plebejus contains 2 types of symbiontic bacteria classified by MüLLER (1949) as t- and a-symbionts. They are lodged near the posterior egg pole as a nearly spherical mass surrounded by periplasm. During anatrepsis of the germ band the volume of the symbiont mass increases considerably. At the same time the a-symbionts are taken up by primary a-mycetocytes which then form a spherical layer around the as yet extracellular mass of t-symbionts. When the mesoderm of the abdominal segments starts to differentiate into organ primordia, a cluster of presumably polyploid mesoderm cells leaves the 2nd abdominal segment and migrates towards the 5th and 6th abdominal segments where the symbionts are lodged. There these cells are filled with the t-symbionts which they harbour through all further stages of embryonic and postembryonic life. The a-symbionts before katatrepsis of the germ band appear to be taken over by a second type of a-mycetocyte which stems from the genital ridge of the 4th-6th abdominal segments and as a rule is binucleate. During and after katatrepsis the t-mycetocytes and secondary a-mycetocytes migrate from their medial position to the lateral walls of the abdomen where they form one compound mycetome on each side.